West, Ian M. 2014. Kimmeridge Oil Shale, oil source rock and potential shale-gas fracking rock. Web site: www.southampton.ac.uk/~imw/Kimmeridge-Oil-Shale.htm. 12th April 2014.
Kimmeridge Field Guide - Blackstone, Oil Shale, at Clavell's Hard, Kimmeridge, Dorset, England
Ian West,
Romsey, Hampshire
and a Visiting Scientist at:
Faculty of Natural and Environmental Science
Southampton University,
Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory
Website archived at the British Library

|Click here - full LIST OF WEBPAGES --, |Kimmeridge - Kimmeridge Bay |Kimmeridge - Oil Shale Fires, and the Lyme Volcano |Kimmeridge - Hobarrow and Brandy Bays, West of Kimmeridge Bay |Kimmeridge - East of Kimmeridge Bay |Kimmeridge Clay Boreholes, Swanworth Quarry |Kimmeridge - Rope Lake Head to Freshwater Steps |Kimmeridge - Egmont Bight to Chapman's Pool |Kimmeridge - Bibliography - Start |Kimmeridge - Bibliography contin. |Petroleum Geology - South of England |Kimmeridge Clay - Fossils

Associated webpages:- KIMMERIDGE BAY - INTRODUCTION -- ; KIMMERIDGE - OIL SHALE FIRES

GO EAST? - KIMMERIDGE - ROPE LAKE HEAD TO FRESHWATER STEPS; --- GO WEST? - KIMMERIDGE BAY TO YELLOW LEDGE

Burning blocks of Kimmeridge oil shale or blackstone, a potential source of shale gas by fracking, seen in a small artificially started fire at Kimmeridge, Dorset, 2013

Tongue-like flames above artificially burning Kimmeridge oil shale or Blackstone, east of  Kimmeridge Bay, Dorset, 2013

Burning blocks of Kimmeridge oil shale, ignited artificially, showing the gasy type of burning

Combustion tube distillation of Kimmeridge oil shale, Kimmeridge, Dorset

Distilled droplets of oil coming from the margins of a small artificial fire of Kimmeridge oil shale or Blackstone, east of  Kimmeridge, Dorset, 2013

A general view of the bed of Kimmeridge Blackstone or oil shale, dipping eastward towards th beach, east of Clavell's Hard, Kimmeridge, Dorset, 14th June 2013

The Kimmeridge Blackstone or oil shale descends to the shore between Clavell's Hard and Rope Lake Head, Kimmeridge, Dorset, June 2013

The Blackstone or Kimmeridge oil shale seen exposed on the shore between Clavell's Hard and Rope Lake Head, east of Kimmeridge Bay, Dorset, 2008

Upper Kimmeridge Clay of Wheatleyensis to Hudlestoni Zones, between Cuddle and Clavell's Hard, east of Kimmeridge, Dorset, 14th June, 2013, BBC

Kimmeridge oil shale high in the cliff between Cuddle and Clavell's Hard, east of Kimmeridge Bay, Dorset, 2013, BBC


Related Field Guides-- | Kimmeridge - Kimmeridge Bay |Kimmeridge - Hobarrow and Brandy Bays, West of Kimmeridge Bay |Kimmeridge - East of Kimmeridge Bay |Kimmeridge - Rope Lake Head to Freshwater Steps, east of Kimmeridge Bay |Kimmeridge - Egmont Bight to Chapman's Pool |Kimmeridge - Bibliography - Start |Kimmeridge - Bibliography Continued | Kimmeridge - Beach Fires, Cliff Fires and the Lyme Volcano |

WAREHAM, DORSET - USE OF SHALE GAS IN 1848!
This Kimmeridge oil shale and adjacent strata are under investigation at Balcombe and adjacent area of the Weald in Sussex for possible use in producing shale gas by hydraulic fracturing or fracking. The Kimmeridge oil shale, favourably thick and buried, underlies the English Channel between the Isle of Purbeck and the Isle of Wight. The oil shale has, at maximum, about 70 percent organic matter. This is one of the highest proportions of organic matter in an oil shale in the world. The use of Kimmeridge Oil Shale for shale gas is not new at all, and is a industrial procedure that took place in Dorset at least 168 years ago. Shale gas from the Kimmeridge oil shale was used to light 130 street lights in Wareham in 1848 by Wanostostrocht and Company. This was a good technical advance for the date and probably a welcome contribution to urban facilities and safety. There was extensive mining of the the shale at Kimmeridge in Victorian times and there were plans in the 1890s to use Kimmeridge shale gas to light the streets of Paris, France. The Wareham Shale factory was adjacent to Wareham Railway Station. Thus, the use of shale gas in southern England is not new at all. Note that the hydraulic fracturing procedure or "fracking" is a relatively cool process compared to the pyrolysis or heating of the oil shale that was used in Victorian times to produce the gas. The mining procedures for shale in Dorset took place in low mines with loose shale roofs and health and safety procedures were probably not good.

(Please note, that this web site, including all the separate webpages, is only intended to provide geological information without bias. It is not a site for environmental controversy. It is not designed to either advocate or oppose on environmental grounds any policy or programme of either the high temperature pyrolysis of oil shale or the modern, low temperature, hydraulic fracturing (fracking) methods of obtaining shale gas in Dorset or other parts of southern England. The web site is similarly only geological with regard to the subject of oil exploration, which is one of the main providers of new geological information. The author is interested in sedimentology, petroleum geology, stratigraphy and in the Jurassic Coast and New Forest etc. His interests are strictly geological. He does not seek to become involved in controversy or opposition to the use of oil shale, hydraulic fracking or matters of shale gas. He will try to provide geological information, though, when he can.)

For information and photographs of the Blackstone west of Kimmeridge Bay at Brandy Bay, please go to:
Oil Shale or Blackstone at Brandy Bay - West of Kimmeridge Bay

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Other Kimmeridge Field Guides
Kimmeridge - Kimmeridge Bay
Kimmeridge - West of Kimmeridge Bay
Kimmeridge - East - Hen Cliff, Yellow Ledge and Cuddle
Kimmeridge - Blackstone, Oil Shale at Clavell's Hard
Kimmeridge - Burning Beach, Burning Cliffs and the Lyme Volcano
Kimmeridge - Rope Lake Head to Freshwater Steps
Kimmeridge - Egmont Bight to Chapman's Pool
Kimmeridge Clay Fossils
Kimmeridge - Bibliography - Start
Kimmeridge - Bibliography Continued

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Click on images for large, high resolution versions!
(do not use browser zoom on the low resolution versions)
INTRODUCTION:

SAFETY; the Hazards of Kimmeridge Cliffs

Rock-fall from the Kimmeridge Clay cliffs, Dorset

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A small rock fall from Kimmeridge Clay cliffs, Kimmeridge coastline, Dorset, 2011

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Small rock fall at the section with the Kimmeridge oil shale or Blackstone, east of Clavell's Hard, Kimmeridge, Dorset, June 2013

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There is considerable risk in this area of being cut off by the tide, and of being hit by falling rocks. See the main Kimmeridge webpage for more details regarding safety. Sadly someone has been killed by a falling rock in Kimmeridge Bay. In general, keep out as far from the cliffs as possible. Some debris falls every day and if you are close to the cliff you are at risk of being hit by a rock-fall.

The significant danger with regard to the Kimmeridge cliffs is perhaps matched only by Liassic shale cliffs of West Dorset. Although the main danger is risk of being hit by a rock fall, there is a second hazard is that of being cut off by the tide. This applies particularly to the cliffs east of Kimmeridge, although it could also happen at Brandy Bay and elsewhere.

Cliff collapse at the old No. 2 Level of 1890 Blackstone workings, just west of Clavell's Hard, Kimmeridge, Dorset

Because of the hazard of rock falls and it is a dangerous place to visit unless it is well-understood and proper precautions are undertaken. The cliffs are vertical and high and subject to erosion by the sea at the base. The shale and mudstone is full of joints and fissures and not stable. Small pieces will tumble off from time to time as you walk along.

More serious are substantial falls like the one shown above. These may occur without warning; suddenly there will be a loud crash and a plume of debris and dust. These happen particularly in certain weather conditions, such as when there is frost or rain and sometimes when the shale has dried in the hot sun. If you are out on the low-tide ledges falling debris would not usually reach you but there is no guarantee of safety. The risk is greatest where the cliffs are highest, where there is joint-separated shale above and where there is evidence of a recent fall in the form of shattered debris.

For more on the hazard from falling debris at Kimmeridge see photograph of Yellow Ledge.

In terms of route, from the east side of Kimmeridge Bay it is always possible to go east by the cliff top. It is only possible to go east by the beach when the tide is low and conditions are favourable.

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The danger of the tide-trap at Clavell's Hard, east of Kimmeridge Bay, Dorset

The tidal trap just west of Clavell's Hard, Kimmeridge, Dorset

Algal slime on the shale ledges at Clavell's Hard east of Kimmeridge Bay, Dorset

Tide problems at the eastern end of the Bay from Cuddle, prevent access to Clavell's Hard, Kimmeridge, Dorset, 4th January 2008

Watch the tide carefully, because a rising tide may push you back towards the dangerous cliffs, and there is no access up them. It is easy to be cut off on the beach here and only proceed if the tide and weather conditions are safe.

For study, examine the low-tide ledges instead of the cliffs whenever this can be done. Take care because the ledges are very slippery and this is especially the case where there is algal slime, as shown above. Do not attempt to climb the slippery waterfall at Clavell's Hard. A way up the cliffs and access down at Clavell's Hard was at one time possible but it is now too difficult to be attempted. There is no route up the cliffs at Freshwater Steps (once actually steps at the end of the path down from Encombe House) or anywhere east of Kimmeridge Bay. There have been several rescues of people trapped to the east of Clavell's Hard.

If you are able to get onto the Clavell's Hard mining ledge (and this should not be done without a safety rope), do not enter the old mine adit (in any case the entrance has recently collapsed). It extends in about 100 metre horizontally but the shale is unstable and unsafe.

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Kimmeridge Safety Addendum:

Photographs shown on this website have been taken over a period of many years by the author and various other people. Some photographs are from organised field-trips, which may be those of the present author or of other geologists. Many, though are from informal, private coastal walks, or from private, research field-trips. The photographs are for geological purposes only and are there to show rocks, not people or techniques. They are not intended to show safety procedures and no activities shown are necessarily intended to be copied. This website is about geology for geologists. The cliff, sea, tide and weather conditions vary greatly so always make your own assessment of the cliffs and conditions on the day, and arrange your coastal procedures in accordance. Always consult tide tables before field work at or near Kimmeridge. No responsibility at all is taken for any activities of field parties or individuals going to the Kimmeridge coast for their own purposes or objectives. As at other geological sites a risk is present and the possibility of an accident, although a rare occurrence, cannot be eliminated.

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BITUMINOUS KIMMERIDGE CLAY -

- OIL SOURCE ROCK OF THE NORTH SEA;
- EQUIVALENT OF THE ARAB FORMATION OF SAUDI ARABIA

Kimmeridgian strata are very well-know to petroleum geologists. Much of the world's oil comes from strata of Kimmeridgian age. The Ghawar Field, discovered in 1948, is in the Kimmeridgian reservoir limestone known as the Arab Formation. It produces 62.5 percent of Saudi Arabia's crude production, 8 million barrels of oil per day. The source rock is Upper Jurasic, but probably Oxfordian (Oxford Clay and Corallian equivalent - beneath the Kimmeridge Clay). The Kimmeridge Clay, seen in the cliffs of Dorset is not a reservoir rock; it is not porous and permeable. It is a potential oil source rock. On land in Britain it is mostly immature (not sufficiently deeply buried and heated) but under the North Sea and elsewhere it has been more deeply buried (and therefore more heated) and has resulted in the formation of large oilfields. Dorset is the type locality and the cliffs around Kimmeridge form the type-section, giving excellent exposures of the potential oil source rocks. In this webpage we go to Clavell's Hard where the Blackstone, the oil shale, was recognised in Victorian times as a great energy resource and was mined in the cliffs.

(To read more on the Kimmeridgian Arab Formation in the Ghawar Field see:
Ghawar, Saudi Arabian: The King of Giant Fields. Geo Expro, September 2010, pp. 24-29.)

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BITUMINOUS KIMMERIDGE SHALES (comparison) -

YEMEN
For comparison with the Dorset and Weald occurences Kimmeridge Oil Shales it is of interest that rather similar oil shales also occur in Kimmeridge strata as far away as Yemen. This is in addition to Kimmeridge oil source rocks in Saudi Arabia, and the Haynesville Shale, used for fracking in the USA. See:
Hakimi, M.H., Abdullah, W.H., Shalaby, M.R. and Alramisy, G.A. 2013. Geochemistry and organic petrology study of Kimmeridgian organic-rich shales in the Marib-Shabowah Basin, Yemen: origin and implication for depositional environments and oil-generation potential. Marine and Petroleum Geology, available online 17th October 2013.
In the Yemen, the Kimmeridgian organic-rich shales have high total organic carbon (TOC) content of up to 9 wt%. The kerogen is characterized by predominantly alginite and marine organic matter. The high amounts of organic matter are mainly due to a good preservation under relatively reducing conditions. The Kimmeridgian Madbi shales have very good oil-generative potential. Pyrolysis S2/S3 yields (in the range of 2.1867.78), kerogen microscopy and biomarker analyses indicate that the organic matter of the Yemen Kimmeridgian shales is a mixture of oil prone Type I and Type II kerogens with minor gas prone Type III contributions.



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INTRODUCTION

General and Clavell's Hard Location

Clavell's Hard to Rope Lake Head seen from the shore ledges, east of  Kimmeridge, Dorset, Hogg group, 2007

Above Clavell's Hard mining ledge, Kimmeridge, in 1998

Clavell's Hard mining ledge, east of Kimmeridge, Dorset, seen from the cliff edge to the NE, 18th July 2008

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A rope to the mining ledge was once present at Clavell's Hard, east of Kimmeridge Bay, Dorset, 14th June 2013; by 31st February 2014 this route has been destroyed and there no rope

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There is now no feasible route up onto the mining ledge at Clavell's Hard, Kimmeridge, 31st February 2014, and the rope has gone - there is danger of being cut off here by a rising tide

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[Tidal trapping: As noted, elsewhere, but emphasised again, there is serious danger of being cut off by a rising tide at Clavell's Hard, a small promontory. The oil shale exposure at beach level is beyond this and thus geologists might be tempted to walk eastward beyond this point to get to it. It should not be done unless there is a low spring tide and good weather conditions. A field trip to this eastern area must be carefully timed and there should be careful observation of the sea level changes, as shown by the seaweed levels. Up to Rope Lake Head, there is still a view of Clavell's Hard, the potential cut-off point. Going beyond Rope Lake Head requires even more careful planning and some fast walking. A change was noticed in February 2014. It had, with some risk and a little difficulty been possible to scramble up onto the remains of the mining ledge at Clavell's Hard from the east. Now the sea has eroded some of the rocks of the scramble and the amount of slimy algae has increased. There is no access onto the ledge now, and this cannot be regarded as an escape route for anyone caught by the tide. In any case there is no route to the cliff top, now, from the mining ledge.]

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Clavell's Hard seen in profile from the shore to the east, Kimmeridge, Dorset, 10th July 2010

Ian West at the Wheatleyensis bituminous shales beneath the Blackstone, east of Clavell's Hard, Kimmeridge, Dorset, 2007

Ian West at the extensional fault at the eastern end of the Clavell's Hard mining ledge, Kimmeridge, Dorset, 2010

David Dodd records the sounds of Kimmeridge oil shale, east of Clavell's Hard, Kimmeridge, Dorset, 2010

The Blackstone, oil shale, east of Clavell's Hard, Kimmeridge, Dorset

Kimmeridge Clay with oil-shale or Blackstone, east of Clavell's Hard, Kimmeridge, Dorset

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INTRODUCTION:

Maps, Aerial Photographs and Publications

The lithological succession, using traditional names, in the Kimmeridge area, Dorset, based on old geological survey maps, with a minor correction on the Jurassic-Cretaceous boundary

Geology of the Kimmeridge coast, Dorset, partly redrawn after a modern BGS geological map

Old geological map of the Kimmeridge Area, Dorset, based on 1895 and 1904 editions

Location aerial photograph of  Kimmeridge Bay and the cliffs to the east as far as Freshwater Steps, Dorset

An aerial photograph of the coast from Clavell's Hard to Freshwater Steps with some zonal information, east of  Kimmeridge, Dorset

Map - Geology of the Isle of Purbeck

Geology of the Isle of Purbeck shown on an old map (part of a map modified from Damon, 1884). Modern changes in the geological mapping of the area have only been of detail. Some place names have changed since Victorian times and Clavell's Hard has been added by the present author. Note an older spelling of "Kimeridge" with one "m".

A very simplified location and geological map of the Kimmeridge Bay area and adjacent coast, Dorset, southern England

The map, modified after Cox and Gallois (1981) shows the solid geology of the Kimmeridge area in particular and locations referred to in this and associated Kimmeridge field trip guides. Initial information is in the Introduction to the Kimmeridge Clay section.

OS location map for Kimmeridge Bay and Kimmeridge Cliff locations east to Freshwater Steps, Kimmeridge, Dorset

The Key Publication on the Kimmeridge Cliff Sections, Dorset, by Cox and Gallois, 1981

A multibeam bathymetry image of the sea floor south of Kimmeridge, Dorset, courtesy of the Channel Coastal Observatory

The multibeam bathymetry image and the geological map of the coast between Kimmeridge Bay and St. Aldhelm's Head, Dorset, linked to show sea floor geology

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INTRODUCTION:

The Blackstone, Oil Shale, East of Kimmeridge

A group from Delft university, researching on oil shale fires, hold a discussion on the cliff edge above Clavell's Hard, Kimmeridge, Dorset, 2008

Aerial view, Clavell's Hard etc. 1997

The Blackstone, the main Kimmeridge oil shale is best seen at and near Clavell's Hard, east of Kimmeridge. It was worked on what is now a rather inaccessible cliff quarry. Underground mining took place from levels or adits, one of which still remains. The oil shale was also mined inland of here.

As noted elsewhere, at Kimmeridge Bay the lower Kimmeridge Clay with dolomite beds, crushed ammonites and cycles of sedimentation is seen. By walking along the coast eastward from here progressively higher parts of the Kimmeridge Clay are seen. The Blackstone occurs in the Upper Kimmeridge Clay above the level of the strata at Kimmeridge Bay. Proceeding eastward firstly Hen Cliff, Yellow Ledge and Cuddle is seen. The oil shale is then reached at Clavell's Hard or Blackstone Point about one kilometre east of Kimmeridge Bay. Here and in the adjacent bay to the west there have been fires in the oil-shale . The cliffs around Clavell's Hard are described in some detail in this webpage. Reference is made to geochemistry of the oil shale and the history of mining. This is then followed eastward by a web page on Rope Lake Head to Freshwater Steps section.

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INTRODUCTION:

Cliff Section

Section of the Kimmeridge cliffs from Broad Bench eastward to Houns-tout, near Chapmans Pool,  Dorset

A low-resolution version of one group of a continuous set of photographs taken from the sea by Richard Edmonds of the Jurassic Coast organisation, Dorset. The Kimmeridge cliffs from Clavell's Hard to Chapmans Pool are shown

Beneath this diagram is shown a low-resolution version of one of many continuous coastal photographs from sea taken by Richard Edmonds of the Jurassic Coast, Dorset and East Devon World Heritage Coast. The full images are very high quality.

A cliff section is shown here for a large part of the Kimmeridge Coast. This is based on Arkell (1933), Cox and Gallois (1981) and other information. Amongst other things, it shows the position of the Kimmeridge oil-shale, Clavell's Hard and the location of the mining in the cliffs.

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BLACKSTONE:

The Blackstone or Oil Shale in the Succession

For reference the Kimmeridge zonal succession is given. The "Bolonian" is the Upper Kimmeridgean in traditional British terms, but is not part of the Kimmeridgean in France. The oil shale is in the Upper Kimmeridge Clay in the hudlestoni zone. Other thinner oil shales occur near this in the sequence.

Generalised sequence of the Upper Kimmeridge Clay in Dorset after Cox and Gallois (1981)

STAGE CHRONOZONE ZONAL AMMONITE
Bolonian
Fittoni Virgatopavlovia fittoni
Rotunda Pavlovia rotunda
Pallasioides Pavlovia pallasioides
Pectinatus Pectinatites pectinatus
Hudlestoni Pectinatites hudlestoni
Wheatleyensis Pectinatites wheatleyensis
Scitulus Pectinatites scitulus
Elegans Pectinatites elegans
Kimmeridgian
Autissiodorensis Aulacostephanus autissiodorensis
Eudoxus Aulacostephanus eudoxus
Mutabilis Aulacostephanus mutabilis
Cymodoce Rasenia cymodoce
Bayliei Pictonia baylei

Kimmeridge Clay with oil-shale or Blackstone, east of Clavell's Hard, Kimmeridge, Dorset

Here is the succession seen directly east from the small promontory of Clavell's Hard towards Rope Lake Head. The main oil-shale, the Blackstone, clearly projects in the cliffs. It is unsafe to approach it here in the cliff and it is best to look for loose material on the shore or an outcrop amongst the low-tide ledges.

The base of the Blackstone marks the boundary between the Pectinatites wheatleyensis Zone below and the Pectinatites hudlestoni Zone above.

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Approaching Clavell's Hard from Kimmeridge Bay

The coast immediately west of Clavell's Hard, view westward, showing fallen blocks that have come from the Rope Lake Stone Band, which is above the position of worked-out Blackstone, oil shale, in the cliff,  Kimmeridge Bay, Dorset, photograph by Alan Holiday, 21st November 2013, at low tide

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Worked-out Kimmeridge Oil Shale or Blackstone in the cliffs, a short distance west of Clavell's Hard, Kimmeridge, Dorset, 31st February 2014

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Blocks of the Rope Lake Head Stone Band, a dolostone showing here some unusual curved fractures, seen on the shore when approaching the Clavell's Hard promontory,  Kimmeridge, Dorset, November 2013

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OCCURRENCE - Introduction:

Oil-Shale - Blackstone and Bastard Coal - and Organic Percentages

Recognising the main Kimmeridge oil shale unit in general is very easy. The casual geological visitor does not necessarily need to consider the short discussion on the thickness which follows, but the specialist would need to do so, in order to avoid confusion.

Although nowadays often assumed that it is, the name "Kimmeridge Coal", was not synonymous with "Blackstone". The distinction may not always have been made clear. For introductory purposes this does not matter much. However, details are now given of the original terminology for clarification.

In general, it would seem that "Kimmeridge Coal" was a broad term for the bituminous strata that were mined here, and it may or may not include the Bastard Coal at the top. The name Blackstone, however, seems to have been used only for oil shale, which is very dark or black and with a total organic content of about greater than about 30% (although there is no accuracy in this, of course). When the section is visited, today, the base is seen as a sharp and obvious change from ordinary shale to black oil shale. It is the top which is less well defined. The thin "Black Dirt" or "Bastard Coal" at the top is not easily recognised in the cliff as being part of the major oil shale unit and can be viewed as a distinct bed. This means that it may or may not be included in a particular description. Provided the reader is aware of the matter this not a practical problem. Just take care with historic records and be aware of possible problems

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Thicknesses of the Kimmeridge Coal and the Kimmeridge Blackstone or oil shale, in the Kimmeridge Clay, near Clavell's Hard,  Kimmeridge, Dorset, with comments on oil source rock potential and approximate percentages

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Here a listing based on Strahan (1898, p. 55), is given below.

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"Black Dirt" or "Bastard Coal" - 6 inches - 15.2 cm. (not very conspicuous and thus often omitted from lists)

Shale - 3 inches - 7.6 cm.
"Blackstone" - 1 foot - 30.5 cm.
Shale - 3 inches - 7.6 cm.
Best "Blackstone" (springs issue from this bed) - 10 inches - 25.4 cm.
Shale beneath

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The old measurements without the Bastard Coal, i.e. the Blackstone thickness, give a total of 25 inches (2 ft, 1 inch) or 64 cm approximately (63.5 cm).

The old measurements including the Bastard Coal, the Kimmeridge Coal thickess, give a total of 34 inches (2 ft 10 inches or nearly 3 feet) or 86 cm (86.36 cm).

Most discussion which follows is on the 63cm of hard dark strata, mostly Blackstone in the original sense, which does not include the Bastard Coal. However, do not forget the poor quality oil shale that lies above. If you examine the magnetic susceptibility log of Coe et al. (1999) you will see that the base of the Kimmeridge Coal is abrupt but the top is more transitional (in an irregular manner).

[Note that Arkell (1953) listed (on p.72) "Hard shale with Blackstone at bottom - 3 feet". He was therefore, in effect distinguishing between bituminous shale in general, and the very bituminous Blackstone.]

Please note that many photographs below indicate the Kimmeridge Oil Shale without including, or pointing out, the Bastard Coal. Like some previous authors I have tended to omit that bed, and in photographs it is not conspicuous.

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Bay east of Clavell's Hard, Kimmeridge, Dorset seen from the cliff top; oil shale in the lower part of the cliff, 2005

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A composite image to show the relationship of the oil shales at Clavell's Hard and east of Clavell's Hard, Kimmeridge, Dorset, June 14th, 2013

The Kimmeridge oil shale or Kimmeridge Blackstone descends to the shore east of Clavell's Hard, Kimmeridge, Dorset, June 2013

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John Moylan of the BBC, discusses for radio the Kimmeridge oil shale or Blackstone, east of Clavell's Hard,  Kimmeridge, Dorset, 14th June 2013

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The cliffs between Clavell's Hard and Rope Lake Head, Kimmeridge, with the Blackstone, oil shale, in the base of the cliff, February 2011

The Blackstone, or Kimmeridge oil shale, downfaulted to the east, immediately east of Clavell's Hard, Kimmeridge, Dorset, 17th September 2012

Oil Shale east of Clavell's Hard, Kimmeridge, Dorset

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No.5 Adit, just east of Clavell's Hard, Kimmeridge, Dorset, as seen on the 31st February 2014

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No.5 Adit is a short tunnel, that was open in the 1950s and 1960s but became closed by a rock fall. It can be located easily because it is near the rusting steel, fin-like object that is present on the beach (some old oceanographic survey equipment?). The adit seems to have been a test excavation that was not connected to other workings. After the severe storms of early 2014, the entrance to this adit became partially open again, although with evidence of a major roof fall. It is a dangerous place and no attempt should be made to enter it, even if the entrance is further enlarged in the future by sea erosion. Notice that in the photograph above, to the far right, there seems to be a possible indication of lensing within the lower part of the Kimmeridge Blackstone or oil shale. However, this may be a false impression caused by some surface coating on the exposure. It needs further examination.

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A vertical sequence of Upper Kimmeridge Clay containing the Kimmeridge Blackstone or oil shale, west of Clavells Hard, Kimmeridge, Dorset

Details of the Kimmeridge Blackstone and underlying oil shales, east of Clavell's Hard, Dorset, with reference to Coe's Log

Oil shale location east of Clavell's Hard, Kimmeridge, old photo

The Kimmeridge oil shale or blackstone descends to the beach east of Clavell's Hard, Kimmeridge, Dorset, 2010

A group of petroleum geologists visits the Kimmeridge Blackstone or oil shale, east of Clavell's Hard, Kimmeridge, Dorset, 21st November 2013

The Kimmeridge Blackstone or oil shale is visited by Ian West in winter after it has been naturally washed clean by storms and rain, Kimmeridge, Dorset, January 2006

A general view of the Kimmeridge Blackstone or oil shale where it descends to the beach between Clavell's Hard and Rope Lake Head, east of Kimmeridge, Dorset

The Kimmeridge Blackstone or oil shale, shown with enhanced colour and contrast to render details visible

Ian West examines the Kimmeridge Oil Shale or Blackstone, near Rope Lake Head, east of Kimmeridge Bay, Dorset, 31st March 2014

Looking under the Kimmeridge oil shale or Blackstone, Kimmeridge Bay, Dorset, Perenco Field Trip, 31st March 2014

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Fossil Fauna of the Kimmeridge Oil Shale

In the field, the typical Blackstone parts of the Kimmeridge Coal or oil shale unit do not seem to contain fossil. The associated, less organic-rich shales may do so. The Kimmeridge oil shale [using the term in general] contains ammonites, and one can be seen in a photograph below in the central shaley band. Discinisca latissima and, according to Strahan (1898), "Ostrea laevuscula" have been found. This apparent occurrence of an oyster, a benthic organism, is difficult to explain in or near an oil shale, that presumably originated in conditions of an anoxic sea floor.

Strahan (1898) reported that beneath the oil shale Serpula, Lingula ovalis and Protocardia striatula are present, together with other fossils, including the remains of fishes and shrimps. This does not seem to equate easily with deep, stagnant conditions. Of course, the kerogen is Type II and there is land plant debris in addition to marine organic matter, so the deposits may not necessarily be those of some huge, deep and black sea.

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Pyrite Nodules in the Oil Shale

The oil shale contains large pyrite nodules and these are conspicuous features (see photographs below). Gallois (1978) pointed out that "Pyrite is common in the Kimmeridge Clay oil shales and formed the basis for a small alum industry in the eighteenth century". He also commented that "The oils produced from Kimmeridge Clay oil shales all have high sulphur contents, generally 4 to 8 percent". The sulphur content has long been the main problem with the Kimmeridge oil shales. Hydrogen sulphide is produced at as low a temperature as 200 degrees centigrade (Gallois, 1978, p. 16). Thus in practice this oil shale produces hydrogen sulphide when heated to some extent well before it actually burns. When it does burn it has particularly strong and characteristic smell. A cliff fire in this oil shale can be detected by smell at one kilometre distance or more. Its odouriferous properties have long been known. Coker (referred to by Arkell (1953)) noted in his Survey of Dorsetshire in 1732 that "in burning it yields such an offensive savoure and extraordinaire blacknesse, that the people labouring about those fires are more like furies than men". Thus the pyrite present in the oil shale is not just the conspicuous nodules but also finely dispersed pyrite within the shale rock. However, the Kimmeridge oil shale is not a "stinkstone" because it does not normally give off an unpleasant odour when struck. Blocks of it retained on display do not give off any odour. Even underground at a depth of as much as 3 kilometres it would not naturally be at the 200 degrees centigrade, referred to above, and probably only at about 100 degrees in round figures.

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Pyrite nodules that have been separated when mining the Kimmeridge oil shale, and then left in old workings, Clavell's Hard, east of  Kimmeridge Bay, Dorset, 31st February 2014

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The jointing pattern in the Kimmeridge oil shale, seen from above, and showing joints crossing pyrite nodules, near Clavell's Hard, Kimmeridge, Dorset, 31st March 2014

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Kimmeridge Oil Shale - Fracking Potential?

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The Kimmeridge Blackstone or oil shale seen high in the cliff, just east of Clavell's Hard,  Kimmeridge, Dorset, 17th September 2012

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Kimmeridge oil shale, fracture pattern as seen at collapsed mine workings, near Clavell's Hard, east of Kimmeridge Bay, Dorset, 31st March 2014

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The Kimmeridge Blackstone or oil shale bed in the mid-cliff, near Clavell's Hard, Kimmeridge, Dorset, showing fracture pattern and possible fracking potential

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Fracture patterns in the  Kimmeridge oil shale or Blackstone, near Rope Lake Head, east of Kimmeridge Bay, Dorset, 31st March 2014

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The images above shows some possible hydraulic fracturing or fracking properties of the Kimmeridge Blackstone or oil shale in the mid-cliff near Clavell's Hard, Kimmeridge. The Blackstone fractures in quite large blocks of one or two metres width. In contrast the associated shales and mudstones have close fracturing patterns. What would happen if hydraulic fracturing or fracking was applied to a sequence like this? Would the fairly continuous oil shale lift at the basal, bedding-plane fracture? Would it break into large blocks above highly fractured mudstone? The fracking prospects of oil shale like this are not well-known at the moment. It must be stressed this oil shale at Kimmeridge is not thermally mature and, therefore, is not suitable for fracking in any case. It will not happen at Kimmeridge and probably nowhere onshore in Dorset. The possibility of fracking the oil shale would be in places where it is thermally mature (i.e. "cooked" to about 100 degrees centigrade). The general area are parts of the central western Weald (i.e. Balcombe, Fernhurst etc) and some part of the offshore Inversion Structure (part of the Portland - Isle of Wight Basin) between the Isle of Wight and Swanage (not yet investigated). Fracking in the English Channel Inversion may not necessarily ever happen though; it is merely a future possibility. In the USA the fracking has been onshore and not offshore.

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Blackstone - Some Technical Details

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Use of inverse magnetic susceptibility, after Coe et al., to indicate beds with high AOM, Upper Kimmeridge Clay, near Clavell's Hard, Kimmeridge, Dorset

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KIMMERIDGE BLACKSTONE AND OTHER OIL SHALES:

Listing of Main Beds in Clavell's Hard Area

The diagram, shown higher above, and based on the work of Coe et al. 1999) shows how oil shales might be indicated by inverse magnetic susceptibility. On this basis the following might be worth further investigation for AOM, amorphous organic matter (mostly kerogen). Some of these beds might have been the source material for archaeological artificacts, although the Blackstone is obviously the most likely source. There are about 12 oil shale bands, east of Kimmeridge Bay.

Sequence - top, downwards to bottom, mostly wheatleyensis and hudlestoni zones.

45/11 (White Stone Band, with some oil shale)
45/19 (thin)
45/17 (thin)
45/6 (thin)
45/6 (thick, hudlestoni - worth investigation)
43/4 (thin)
42/29 (base - thin)
42/12 (Blackstone - in two major parts, upper and lower with calcareous clay in the centre; with calcite-pyrite nodules)
42/10 (Bubbicum, significant)[or Rudicum, see Pringle in Whitaker and Edwards (1926)]
42/5
41/10
39/1
38/2

(there are few others)

Note that the maximum gamma peak, with relatively high uranium and potassium is in 42/6, calcareous mudstone beneath the Bubbicum.

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BLACKSTONE:

Further Details

The Blackstone or Kimmeridge Coal, 42/12, the main oil shale bed descends to the shore east of the Clavell's Hard mining ledge. It is about 86cm thick. It forms a projecting band in the cliff but it is not as conspicuously dark as might be expected because it is often coated with white sulphate, carbonate and clay encrustations washed down from the cliff above. In the field you will notice by looking at the Blackstone in the cliff that there are quite large nodules of pyrite, visible in cross-section.

Oil shale of the Kimmeridge Clay, source rock for North Sea oil

Debris frequently falls from the cliffs here and it is safer to study the oil shale as loose blocks on the shore rather than in the cliff. It is a dark chocolate brown, sometimes appearing almost black. When polished by the beach debris or by human activity is a waxy-looking material. When fresh surfaces are broken is it is a mat dark grey, well-laminated and quite strong. As noted above, joint surfaces are often coated in contrasting white calcite, as in the photograph here. It can weather to a paper-shale.

The oil-shale is of relatively low density and rather tough and resistant to disintegration. As a result small, flat, ovoid pebbles of oil-shale with a rather brownish colour are commonly washed along the shore and appear on the beach at many places where Kimmeridge Clay outcrops nearby.

The Blackstone, oil shale, east of Clavell's Hard, Kimmeridge, Dorset

Students visit the Blackstone near Clavell's Hard, east of Kimmeridge Bay, Dorset, England

The Kimmeridge Blackstone, oil shale, forms a small shore ledge just west of Rope Lake Head, near Kimmeridge, Dorset

Shore ledge of oil shale or Blackstone, Upper Kimmeridge Clay, west of Rope Lake Head, Kimmeridge, Dorset, 17th September 2012

The Blackstone can be seen as a smooth surface between the beach boulders, a short distance to the west of Rope Lake Head. It is strange that the oil-shale occupies this narrow gap between the blocks, but probably it is projecting upwards to some extent above the level of the adjacent shale. The boulders are from the Rope Lake Head Dolomite Bed. In the distance, on a wet and rather misty day, is St. Aldhelm's Head, a peninsula of Portland Stone.

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Geologists examine the shore exposure of the Kimmeridge oil shale or Blackstone, near Rope Lake Head, east of  Kimmeridge Bay, Dorset, 31st March 2015

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Walking on Kimmeridge Oil Shale or Blackstone, with pyrite nodules, between Clavell's Hard and Rope Lake Head, Kimmeridge, Dorset, 31st February 2014

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BLACKSTONE

Pyrite Nodules (more)

 Walking on the Kimmeridge Blackstone or oil-shale, with pyrite nodules, near Rope Lake Head, Kimmeridge, Dorset, 17th September 2012

Carbonate/pyrite nodules in the Blackstone, oil shale, near Rope Lake Head, Kimmeridge, Dorset

A large pyrite nodule seen from above in the shore exposure of the Kimmeridge Blackstone or oil shale, between Clavell's Hard and Rope Lake Head, east of Kimmeridge Bay, Dorsetm, 2008

(see also the cross-sections of nodules shown above in photographs of the vertical exposure of oil shale at the foot of the cliff)

In the area between Clavell's Hard and Rope Lake head the large pyrite nodules of the Blackstone, mentioned above, are accessible for study. On the shore they are visible, not in section but partly eroded out of the flat surface of the oil-shale. They are quite large, up to about 60cm or more, and elongate and rather irregular in shape. Although pyrite is common throughout the Kimmeridge Clay, these are the largest pyrite nodules in the formation and, of course, it is not surprising that they are associated with the greatest accumulation of organic matter. The pyrite is the result of sulphate-reducing bacteria living on the organic matter of the oil-shale shortly after deposition and reducing the sulphate of seawater to sulphide. This then has reacted with available iron to produce the ferrous sulphide - pyrite (FeS2). The oil-shale gives an impression of curving a little against the nodules and this probably because there were of early origin and compaction has taken place after their formation. In section they can often be seen to have flanges or "wings". Presumably there was some soft sulphide precursor devopment in the very soft, organic-rich mud prior to compaction.

Sulphate reduction (diagenetic zone 2) is typically an early process taking place in marine sediments before burial to the fermentation zone (3) where the Kimmeridge dolomite beds originated Irwin et al. (1977).

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BLACKSTONE:

Organic Content General

Details of the Blackstone sequence and details, emphasized and shown with a scale, Kimmeridge Bay, Dorset

Variation in combustible organic matter in the Blackstone and associated strata, Clavell's Hard, Kimmeridge, Dorset

The Kimmeridge Blackstone or the main Kimmeridge Oil Shale is not a uniform bed. Some old analyses have been published on the basis of random samples from it, without reference to their detailed stratigraphic horizon. Percentages of organic matter given are not consistent. The reason is that it is quite variable vertically on a scale of a few centimetres. In total the bed is about 64 cm thick in the main section between Clavell's Hard and Rope Lake Head. The maximum total organic matter (TOM, not TOC) is about 70%. The diagram above, based on the work of Ioannides et al. (1976) and of Gallois (1978) shows some of the variation well. See also the magnetic susceptibility log of Coe et al. (1999) because this reveals a good inverse relationship to the TOM of the Blackstone and Bubbicum. The data above should be of some use in determining the source of archaeological samples of Blackstone.

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The Kimmeridge Blackstone or Kimmeridge Oil Shale, shown with colour variations emphasised and surface calcite artificially removed from the photograph, 2014; compare with unaltered images

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This particular image, above, is not original and unaltered. It has been intentionally modified so as to remove some patchy white calcite veins which partly obscure the surface. The variations in shades of grey within the oil shale in this picture have been emphasised. The purpose of this is to shown that the Blackstone is very variable, vertically, in composition, and also to show the distribution of lenticular pyrite nodules. There seems to be a crenulate ross-section of an ammonite in the central bituminous shale, which is lower in organic content than the other parts of the Blackstone. In a sense, there are two rich beds of oil shale with less-bitumous shale in the central part. This is also clear from an illustration further above.

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BLACKSTONE AND KIMMERIDGE CLAY IN GENERAL - ORGANIC CONTENT

TOC - Total Organic Carbon

Note that TOC is the Total Organic Carbon in a source rock. Note that it is not the TOM, total organic matter, which is higher. For the Kimmeridge Clay figures for both TOM and TOC have been given by various authors. Carbon constitutes 75 to 95 wt % of hydrocarbons by molecular weight, with an average value near 83 weight % ( Jarvie, 1991). Thus regard the TOC, the total organic carbon, as indicating a large proportion, about 83 wt% of the total organic matter, when considering the meaning of the data given (or to be given) below.

To understand the Kimmeridge Clay Formation in broad terms the zonal averaging scheme of Tyson (2004) is very useful. Bulk geochemical data from the NERC Rapid Global Geological Events (RGGE) project were used to appraise the pattern of variation in mean organic content through the cymodoce to rotunda zones of the type Kimmeridge Clay Formation in southern Dorset. There is a symmetrical stratigraphic pattern in mean and modal total organic carbon (TOC) values, which increase from 1-2% at the top and base to a peak of 8-9% in the middle of the formation. (Note that these averaged values do not seem very high in comparison with the extreme figure of 70% TOM for part of the Blackstone.)

Regarding the Dorset Coast, consider about 8% TOC for a favourable zone (near the middle) and about 40% TOC for the oil shale bed, the Blackstone, as the type of figures involved (generalising).

In comparison, the American Kimmeridgian, Upper Haynesville/Basal Bossier shales contain about 1.5 -5.0 wt% TOC. These figures are not very different from those of the Kimmeridge Clay basinal facies of Dorset (but the thermal maturity is much greater in the USA sequences).

West of Shetland, the offshore Kimmeridge Clay Formation has an average TOC of 7% (maximum 15%) (R. Jowitt et al. 1999, in Fleet, A.J. and Boldy, 1999. Petroleum Geology of Northwest Europe, Proceedings of the 5th Conference), Vol. 2, published by the Geological Society of London.)

In both Dorset and Somerset the Liassic black shales have TOC values between 0 and about 12%, with most figures near 3%. In general these are not very different from Kimmeridge Clay figures, excluding the exceptional Blackstone.

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OCCURRENCE

Comparison - Oil Shale in the Weymouth Relief Road Cutting

A specimen of the Kimmeridge Clay Blackstone or oil-shale that has been heated by the sun on one side and, losing organic matter and water, has begun to coil - Weymouth Relief Road, Dorset, 30th June 2009

Kimmeridge oil shale occurs at Burning Cliff near Ringstead, and in the inland area north of Weymouth. When weathered, as in the example from the Weymouth Relief Road above, it may appear very laminated, and, rarely, distorted by head of the sun.

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BLACKSTONE:

Details of the Blackstone Bed and the Associated Oil Shales

Blackstone, oil shale - details of succession seen up in the  cliff near Clavell's Hard, Kimmeridge

The Kimmeridge Blackstone or oil shale in the cliffs between Clavell's Hard and Rope Lake Head, east of Kimmeridge Bay, Dorset, UK, photo 2006; note the carbonate/pyrite nodules

Details of the Blackstone, the oil shale, east of Clavell's Hard, Kimmeridge, Dorset

The Blackstone is sometimes treated for simplicity as though it is a single bed and relatively homogeneous, but, in fact, it is not. As shown in the photograph, it really consists of two main beds separated by bituminous shale. In addition it contains numerous lenticular sulphide-carbonate nodules, that probably originated in the early sulphide reduction zone. The presence of the nodules means that analyses of the Blackstone should be treated with caution, because, although valid for the separated oil shale, these usually ignore the presence of the nodules. Thus they therefore grossly underestimate the real iron and sulphur content of the bed, considered in bulk.

Details at Blackstone Point or Clavell's Hard were given by Strahan (1898) and are reproduced below with modification and additions. Strahan's measurements, given in inches, have been converted to metric figures and probably not highly accurate. I have shown, though, how the bed listing ties in with the photograph. However, there is some doubt as to whether the Bastard Coal is the uppermost part of the main oil shale shown and not the base of the mudstone above. It is clear from analyses that, not surprisingly, the Best Blackstone is the richest in total organic matter and the Blackstone deteriorates upward. Note that the photograph has been taken at a point just west of the mining ledge and the oil shale cannot be reached from the beach just here. (To recognise the Blackstone easily, look for the white veining on a black rock with angular fractures)

Sequence
(with bed numbers from Coe et al. 1999)

43/3. 'Short Joint Coal', not worked but picked up by the villagers - 0.15m (6 inches). Coccolith rich limestone within oil shale (Cox and Gallois, 1981)
Shale - 3.05m (10 ft)
42/24. Hard calcareous stone band, formerly used for cement-making [Rope Lake Head Dolomite Bed]. - 0.46m (1ft, 6 ins)
Shale - 4.57m (15ft)

------------------------------------- top of Kimmeridge Coal - The Oil Shale or Blackstone
(This is given as approximately 60cm by Cox and Gallois, 1981 and by Coe et al., 1999, and does not include the Bastard Coal)
42/14 & 16). Black Dirt or Bastard Coal' - 0.15m (6 ins) (inferior bituminous shale, not always included in the Blackstone)
Shale - 0.08m (3 ins)
42/12 upper part: Blackstone, upper Blackstone - 0.30m (1 ft)
Shale - 0.08m (3 ins)
42/12 lower part: Best Blackstone or lower Blackstone (springs issue from this bed) - 0.25m (10 inches)
--------------------------------------- base of Kimmeridge Coal or Blackstone
(total Blackstone thickness - 0.86m including the Bastard Coal. True Blackstone without Bastard Blackstone totals about 63 cm, according to this list).

Shale - 1.83m (6 ft)
(end of sequence list)

Below the Blackstone (42/12), about 1.4m down is another oil shale known as the 'Bubbicum' or 'Rudicum'. This is bed 42/10. It has lower organic matter (32%) according to Dunn (1973). Like the Blackstone it contains pyritised plates of Saccocoma. Further down, Bed 42/5 is a laminated, cemented oil shale with Saccocoma. See Cox and Gallois (1981, p. 42) and Coe et al. (1999).

[Bubbicum or Rudicum - more notes. This is referred to in three boreholes (107, 108 and 109 at Kimmeridge by Whitaker and Edwards (1926). It is given as 10 inches [0.25m.] in two borehole, and at 8 inches [0.20m.] in another. The separation from the Blackstone is given as 5ft 6 inches (1.68m), 5ft 1 inch 9 (1.55m) and as 5ft 9 inches (1.75m). What is the meaning of "rudicum" and why has it been used for this bed. Is there any connection to Roman use of Kimmeridge oil shale?]

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BLACKSTONE continued:

Kimmeridge Oil Shale - General Appearance

The Blackstone is actually a rock of a very dark chocolate grey colour (the ordinary Kimmeridge shales are dark grey, but lack the chocolate tinge). It is very well-laminated, and when eroded occurs on the beach as flat pebbles, often with a rather brownish rim. Oil shale density is much less than associated sedimentary rocks and can be as low as 2.2 (and isprobably lower in the case of the Best Blackstone). A result of this low density and the occurrence of thin, flat slabs is that is often thrown up high on beaches by wave action. Because it outcrops at the Isle of Portland it is often washed up at the Portland end of the Chesil Beach, and found high on the beach.

Clay minerals in Kimmeridge oil-shales are dominantly illite, but also with some kaolinite, some mixed-layer mineral and some chlorite (Gallois, 1978).

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BLACKSTONE continued:

Composition and Geochemistry of the Kimmeridge Blackstone and Associated Shales

It is recommended to have available a pdf or printed copy of the Kimmeridge Clay Composite Graphic Log for Coastal Exposures, by Angela L. Coe, Stephen P. Hesselbo, Hugh Jenkyns, Helen Morgans Bell and Grapham P. Weedon. 1999. This is part of the REGGE, Rapid Global Geological Events Project on Anatomy of a Source Rock. This excellent log provides the following data for almost the whole of the Kimmeridge Clay cliff section:

Magnetic Susceptibility,
Total Gamma Ray,
Uranium (ppm.),
Potassium (%),
Thorium (ppm.),
Th/K,
Th/U,

The log is essential for anyone making a serious study of Kimmeridge Clay composition. It is an impressive log and remarkable that samples have been studied at 10cm intervals throughout almost all of the Kimmeridge Clay of the Kimmeridge coast. This is a remarkable achievement.

Some older, but very useful, geochemical data, modified after Dunn (1974) is now provided to give, in particular, total organic contents and trace element data for comparison with recent work and for general discussion. The paper of Dunn (1974) should be consulted for information on analytical methods and the application of the data to the study of sedimentary cycles. Dunn used a simple Fourier analysis to find harmonics. Most notable is his discovery of a dominant periodicity of about 40,000 years at 3.3 to 5 metres. This is the well-known obliquity cycle (axial tilt 21.5° to 24.5 °). [For more recent work on time-series analysis see Weedon et al. (1999) who obtained similar results in recognising a dominant 38 ka obliquity cycles.] Dunn's (1974) paper is an early and good paper of interest with regard to trace element geochemistry. It was undertaken mainly by optical emission spectrograpy and is semi-quantitative. The tables below are based on his work. Some results that are significantly different from the average or of special interest are highlighted here. Note that the sequence in the tables in inverted and they young downwards. The data given in the later and very detailed work of Coe et al. (1999) is not the same. Sampling was closer in the later work (10cm as opposed to Dunn's 25cm), and gamma ray and magnetic susceptibility have been measured, together with data for U, K, Th etc. Total gamma ray in Coe et al. corresponds quite well with V [vanadium] in Dunn, but otherwise there is little direct relationship or overlap, and the two works are complimentary. To understand the comparison and for cross-referencing purposes see the following diagram which shows the logs of RGGE and of Dunn alongside.

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[Extract from Dunn (1974)summarising the trace element geochemical distribution:
"Whereas the mineralogy of the sequence is quite simple (organic matter, dolomite, calcite, illite, kaolinite, extremely fine-grained quartz, pyrite and rarely siderite), the trace-element geochemistry varies appreciably (Table 1). Statistical analysis of the data has shown Mo, P and some of the Cu and Ni to be associated with the organic fraction. Most of the other elements, including the remainder of the Cu an Ni, are structurally incorporated in, or absorbed on, the clay minerals. Mn is mostly associated with carbonate fraction. Lithological variation indicates no apparent cyclicity over this interval, and no clear cycles emerge from the geochemical data." ]

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Comparision of logs, Dunn and RGGE, Kimmeridge oil shales, Clavell's Hard, east of  Kimmeridge Bay, Dorset

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First part of a table of geochemical data modified after Dunn (1974) and showing details of the strata beneath the Blackstone, east of Kimmeridge, Dorset

Second part of a table of geochemical data modified after Dunn (1974) and showing details of the strata above the Blackstone, east of Kimmeridge, Dorset

Third part of a table of geochemical data modified after Dunn (1974) and showing details of the strata above the Blackstone and in the vicinity of the Short Joint Coal, east of Kimmeridge, Dorset

Another table of trace-element geochemical data is provided by Cosgrove (1970), Table 1, p. 832. Twenty samples were analysed (including three not from the Kimmeridge Clay). Most of these are from Blakes Bed 26, a broad unit which includes the lower part of the hudlestoni Zone. Within this is the Blackstone, the Rope Lake Head Stone Band and the Short Joint Coal. The samples are not well localised within the unit. Two samples for comparison are from scitulus Zone shales. With regard to the samples collected in the field from the hudlestoni zone the sample with highest organic matter (26C1)is listed as containing 55.3% organic C% (not organic carbon).

A museum specimen of Kimmeridge Coal from Clavell's Hard, Dorset, gave, according to the table an organic carbon content (i.e. - TOC) of 71.4%. This is anomalously high for organic carbon from the Blackstone, but just feasible for total organic matter. [total organic matter - TOM can be obtained from total organic carbon - TOC by multiplying by 1.2, assuming that the organic matter is 83 wt% carbon.]. Converting the figure to TOM gives 85.68%, which is much higher than anyone else has reported. However, the details of origin of the museum specimen are not known. It remains to be seen whether samples of such high organic content can be obtained from the Blackstone in the field. It is possible that they occur at a particular level.

If the highest figure for the field samples for TOC, 55.3 and very likely from the Blackstone is converted to TOM, the figure is 66.36%. This is remarkably close to the maximum figure for TOM given by Dunn (1974) which is 61.6%. Thus the TOC figures for the field samples of Cosgrove (1970) seem to be appropriate for the bituminous strata involved. However, there is not a very good correspondence between the trace element analyses of Cosgrove and of Dunn. However, very different analytical techniques were being used, and they were presumably not as advanced as methods available now. The matter is not simple in any case since the sampling was different.

The main point of the Cosgrove paper is not the general range of trace elements, discussed above, but the discovery of a relatively high iodine content in Kimmeridge oil shale. The museum specimen gave a value of 72 ppm I and also 16 ppm Br. A field sample (26C1) that is likely to have come from the Blackstone gave an I value of 34 ppm and a Br value of 14. The important and original conclusion of Mike Cosgrove's paper is as follows:

"The bituminous Kimmeridge shales of Dorset are particularly I-rich sediments. With an average value for I of 17 ppm and highest values in excess of 30 ppm they are amongst the most I-rich sedimentary rocks yet recorded. The Kimmeridge "Coal" with a value of 72 ppm. I must likewise be amongst the most I-rich of combustible rocks recorded.
The abundance of microplankton in those rocks, and the identifiable celular structure in the kerogen points to the organic C as being derived from these organisms. Further, the known enrichments of I and Br by various marine plants suggests that these elements are also derived from the same source. The relationship between organic C, I and Br is confirmed by the high correlation obtained between these elements during statistical analysis of the results.
Comparison with other shales indicates that while Mo enrichment can be related to the marine environment, I accumulations requires the presence of plant material during deposition, Br being enriched at the same time. Comparison with an oil shale deposited in fresh water emphasises the presence of Mo, I and high Br content as marine indicators."

Further Discussion of Blackstone Geochemistry

As a potential oil-source rock, more discussion will be given to the geochemistry of the main oil-shale, the Blackstone. Various analyses have recorded a total organic content of between 20% and 74% (e.g. Cosgrove (1973), Dunn (1974), and Legg (1984)). In a very old report, Strahan (1898) referred to an analysis which gave 61% volatile matter and 13.15 % carbon (coke). Thus the total organic content was 74.15 % with ash of 28.85 %. This seems compatible with a general impression that the purest, high quality Blackstone can be considered as consisting of about 70% organic matter in the form of kerogen (although see the report by Cosgrove (1970), discussed above, on the museum specimen.

From the tables of Dunn (1974), above, a very organic-rich, sample of the "Best Blackstone" (at the base) which has been analysed for trace elements contains 61.6% organic matter and only 5.6% CO 2 . There are the following trace elements in ppm: V - 25; Cr - 20; P - 1580; Zr - 7; Ni - 35, Cu - 25, Mo - 20, Ga - 3, Mn -20, Li - 15, Rb - 50, Cs - n.d., Sc - 2. The trace element composition is not unusual for a Kimmeridge Clay rock with low clastics. V and Cr are low compared to adjacent strata while P is high for the Kimmeridge Clay. Zr is as low as would be expected for a rock with low clastics, Ni and Cu are similar to values for the shales, and Mo and Ga are not unusual. Mn is decidedly low, probably because of deficiency of carbonate. Li is very low and Rb, as would be expected, is much lower than in the shales. Cs and Sc are not present in significant quantities. Generally then, elements contents are low compared with the adjacent bituminous shales probably because of dilution by organic matter trace. The high phosphorus is the notable feature.

Additional trace element data has been provided by Cosgrove (1973). He analysed various Kimmeridge Clay samples including a museum specimen of Kimmeridge Coal or Blackstone. This particular specimen gave 71.4% 'organic C' (but this is presumably this total organic matter, not just carbon). CO 2 was 3.7%, and these figures are not very different from the values given by Dunn (1974) for the richest oil-shale. The trace element composition is broadly similar to that given by Dunn (1974). There are some additional elements, including Zn - 65, Br - 16, Sr -154 and Y - 13 ppm. The most notable feature is the iodine content of 72 ppm which makes this one of the most iodine-rich combustible rocks known. Other Kimmeridge rocks have lower iodine contents but they are still quite high for shales. The iodine was attributed (Cosgrove, 1973) to a marine plant origin. It was not detected in Carboniferous oil-shale from Broxburn, Scotland, which is of freshwater origin. More trace element data is available in Gallois, (1978). In addition to comparable results to those discussed above, Ba is unremarkable being near 250 or 300 and U is very low and not concentrated in the oil-shale.

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BLACKSTONE:

Kimmerige Oil Shale - Thermal Maturity

Diagenetic history of the Kimmeridge Blackstone or oil shale in comparison to that of the Kimmeridge Basalt Stone, a dolomite microsparite, strata of the Upper Kimmeridge Clay, east of Kimmeridge Bay, Dorset

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In terms of organic maturity the Kimmeridge Clay of the Kimmeridge area is still immature at the Kimmeridge coastal exposure. It has R o (estimated vitrinite reflectance) of about 0.45, higher than the value for the Kimmeridge Clay of Ringstead of 0.34 (Ebukanson and Kinghorn, 1986). See this work for information on aliphatic hydrocarbons in the Kimmeridge Clay.

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BLACKSTONE

Kimmeridge Oil Shale - Gas Production

Gas production from Kimmeridge Blackstone has been stated, in old work, to be 11,300 cubic feet per ton, after purification of the gas. This is said to be only 700 cubic feet less than the output from a ton of coal. This information is from Legg, (1984) quoting Dr Ted Ward, who, in turn, was referring to the work of the German chemist August Wilhem von Hoffman in the 19th Century. Strahan (1898) stated that one ton yielded 9,000 cubic feet of gas, a comparable figure.

At 60° C methane is the gas produced. At 100° C methane, ethylene, ethane and propylene are produced. C 4 and C 6 hydrocarbons are also produced at high temperatures (Gallois, 1978). A potential hazard is that hydrogen sulphide is produced at 200° C. Thus the strongly smelling fumes are not only extremely unpleasant but could be toxic, to some extent at least. As will be noted elsewhere, the high sulphur content of the Kimmeridge oil shale is a particular problem, and oil produced is quite sulphurous.

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BLACKSTONE:

Oil Yield

A study of oil yield has been made by (Gallois, 1978). Up to 20.9 US gallons per short ton are the potential oil yields from samples in various boreholes through the Kimmeridge Clay in southern England. The Blackstone at Kimmeridge has given oil yield of 15 wt/wt %. Hydrocarbons produced by the pyrolysis of Kimmeridge Clay oil shales are mostly viscous oils and tars with a high sulphur content (Gallois, 1978).

Incidently, it is interesting to note that ammonia was a by-product of distillation, and salammoniac has been recorded as produced by an oil-shale cliff-fire (Cole, 1975) .

Gallois (1978a) has considered the practicalities of large-scale oil shale distillation. The oil shale has to be retorted to about 500° C and the volume of spent shale after retorting has a similar volume to the raw shale. There is therefore much to dispose of and it has few uses and may include small quantities of carcinogens. Its disposal therefore presents formidable problems.

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BLACKSTONE:

Organic Geochemistry

There has been some important work on the organic geochemistry of various Kimmeridge lithologies, including the Blackstone, by Van Kaam-Peters et al. (1998) . They found that organic matter had been preserved by sulphurisation. The organic matter of all sediments was deposited under euxinic conditions as revealed by the occurrence of isorenieratene derivatives indicating (periodic) photic zone euxinia. They suggested that at times of reduced run-off from the hinterland, represented by so-called condensed sections, the flux of reactive iron was relatively small compared to the flux of reactive organic matter, which resulted in the formation of relatively small amounts of pyrite and an excess of hydrogen sulfide capable of reacting with fresh organic matter. Within the condensed sections, variations in the degree of sulfurisation of organic matter are probably due to both differences in primary production and differences in the supply of reactive iron. Their findings demonstrate that climatic changes, probably driven by Milankovitch cycles, can have a large impact on the molecular and carbon isotopic compositions of the sedimentary organic matter in an otherwise relatively stable stratified basin. They also show that large amounts of labile carbohydrate carbon may be preserved through sulfurisation.

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BLACKSTONE:

Lack of Beef

At Kimmeridge the Blackstone is not associated with beef, the diagenetic fibrous form of calcite, commonly in deep-buried organic-rich shales. There is, of course, a striking contrast with the Lower Lias of the Dorset coast where probably the most organic-rich part is the Shales-with-Beef. The Chief Beef Member in the Purbeck Group is also organic rich. There is a very small amount of beef in the Autissiodorensis Zone of Kimmeridge Bay, but it is of little significance. The main reason for the absence in association with oil shales is probably very simple. Dolomite, rather than calcite is the dominant carbonate, and, crystallographically, this does not normally elongate on the c-axis. Dolomite beef, while not necessarily an impossibility, is not a common feature of organic-rich shales, as is calcite-beef.

Following such a generalisation, it might be asked as to why beef is not common in the Pavlovia zones (which are less dolomitic) of the Upper Kimmeridge Clay. The probable reason is that these strata are less organic-rich. The dominant carbonate diagenesis of the bituminous Kimmeridge Clay is that of the fermentation zone. Beef is probably more characteristic of the abiotic zone 4. These comments apply to the type-section at Kimmeridge. Beef could develop elsewhere in the Kimmeridge Clay in different conditions of geochemistry, burial and temperature.

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BLACKSTONE

Ringstead, Portesham (Portisham) and other localities

At Brandy Bay to the west of Kimmeridge, the Blackstone and the underlying Bubbicum are broadly similar to the section at Clavell's Hard but slightly thinner.
See the webpage:
Kimmeridge West, Brandy Bay and Gad Cliff

Cox and Gallois (1981) reported the Blackstone as having a thickness of 50cm (compared with 60 at Clavell's Hard) and the Bubbicum as 40cm (which is about the same as at Clavell's Hard). The Blackstone at Brandy Bay is very similar to that at Clavell's Hard in that it contains pyritic concretions and pyritised Saccocoma.

For Ringstead Bay Cox and Gallois (1981) showed the Blackstone to be just over 20cm thick and the Bubbicum just over 10cm. The Blackstone contains calcareous concretions. The whole Kimmeridge Clay sequence is reduced in thickness and the oil shales thin roughly in accordance with this.

North of Weymouth there is a type of "back-basin", where the Upper Jurassic thickens again until affected to the north by the Abbotsbury-Ridgeway fault system. In this region the Kimmeridge oil shales are well-developed again. They are best know at Portesham.

The Kimmeridge Blackstone was discovered at Portesham (or Portisham), northwest of Weymouth, in 1856. Apart from at Kimmeridge coastal area, this was the only place where it was worked. The location was at Portesham Dairy about 100 metres west of the old station (Strahan, 1898). It was found at only about 4.3m. depth. In 1877 the railway cutting disclosed the outcrop. The Kimmeridge "coal" was mixed with clay and 2000 tons were burnt to make ballast for the railway (no longer present). Various shafts were made by were overpowered by water. The Manfield Shaft was more successful. At 14.5 metres it encountered a very rich oil shale. This proved to be at about 10 metres above the true Blackstone. Comparison could perhaps be made with the Short Joint Coal which is about 8 metres above the Blackstone in the main Kimmeridge Cliff Section. Near the horizon of the Short Joint Coal at Kimmeridge a thin oil shale containing 38.5% organic matter has been reported by Dunn (1974). It should not be assumed, though, that the higher Portesham oil shale actually is proven to be the exact equivalent of the Short Joint Coal. It could be another (higher) bed in the hudlestoni Zone.

The principal or Main Bed bed of oil shale which is presumably the Blackstone was found at a depth of 41.76m (Strahan, 1898). The main oil shale is about 61cm thick. Beneath it is a clay or marl of 30cm thickness. The lower oil shale is 25m.

It is not absolutely proven that the Main Bed of oil shale, the 61cm, corresponds to the whole Blackstone at Kimmeridge, but it probably does. See also (Strahan, 1918).

More details on the Blackstone and associated strata at Portesham have been given by Whitaker and Edwards (1926), p. 91 et seq. They come from the Corton Farm No.2 Borehole, 1917, 400 metres SE. Height above OD. - 47m. In addition there is data from Corton Farm, No. 2a borehole, Waddon No.3a borehole, and Near Manfield Shaft, No. 4 borehole. These records can be summarised as follow:

The Main Bed (the Blackstone) ranges from 1ft 9 inches (53cm) to 2ft (61cm) to 2ft 1 inch (63cm). Thus the Blackstone at Portesham seems to be of about 60cm thickness, as at Clavell's Hard. The substantial thickness at Portesham is the reason for the old attempts by Mr. Manfield to work the oil shale in that area. The Main Bed, the Blackstone, contains Saccocoma.

Dark clay, 58cm thick, with calcareous nodules was found beneath the Main Bed at Corton Farm No.2 Borehole. Large specimens of uncrushed ammonites are an interesting feature that are rare elsewhere. They are probably important palaeontologically. Whitaker and Edwards (1926) comment that they appear to be nearly identical with Pseudovirgatites scruposus (Oppel). Neaverson (1924) had earlier reported that: "Mr. J. Pringle has obtained fragmentary specimens of large ammonites from the Oil shales of Corton, and these have been recorded as Pseudovirgatites scruposus Vetters". According to Coe et al. (1999), the equivalent bed (42/11) between the Bubbicum and the Blackstone at Clavell's Hard is in the Pectinatites (Virgatosphinctoides)wheatleyensis Subzone [the top one] of the Pectinatites (Virgatosphinctoides)wheatleyensis Zone of the Bolonian (sensu gallico) or Kimmeridgian (sensu anglico).

Underneath the ammonite clay a thinner oil shale of 1 foot (30cm) thickness was recorded at Corton No. 2. This may be the Bubbicum. It seems not have been regarded as important and is not listed separately in the other boreholes at Portesham.

At Friar Waddon Dairy House No.1 Borehole, Upway (Upway) the Main Bed, the Blackstone is again 60 cm. thick. Black shelly clay beneath (the ammonite bed) is 53cm. The presumed Bubbicum is listed as "Brown oil-shale, inferior". It is 53cm thick, a little thinner than at Portesham.

In a borehole at Littlemoor the lower limit of the Blackstone was not well-defined. Also at this locality the Blackstone was observed in the Weymouth Relief Road but it was in poor, weathered conditions and the details were not worked out.

The Kimmeridge Blackstone is also present at the Isle of Portland. The details are not well-known, but pieces of the oil shale can be seen washed up on the southeastern end of the Chesil Beach.

Thus, in conclusion, it can be stated that the Kimmeridge Blackstone is about 60 cm thick at Clavell's Hard Kimmeridge and also in the Portesham area. It is a little thinner at Brandy Bay and much thinner at Ringstead. The Bubbicum, an inferior and lower oil shale can also be recognised at most localities.

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BLACKSTONE - PALAEONTOLOGY:

Fauna - General

The fossil content of the Blackstone beds if of special interest because there has been much discussion about the palaeoceanography. Organic matter is best preserved on seafloors if the conditions are very reducing but then the conditions are unfavourable to benthic life. The Kimmeridge seafloor has provoked much discussion.

Remains of ammonites and fish, part of the nekton, occur and planktonic crinoid debris is present. The benthic fauna is limited but not totally absent. Records come the 'coal' and associated shale (Strahan, 1898). Particularly interesting is the occurrence of brachiopods - Discinisca latissima Sow. and Lingula ovalis Sow. There are some bivalves including what were termed Astarte mysis (?) and Cardium striatulum (is this the Protocarda so common in the Kimmeridge Clay?). There are remains of fishes and shrimps and serpulids. There is a strange record of Littorina (?) (Strahan, 1898).

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BLACKSTONE

Saccocoma, the Oil Shale Crinoid

Small pyritised plates (brachials) of an unusual crinoid Sacoccoma are conspicuous in Kimmeridge oil shales (Arkell, 1947; Gallois, 1978). The distribution has been discusse by Cox and Gallois (1981). Saccocoma was first recorded by Bather (1911).

In more detail brachials are present in the Kimmeridge Blackstone (KC 42/12), the Bubbicum (KC 42/10), bed KC 42/5 and some nearby, lower oil shales Cox and Gallois (1981). It is mainly in the upper part of the Pectinatites wheatleyensis Zone, and is usually found in the oil shales of this part of the sequence. The pyritised plates are very small but can be seen by splitting a piece of oil shale and tilting it so as to reflect the sun. Small sparkling patches of yellow will be seen. A hand-lens helps. The pyritised brachials are also present in the Blackstone in Brandy Bay (Cox and Gallois, 1981). They do not seem to have been noticed in non-oil-shale beds, although it is possible that they are only pyritised in the very reducing oil shales. Perhaps they ought to be present in the coccolith limestones of the hudlestoni Zone but they do not seem to be mentioned in the literature.

The most detailed description of this brittle-star-like crinoid has been given by Jaekel (1892).

The mode of life of Sacoccoma is disputable. It has generally been regarded as a pelagic crinoid, associated with the plankton blooms. However Milsom (1994) has argued for a benthic origin. She has shown a reconstruction of the organism (redrawn from Jaekel, 1892). The brachials, on the inner parts of the arms are flanged. It is these flanged brachials that are most frequently preserved in the Upper Kimmeridge Clay oil shales. Milsom (1994) refers to the species at Kimmeridge as: Saccocoma tenella.

This crinoid, Saccocoma tenella is the most numerous macrofossil in the famous Solnhofen Limestone of Germany. It occurs particularly in the Upper Solnhofen Plattenkalk (laminated limestone) which is of early Tithonian age (ti 2b). S. tenella (including its juvenile stage S. pectinata) are most abundant in the Eichstatt quarries where slabs covered with the crinoid occur. According to Milsom (1994), the abundance of Saccocoma decreases in a southwesterly direction towards the Solnhofen quarries where specimens are relatively rare.

S. tenella, which is apparently the Dorset, Kimmeridge oil shale species, occurs in the Kimmeridgian at Talloires near Lake Annecy in the Haute-Savoie district of France. It is present in the Malm 6 (= Middle Kimmeridgian (ki2)) of Wurtemburg, Germany and in the Late Jurassic of the North American Atlantic seaboard (Hess, 1972, Deep Sea Drilling Project, Initial Report 11). See Milsom (1994) for more information and for literature references.

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Oil Shales and Bituminous Shales below the Blackstone

Oil shales and bituminous shales beneath the Blackstone at Clavell's Hard, Kimmeridge, Dorset, labelled photograph, 2005

Thin oil shale beds in the high wheatleyensis Zone, Upper Kimmeridge Clay, below the Clavell's Hard mining ledge, east of Kimmeridge Bay, Dorset, October 2011, labelled photograph

The succession just beneath the Kimmeridge Blackstone or oil shale, seen in the outer cliff of Clavell's Hard, from the shore platform of the Clavell's Hard Dolomite Bed, or Stone Band, east of  Kimmeridge Bay, Dorset, 14th June 2013

In front of the Clavell's Hard mining ledge is a fairly low but steep and slippery cliff. The Clavell's Hard Dolomite Bed or Stone Band (Bed 41/22) forms a convenient shore platform to walk on (at low tide). The cliff section here shows four bituminous beds in the upper part of the wheatleyensis Zone. Some geochemical data relating to these strata, and after Dunn (1974) is also shown here. Two of these beds, 42/5 and 42/10 (the Bubbicum)are particularly rich in organic matter. They are oil shales, with Saccocoma, and resembling the Blackstone, but in fact of inferior quality. The organic content is about 30% rather than about 60% as in the Best Blackstone (lower Blackstone). These beds can also be seen beneath the mining ledge, as shown above, and also in the cliff recess immediately to the east of Clavell's Hard. However, this is not a safe place, and there is risk of rock fall. Notice, incidently, that the two cemented or calcareous oil shales are phosphatic to some extent. This might indicate a pause in sedimentation, a minor diastem, although they are not at a zonal boundary.

The tidal trap at Clavell's Hard, east of  Kimmeridge Bay, Dorset, 14th June 2013, with rising tide

Warning - Get out westward from Clavell's Hard before the tide rises or you will be trapped! See the photograph above!

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Faulting at Clavell's Hard

Faulting at Clavell's Hard, Kimmeridge, Dorset, shown large

There is extensional faulting with drag and with some brecciation at Clavell's Hard. This is immediately to the east of the mining ledge. At the same site as the faulting calcareous tufa has been deposited, probably more over moss than algae. This is result of calcareous water trickling over or out of the cliff here.

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HISTORY:

Blackstone, Oil-Shale Mining
(See also information in a separate webpage on the cliff fire at Clavell's Hard.)




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HISTORY OF THE OIL SHALE USE:

1. Iron Age and Roman Use

Blackstone as coal money and polished Blackstone and coal money again

The Kimmeridge oil shale was used extensively by Iron Age British tribesmen and the Romans for armlets. These were made on a primitive lathe. The residual central disks are known as Kimmeridge coal money. Large numbers have been found. With one of them is shown a beach pebble of oil shale that I polished with carborundum paper and metal polish. This gives an idea of the polished appearance. It is easy to polish and the material seems to have been much valued in antiquity.

Stages in the manufacture of armlets by the Romano-British at Kimmeridge, Dorset

A Roman mosaic at Kourion, near Akrotiri, Cyprus, showing the young woman KTICIC, the Founding Spirit or Creation,  wearing an armlet and carrying a measure of the Roman foot

The upper images shows stages in the manufacture of Kimmeridge Blackstone armlets, as illustrated by Calkin (1955). The lower image shows a young lady, KTICIC, the Founding Spirit or Creation, wearing something much like a Kimmeridge Blackstone armlet on her right wrist. This is from a Roman mosaic at Kourion in Cyprus, so it is not necessarily a Kimmeridge armlet that was the model or inspiration for this ancient work of art. However, perhaps something resembling the Kimmeridge Blackstone armlets was also worn in the Mediterranean area.

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HISTORY OF THE OIL SHALE USE:

2. 17th Century Use of the Oil Shale or Blackstone

A burning flake of Kimmeridge oil shale or Blackstone, Kimmeridge, Dorset

Burning blocks of Kimmeridge oil shale, ignited artificially, showing the gasy type of burning

The Blackstone or Kimmeridge Coal has been used as fuel in local cottages in the past, up to quite recent times, but was clearly a very unhealthy means of heating. It gives off very unpleasant sulphurous fumes, including hydrogen sulphide, and leaves much ash.

(Abominable Odour of Burning Kimmeridge Oil Shale:
notes from Damon (1884):
"Referring to the inflammable shale of the Kimmeridge Clay, Dr. Mills, a former Dean of Exeter, says, it is not unlike the bitumen of the Dead Sea, and that the smoke arising from it resembles that of the latter - Phil. Trans, vol 61. Dr. Pocock, 'Travels to the East' vol. 2, p. 30 bears similar testimony.
In further confirmation of which I may add that specimens of bitumen coated with sulphur, which I gathered on the shore of the Lake, on being exposed in a flame, emitted an odour resembling that of Kimmeridge Shale when burnt. - Robert Damon.")

Smedmore House, Kimmeridge, Dorset, once the home of Sir William Clavell, but since extended

In the early 17th century Sir William Clavell was the landowner and he lived at Smedmore House (since much extended), north of the oil shale outcrop. (described by Lord David Cecil, 1985). He was an adventurous type of man, knighted for having put down a rebellion in Ireland. In Dorset, he was a pioneering hydrocarbon industrialist. He used the pyrite, which is abundant in Kimmeridge Clay, in an attempt to manufacture alum. The work had been started by his father John Clavell. Sir William, however, worked with Lord Mountjoy who ran an alum and copperas industry in the Bournemouth - Poole area, including Alum Chine (Cochrane, 1970).

[Note that in historic times there was some confusion regarding the use of the term "alum". It may apply to true alum, with a double salt of ammonium (or potassium) sulphate and aluminium sulphate, or in the past it was sometimes used for melanterite or copperas which is ferrous sulphate. True alum is difficult to produce, whereas pyrite can break down naturally into greenish melanterite or copperas, or yellow jarosite. There is a record for 1536 that no issues of alum had happened at Durleycliff (near Durley Chine, Bournemouth) in that year. This has to be a reference to the natural weathering products of pyrite, i.e. melanterite (copperas) and jarosite, not true alum. Because of this confusion old references to alum must be treated with care.]

A waste tip of burnt Kimmeridge oil shale, probably from the 17 century works of Sir William Clavell, southeastern corner of Kimmeridge Bay, Dorset, photo 2006

Sir William Clavell also made salt by boiling sea water, using the oil shale as fuel. Finally he set up a glass works, again using the oil shale or "Kimmeridge Coal" as fuel. I do not know the source of the glass sand. The best glass sand in the region is in the Barton Sands of Alum Bay and Headon Hill, Isle of Wight, but he might have used more local Wealden or Tertiary sand.

More on the Air-polluting Industry at the "Close Stool" of Kimmeridge, the Isle of Purbeck

A simplified outline of the probable scheme of alum production at Kimmeridge Bay, Dorset in the 17th century

The Reverend J. Coker in 1732 gave more details of the works and a colourful impression of their unpleasant and unhealthy nature:

p.46 etc.

"The next place that offereth itself is Smedmore, where Sir William Clavile, descended of antient Gentry (as you have allready hearde) built a little newe House and beautified it with pleasant Gardens. The place long sithence had Lordes of the same name, from whom by Dolfin it passed hereditarilie to the Claviles; neare adjoining to the Sea: And not far hence, the nowe Owner, beeing ingenious in diverse Faculties, put in tryall the making of Allom, which hee had noe sooner, by much Cost and Travell, brought to a reasonable Perfection, but the Farmers of the Allom Works seized to the Kings Use; and beeing not soe skillfull or fortunate as himselfe, were forced by Losses to leave it offe, and now it rests allmost ruined.

But in place of it Sir William Clavile, whom one disaster, dismayed not, has sitence sett up a Glasse House (which is come to Perfection and is likely to redounde to a great Benefit) and Salt House.

For Transportation of these Comodities, as also of White Salt (where it is made in great Abundance by boiling it out of the Sea Water) he hath at his own Charge, with Great Rocks and Stones piled together, built a little Key in imitation of that at Lime [the Cobb at Lyme Regis], for small Barkes to ride, invironed on the East Side with a Hill yeelding Myne [i.e. mineral or ore, presumably in this case - pyrite or pyritic shale, alum-shale] (as they call it) for the Allom Works [this description seems to suggest Hen Cliff, which is a hill],

(An early environmental protest! A handwritten note has been added here in old English writing style, with the old type of S. Presumably this is by Coker.

"A very particular situation in a small flat surrounded with high hills towards the land side and open to the Sea. A relation of the Cullifords called it the close stool [i.e. night commode] of the Island."

[The Cullifords were the neighbours of the Clavells on the east side. They held the Encombe Estate. At a later date, Lord Eldon was the owner and built Encombe House. The Cullifords were living downwind of the air pollution coming from the works. It is not surprising that they made the comparison to a pre-plumbing lavatory.]

and a kind of bluish Stones that Serve to burne [i.e. unweathered Kimmeridge oil shale or Blackstone] for maintaineing fire in the Glasse House; but in burneing yeelds such an offensive Savour [odour] and extraordinairie Blackness, that the People labouring about those Fires are more like Furies than Men."

To read more about alum and its production see: Osborne (1999). He discussed the history of the alum workings in the alum shales of the Lower Jurassic, Toarcian, Lias of the Yorkshire coast from 1620 to 1870. Millions and millions of tons of rock were taken out by pickaxe and wheelbarrow. By the early part of the nineteenth century production there was over 3,000 tons of alum per year. This could have happened at Kimmeridge as there is adequate bituminous shale and pyrite, but the seizing of Sir William Clavell's alum works for the king (King James I), stopped the work. Incidently the reference to a floating egg is with regard to a method of establishing the concentration of the alum solution during the manufacturing process.

Useful dates have been given by The Purbeck Mineral and Mining Museum (online). They give the approximate date of Sir William Clavell's alum workings as about 1600, following Lord Mountjoy's discovery of "Allom Myne" there in the 1560s. Was the "Allom Myne" the yellow jarosite of Hen Cliff. It is not large in quantity but very obvious to anyone walking along the foot of the cliff. It might, however, have been the shiny pyrite so easily observed in Kimmeridge Bay in the autissiodorensis Zone. Regarding Lord Mountjoy and the jarosite and melanterite of Brownsea Island see my Brownsea Island webpage. (Although Brownsea Island has no Kimmeridge Coal it has much lignite in the Parkstone Clay.

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HISTORY OF THE OIL SHALE USE continued:

3. The Oil Company Mining of the 1880s and 1890s

Map of oil shale or Blackstone workings of 1890 at Clavell's Hard, east of Kimmeridge Bay, Dorset

About 1.6km east of Kimmeridge Bay is the small headland once known as Blackstone Point but usually referred to as Clavell's Hard (map ref. SY920777). The Kimmeridge oil-shale or Blackstone was worked here and further the east, near Rope Lake Head, it descends to the beach.

Old workings of the Kimmeridge oil shale or Blackstone workings at Clavell's Hard, east of Kimmeridge Bay, Dorset, in the 1890s

Above is an old photograph of the oil shale workings at Clavell's Hard. The mining ledge has since been reduced in size by coast erosion. No. 3 level has collapsed or is concealed. No. 4 level can still be seen but is collapsing. Photographs below show the mining ledge at Clavell's Hard in recent years.

Clavell's Hard or Blackstone Point seen from the west in the year 2000, east of Kimmeridge Bay, Dorset

The small stream at Clavell's Hard, Kimmeridge, Dorset, with upblowing water during a gale, February 2011

Waterfall with tufa and algae at the mining ledge of Clavell's Hard, east of Kimmeridge Bay, Dorset

The mining ledge at Clavell's Hard, east of Kimmeridge Bay, Dorset, as seen on 4 October 2005, with Halfdan

The eastern part of the Clavell's Hard mining ledge, with evidence of the cliff fire, east of Kimmeridge Bay, Dorset, 17th October 2008

The old No. 4 Level for oil shale at the Clavell's Hard Mining Ledge, Kimmeridge, Dorset A general view of the entrance to No. 4 Level at the Clavell's Hard mining ledge, east of Kimmeridge Bay, Dorset, 2005

The entrance to No.4 Level in the Kimmeridge Blackstone or oil shale, after the oil shale fire, Clavell's Hard, east of  Kimmeridge Bay, Dorset, old photograph

Inside No.4 Level, Kimmeridge Blackstone Mining Ledge, Clavell's Hard, east of Kimmeridge, Dorset

Clavell's Hard refers to a landing beach, and the name is first recorded in 1787 (Legg, 1884). The Kimmeridge Oil and Carbon Company reported that in 1890 there were five thousand feet of underground tunnels or levels at Kimmeridge (see Legg, 1884, p. 23 for a map of industrial Kimmeridge showing the various levels and shafts). Two of the levels, No. 3 and No. 4 were opened onto this ledge. The old photograph shows the state of the ledge in the 1890s (based on Strahan, 1898). Next to the hut is No. 3 Level, covered by debris now. The oil shale, which just above the floor of the ledge or platform, is almost a metre thick (0.86m according to Strahan, 1898). Slabs of this have been piled up ready for removal. You will notice the railway track and some wooden posts, probably part of a pier or jetty, no longer in existence.

The five levels or adits in the cliff are from the 1890 workings of the Kimmeridge Oil and Carbon Co. Ltd. in the final phase of operation (Legg, 1984). Galleries branched off the main levels, and between No. 1 and No. 2 the Blackstone was worked out for the whole of the 150 feet closest to the cliff (Legg, 1984). Worked out shale can be easily seen close to Clavell's Hard.

The mining ledge or old quarry is now much eroded and inaccessible, but the relics of the workings are clearly visible. A photograph above shows the deserted coast with the remains of the mining in the cliff. Clavell's Hard or Blackstone Point is seen from the beach to the east, at low tide on a wet day. The flat boulders on the beach are fallen blocks from the Rope Lake Head dolomite band which descends with a gentle dip to the east at Rope Lake Head, a short distance east. Note that you can get cut off here by a rising tide and forced back to the cliffs from which debris regularly falls! The ascent to Clavell's Hard mining ledge is slippery and dangerous and there is no safe route to the top.

Two levels entered the cliff from the mining ledge at Clavell's Hard. One of these, No. 4 was still open recently but like no. 3 (the western) it has now collapsed. No. 4 used to be visited regularly by geologists, and in the 1950s the ledge was easily accessible by a path from the cliff top. There was also a rope-assisted route up from the eastern end of the ledge. It was possible to walk into the tunnel for about 50 or 100 metres before coming to a dead end. I did not see any branches that could be entered. The level was still accessible at the time of the 1973 cliff fire and Dr D. Cole and myself inspected it from time to time to see whether the fire was likely to extend into it. There was some fumes and heat in the tunnel but the fire did not gain a hold here. After that there were rock falls at the entrance and it became increasingly dangerous, and it should not be entered now. No. 5 level was also open in the 1950s but it was at a height in the cliff that was difficult to reach, and I never went into it.

On the cliff top around here there are some old tramway lines remaining but there has clearly been much coast erosion and parts of the old working have been lost to the sea. Some relics of the mines can be seen from the beach west of the mining ledge, and sometimes to the east. The name 'Clavell's Hard' referring to a hard or landing place of Sir William Clavell, the owner of Kimmeridge around about 1600, and an early exploiter of oil-shale. At Clavell's Hard the oil shale was probably worked on various occasions from the the 17th Century to the 19th Century. The Blackstone was used as either a fuel or a source of oil. The main workings on the ledge were from the 19th Century and there was some sort of pier extending seaward from this from which shale could be loaded onto boats.

It is of interest to note how Mr C.E. Robinson walking along the cliff-top from the east to Kimmeridge Bay in 1882 encountered the cliff quarry and mines.

"Approaching Kimmeridge Bay, on the side of the cliff is to be found an excavation or quarry of the bituminous Kimmeridge shale, which is used by the cottagers for fuel, and but for the peculiar smell, like that of melted guttapercha, which it emits while burning, might be sold in more distant markets. The rude crane which is used to lift the extracted material to the top of the cliff has its presentment in one of the cuts illustrated this chapter."

Mansell (1967), and quoted in Legg (1984), summarised the history of the mining ledge:

"The most obvious remains of the ninteenth century enterprises are to be seen in the cliff at Clavell's Hard, a mile to the southeast of the bay. Here were tunnels running into the cliff about thirty feet [10m] above the shore level. One such tunnel [No. 4 level] still remains, though its mouth is gradually being closed by falling shale. They were connected by a ledge, presumably artificial, which still remains, though in an unsafe and dilapidated condition. Along the ledge once ran a railway from tunnel to tunnel. Below the ledge, at low tide, one can find circular cuttings in the flat rocks, usually covered with seaweed or full of stones. In these are the metal bases of the uprights of which the purpose was presumably to carry some sort of pier."

Old tramway lines from the oil shale workings on the cliff top, west of Clavell's Hard,  Kimmeridge, Dorset, as seen 9 Oct 2005

Maxwell (1927) also mentioned the tramway that once carried trucks from the tunnels to the quay or pier. For more information see Legg (1984).




----- No. 1 Level

Remains of No.1 Level in the Blackstone, between Yellow Ledge and Clavell's Hard, east of Kimmeridge Bay, Dorset

The Kimmeridge Blackstone or Oil Shale has been mined in the cliffs near No. 1 Level, between Cuddle and Clavell's Hard, east of  Kimmeridge, Dorset, January 2008

What are probably the collapsed remains of No.1 Level can be seen in the middle of the bay between Yellow Ledge (Cuddle) and Clavell's Hard, and about half-way up the cliff. Tramway rails can project from the remains of the tunnel. To the east of this the oil shale has been worked out and only the debris of shale that is less organic-rich fills the former mining excavations.



----- No. 2 Level

Worked-out oil shale and collapse of No.2 Level, with an old buried waggon on rails, west of Clavell's Hard, Kimmeridge, Dorset, 2010

Worked-out and collapsed mining level, west of Clavell's Hard, Kimmeridge, Dorset, 2011

Cliff collapse at the old No. 2 Level of 1890 Blackstone workings, just west of Clavell's Hard, Kimmeridge, Dorset

The No. 2 Level is not open at present. However its location is visible in the shallow embayment a short distance to the west of Clavell's Hard. There is a stretch of worked-out Blackstone which has been backfilled by small shale mining debris. If you look closely you will see an old tramway rail hanging down from the old workings in the cliff in the area of dark shale above the right shoulder of the petroleum engineering student.

From here for a distance along the cliff of about 300 metres northwest is a stretch marked as "Line crawl underground shale taken out" on the map given by Legg (1984), p. 23, and based on the original plans of the Kimmeridge Oil and Carbon Co. Ltd. Legg mentioned that this area is "a thousand feet by a hundred and fifty feet, which worked to their standard height of seven feet would call for the extraction of a million cubic feet of shale and clay". I have not studied the working to determine the height. I wonder whether "line crawl" suggests that the working were not full height and that only the thin Blackstone and adjacent shale was extracted. The Blackstone is only about 0.86 m., less than half the seven foot working height (i.e. about the height of the main levels). Examine once more the backfilled mine in the photograph.

The cliff fall, shown, is recent and seems to have taken place in April 2006. It indicates the dangers of these cliffs. Two planar sloping surfaces in the cliff are probably fault planes.




----- No. 3 Level

No.3 Level at the oil shale mining ledge, Kimmeridge, Dorset

No. 3 Level is now filled with debris and shows some roof collapse. The sea has eroded away the western end of the mining ledge just here. This level, when in work, is shown in an old illustration further above. This tunnel was at the western end of the mining ledge adjacent to the hut and to the ladder up the cliff. The location is not of easy access.



----- No. 4 Level

The old No. 4 Level for oil shale at the Clavell's Hard Mining Ledge, Kimmeridge, Dorset

Old photograph of the entrance to No.4 Level Blackstone mine at Clavell's Hard, Kimmeridge, Dorset

The entrance to No. 4 Level of the old oil shale or Blackstone workings at Clavell's Hard east of Kimmeridge Bay, Dorset, as seen in 2005

This is No. 4 Level at the east end of the ledge. It is still open, but now highly dangerous. It was said on the old plans to be not much worked, but this one alone has survived. It is not safe to go into because the roof is loose and there have several falls at the entrance. The entrance is on sloping shale debris at the edge of cliff and a close visit is not recommended. This particular adit extends horizontally into the cliff for about a hundred metres and there is nothing special to see inside. Note the Rope Lake Head Dolomite Bed, with a thin limestone beneath, just above the adit. This dolomite is 4m above the Blackstone. At the entrance of the level calcium carbonate is precipitated as calcite over moss by dripping water and "moss-tufa" is formed.

----- No. 5 Level

No.5 Level in the Blackstone or oil shale, east of Clavell's Hard, Kimmeridge, Dorset

No.5 Level was visible in the mid-20th century as a small opening in the cliff at about the level of the Blackstone. Then it became covered by the debris of a cliff fall. This photograph shows what may be the No. 5 level in the centre of the photograph. The aperture is small and seems to be largely filled by the fallen debris. With future coast erosion it should become clear as to whether this is No.5 level.

For more on the history of the oil-shale workings see Legg (1984) and Mansell-Pleydell (1893; 1894). For more modern and technical information on the oil-shale see Gallois (1976;1976a; 1978; 1978a). See Cole (1975) for a record of a cliff fire here.

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BLACKSTONE continued:

Oil Shale Working - Victorian

The Blackstone or the main Kimmeridge oil shale which is discussed here, tends now regarded as a single bed of about 60cm thickness (two feet). Study of the detail given here will show that it actually consists of two rich beds of oil shale with a band of less-bituminous mudstone in the centre, and it was regarded as such in Victorian times ( Damon, 1884, p. 60.). The mining was confined to this 60cm (or 2 feet) of Blackstone. The Short Joint Coal above, Bed 43/3 and of the lower part of the hudlestoni Zone, is 15 to 20cm thick. It was too thin to be economical to mine. It was, however, used locally by villagers.

Oil was produced from the Kimmeridge oil shale, but so too was gas. One ton yielded 11,300 ft. of 20 candle gas ( Damon, 1884) compared to 15,480 ft of 52 candle gas from Boghead Coal, torbanite (algal oil shale) from West Lothian, Scotland.

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BLACKSTONE contiued

History - deodoriser etc.

When Strahan was investigating Kimmeridge for the geological survey in the 1890s he visited these works and wrote " Of late years the mineral property at Kimmeridge has been leased to the Kimmeridge Oil and Carbon Company, by whom the coal has been used for fuel, for improving the illuminating power of coal-gas and for the manufacture of paraffin. It is stated by them that the residual coke or carbon after the distillation of the oil possesses the properties of animal charcoal, and can be employed as a deodoriser, distinfectant, and decoloriser and that it also serves as a manure.

From the oil an insecticide and a preparation for the prevention of mildew, Oidium , etc, on vines and other plants has been made. The Blackstone of Kimmeridge Bay, by the same account, it the richest seam yet discovered, and in the laboratory has been known to yield 120 gallons of oil to the ton, or , when distilled on a large scale, 66 gallons to the ton, while common shale gives 33 gallons to the ton."

"Notwithstanding, however, the variety of uses to which the coal has been put, it has never paid the expense of extraction, and at the time the resurvey of the district was in progress the works were almost at a standstill. "

Fires have occurred in the oil-shale at Clavell's Hard and on the beach east of Cuddle. For pictures and information on the oil-shale fires see the webpage - Burning Beach, Burning Cliffs and the Lyme Volcano.

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BLACKSTONE continued:

Oil Shale Mining History - A Summary

The sequence of mining here is given, based on information from Strahan (1898). ;Legg (1984); Arkell (1947) and other sources.

1560s. Lord Mountjoy discovered that the land at Kimmeridge was 'full of allom myne' and he obtained a patent to make alum there ('myne' means ore not mine). Mountjoy's plan was unsuccessful.

Around 1600. Sir William Clavell, owner of Kimmeridge, took up alum manufacture. When the process was perfected the alum works were seized by London merchants who had King James 1' sole patent for making alum.

1617. Abraham Bigo with Clavell used oil shale to make green drinking glass at a glass-house just north of the fishermen's huts. Royalties were unpaid and later Clavell went to prison. He had large losses and died in 1644.

Until 1620. Sir William Clavell built a pier 100 feet long in imitation of the Cobb at Lyme Regis. It was destroyed by storm in 1745. Some remains are still visible.

1848. Bituminous Shale Company built a tramway and worked oil shale. This was used for production at Weymouth of varnish, grease, pitch, naptha, dyes, wax, fertiliser and other by-products. In 1854 the Weymouth factory was condemned as a public nuisance because of the smell and the company went into liquidation.

1848. Wanostrocht and Company used Kimmeridge oil shale to light the streets of Wareham with 130 gas lamps. There was building of a new works near the Wareham railway station in 1852.

1855-1858. Ferguson and Muschamp established an oil shale plant at the site that later became the Wareham pottery works, but it closed in three years.

1858. Wanostrocht and Company reopened the Wareham shale plant. Fifty tons of shale oil were produced there each month and there was a contract to light the streets of Paris with gas from this product.

1862. Wanostrocht and Company were struggling and taken over by the new Wareham Oil and Candle Company. The works were southwest of the railway station.

1871. West of England Fireclay Bitumen and Chemical Company was incorporated. Ten thousand tons of Kimmeridge shale were to be processed annually at Calstock, Cornwall. It went into liquidation in 1876.

1872. Works of the Wareham Oil and Candle Company were burnt down and the firm closed.

1876. West of England Chemical Company went into liquidation.

1876. Sanitary Carbon Company at Wareham was established to convert shales into a coke that could filter sewage. The company later became the Kimmeridge Oil and Carbon Company.

1883. A longer tramway was constructed at Kimmeridge. The Manfield Shaft mine was dug.

By 1890. Four main levels or adits had been dug, two of these coming from the mining ledge at Clavell's Hard. Galleries branched off. No. 5 level was marked out for cutting. There were 5,000 feet of tunnels.

Late 1890s. Oil shale mining came to an end.

1898. Strahan stated that "Notwithstanding, however, the variety of uses to which the coal has been put, it has never paid the expense of extraction, and at the time the resurvey [geological survey] of the district was in progress the works were almost at a standstill."

1917-1918. A series of test boreholes were sunk around Kimmeridge and Portesham to determine the total extent of the oil shale field in case exploitation should become necessary because of shortage of other oils.

1947. Arkell stated that because of the high content of sulphur ( 6 to 7%) and the thinness of the workable seam, the cost of extraction has always been too high to allow the project to become an economic success. An annual royalty for mineral rights was said to be still paid to the landowner at Smedmore.

1959. The BP oilwell west of Gaulter's Gap was established. The reservoir is Cornbrash at 564m (1850 feet).

1978. R. Gallois published ' A pilot study of oil shale occurrences in the Kimmeridge Clay'. This was intended to determine whether a full-scale study was warranted. The oil shales in the Kimmeridge Clay in Norfolk, Lincolnshire and Dorset are comparable to the Jurassic oil shales which have been worked by opencast methods in France. There is, however, a major problem of disposing of useless (and possibly harmful) residue which is just as bulky as the original oil shale which is retorted.

2000. No plans are known, at present, for oil shale production. Opencast mining or some method of burning the shale underground seem remote possibilities in the event of an energy crisis, but even then not necessarily in Dorset since, as noted, the oil shale extends as far as Norfolk. It continues under the North Sea and the Kimmeridge Clay with its bituminous matter is the major oil source rock for North Sea oil.

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PALAEONTOLOGY:

Ammonites from the Clavell's Hard Area.

Subplanites cf grandis  - ammonite Crushed ammonite under water

Left: Subplanites cf. grandis (Buckman), according to Arkell (1947). The specimen is from between Clavell's Hard and Rope Lake Head, 2 - 3m below the Blackstone or Kimmeridge Coal. Image modified after Arkell (1947), which should be seen for details and discussion.

Right: Ammonite under shallow seawater at Clavell's Hard. This is probably from the Wheatleyensis Zone between the Grey Ledge and the Blackstone.

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MINERALS:

Pyrite

Crook-shaped pyrite in the ledges of Kimmeridge Clay, west of Clavell's Hard, Dorset

Pyrite is very abundant in the Kimmeridge Clay. This is particularly the case in or close to oil shales or bituminous shales. The stagnant reducing conditions with much organic matter were favourable for sulphate-reducing bacteria to reduce the sulphate ions of seawater to sulphide ions which then reacted with any availale iron to form pyrite FeS 2 .

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MISCELLANEOUS:

Calcareous Tufa on the Cliffs

Calcareous tufa on the cliff at Clavell's Hard, Kimmeridge, Dorset, 2000

Boulder of calcareous tufa on the beach, east of Rope Lake Head, Kimmeridge, Dorset, 2010

Calcareous tufa forms on moss or algae on the cliffs in the Clavell's Hard area, Kimmeridge. Small streams or rivulets flowing toward the cliff are calcareous, probably not only because of carbonate content in the Kimmeridge Clay, but also because Portland Stone and Portland Stone debris occur on the hills upslope and inland. Cliff erosion may have increased recently and as a result blocks of such tufa occur on the beach in the area of Rope Lake Head (in the down direction of longshore drift from Clavell's Hard).

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LOCATION:

Rope Lake Head

The cliff at Rope Lake Head, east of Kimmeridge Bay, Dorset, with hudlestoni Zone, Upper Kimmeridge Clay, March 2006

The Kimmeridge Ledges of dolomite within the Kimmeridge Clay, near Rope Lake Head, east of Kimmeridge Bay, Dorset

Walk on eastward to Rope Lake Head and Freshwater Steps to see the White Stone Band and associated strata of the pectinatites Zone, Upper Kimmeridge Clay?

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COMPARISON:

Kimmeridge Blackstone at Brandy Bay

A research team working on the Kimmeridge oil shale or Blackstone at Brandy Bay, Kimmeridge, Dorset, October 2011

For more information, please go to webpage:
Kimmeridge - Hobarrow and Brandy Bays, West of Kimmeridge Bay

Ledges of Kimmeridge Oil Shale in the Kimmeridge Clay, below Gad Cliff, west of Kimmeridge,  Dorset, photo by Alan Holiday, 2010

Oil shale at about the level of the Blackstone, Kimmeridge Clay, Brandy Bay, west of Kimmeridge Bay, Dorset

Typical Blackstone exposed on the shore at Brandy Bay,  west of Kimmeridge Bay, Dorset, UK

The main Kimmeridge oil shale or Blackstone is exposed at two localities in the Kimmeridge area. The main section east of Kimmeridge and between Clavell's Hard and Rope Lake Head is the one described above. There is, in addition, another exposure of the Blackstone, slighty thinner at about 50cm thickness, at Brandy Bay, west of Kimmeridge. Access to this locality is only possible when the Army Range Walks are open, usually at weekends (but not every weekend).

The oil shale is exposed on the shore as at the exposure east of Kimmeridge, it tends to be free of fallen rocks, which presumably slide off. The oil shale is pyritic here too.

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Comparison - Kimmeridge Oil Shales and Liassic Oil Shales.

Lower Lias Shales - Source Rocks for Oil and Gas

Millimetre scale lamination in laminites or  bituminous shale, source rocks, the Shales-with-Beef, Lower Lias, Lyme Regis, Dorset

This section is to give some brief comparison, with the Lias oil and gas source rocks, the Shales-with-Beef, as seen on the coast at Lyme Regis, and the well-known Kimmeridge Clay oil shales. [more notes will be added]

The Shales-with-Beef of the Lower Lias are sufficiently bituminous to have caught fire, as have the Kimmeridge oil shales. The most well-known fire is that of the Lyme Volcano of 1907-8. The Shales-with-Beef have some interesting similarities and differences in comparison with Kimmeridge oil shales.

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COMPARISON:

Kimmeridgian Shale of the Kimmeridgian Total Petroleum System of the North Sea.

See particularly: Gautier, D.L. 2005. Kimmeridgian shales total petroleum system of the North Sea graben province.

The Kimmeridgian Shales Total Petroleum System of Gautier, 2005 was defined to include all the oil-prone areas of the North Sea Graben. The maximum rifting in the North Sea was during the Late Jurassic and the earliest Cretaceous times. In comparison in the Dorset and English Channel area the rifting was intensive during deposition of the Kimmeridge Clay, resulting in an almost double thickness in the basin at Kimmeridge and offshore. However, rifting was slowing during sedimentation of the lagoonal and lacustrine Purbeck Group (late Tithonian to Berriasian)and the subsequent early Cretaceous, and came to an end by about the beginning of the Aptian.

Thinner Kimmeridge Clay in Dorset
As in south Dorset and the English Channel, rapidly deposited marine mudstones, rich in organic matter, accumulated widely throughout the rift basins of tghe North Sea. The depositional thickness locally exceeding 3,000m contrasting with the much lesser thickness of about 500m for the Kimmeridge Clay of the Dorset coast at Kimmeridge (the landward part of the Portland - Isle of Wight Basin, or Inversion Structure). There is more organic-rich mudstones, such as part of the Oxford Clay lower in the Dorset and Portland - Isle of Wight Basin, although this is separated from the Kimmeridge Clay by the shallow facies Corallian. Thus, although the south Dorset coast (Kimmeridge area) sequence is much thicker than the general English mainland succession of Kimmeridge strata, it is still thinner than in the North Sea. This makes most parts of it less likely to be thermally mature.

In the North Sea, the organic-matter-rich facies of the late Jurassic and early Cretaceous contain from 2 to more than 15 weight percent total organic carbon (TOC). They are easily identified from their high gamma-ray values on logs, but this does not seem to be the case for the Kimmeridge Clay of Dorset (see the RGGE gamma ray logging both of cliffs and boreholes). In the North Sea the stratigraphic intervals are often called the "Hot Shales" of the North Sea. This term does not seem to have been used in Dorset. The North Sea formations with "Hot Shales" include the Kimmeridge Clay in the Moray Firth/Witch Ground (Gautier, 2005).

Kerogen in the "Hot Shales" of the North Sea is the Type II, degraded mixture of terrestrial and planktonic marine origin, and the situation is the same in Dorset with regard to the Kimmeridge Blackstone and other bituminous units. These organic rich units contain 2 to more than 15% TOC (total organic carbon). The Kimmeridge Blackstone can contain particularly high levels but the average for the Kimmeridge Clay was given by Weedon et al. (2004) as 3.8% TOC.

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Kimmeridge Clay - Thermal Maturity Comparisons

(1) At most place onshore in England, excluding the central Weald of Sussex and Kent, the Kimmeridge Clay Formation is thermally immature.

(2) The Kimmeridge Clay has Ro of about 0.34 at Ringstead (Ebukanson and Kinghorn, 1986).

(3) The Kimmeridge Clay at Kimmeridge has an Ro of about 0.45 (Ebukanson and Kinghorn, 1986).

(4) The lower parts of the Kimmeridge Clay Formation in the Arreton No. 2 Borehole of the Isle of Wight are marginally mature with an estimated Ro: 0.50 to 0.60% (Ebukanson and Kinghorn, 1986).

(5) In the North Sea, in shallowest core samples, the Kimmeridge Clay is thermally immature to marginally mature with vitrinite reflectance (Ro) values of ~0.6%. (Fishman et al, 2012).

(6) In the North Sea, in the deepest core samples the Kimmeridge Clay is thermally mature with vitrinite reflectance (Ro) values of ~1.2% (Fishman et al, 2012).

(7) The Kimmeridgian Haynesville Shales of the USA reach extreme thermal maturity with vitrinite reflectance (Ro) values of up to ~2.0 - 2.8. (Kornacki, 2010).

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Possible Kimmeridge Limestone Reservoirs - Short Joint Coal

The Short Joint Coal at Rope Lake Head, east of Kimmeridge Bay, Dorset, showing a central calcite rhomb limestone with oil shale above and below, 2010

Discussion of the Short Joint Coal by Celtique Energie, in relation to possible Kimmeridge Clay internal limestone reservoirs

The Short Joint Coal or oil shale at Rope Lake Head, east of  Kimmeridge Bay, Dorset, 2010

The Short Joint Coal at Rope Lake Head, east of Kimmeridge Bay, Dorset, showing a central calcite rhomb limestone with oil shale above and below, 2010

Early calcite rhombs in the limestone of the Short Joint Coal, Rope Lake Head, east of Kimmeridge Bay, Dorset, seen in SEM (notes to be added)

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KIMMERIDGE BLACKSTONE - UNCONVENTIONAL GAS RESOURCES?

Oil Shale Fracking (Hydraulic Fracturing) - Introduction

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Diagenesis burial zones and fracking potential of British Jurassic bituminous shales and associated strata, shown in a simplified diagram

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Kimmeridge Oil Shale - Hydraulic Fracturing (Fracking)

The Portland - Isle of Wight Basin

The diagrams which follow are now concerned with the thickness of Jurassic and other strata. It is unlikely that the Kimmeridge oil shales can become useful underground strata for unconventional energy resources, i.e. shale gas, unless there is sufficient burial for thermal maturity (a topic discussed briefly elsewhere in this webpage). It is again emphasised the Kimmeridge Clay is not thermally mature at the coastal section of Kimmeridge shown here. It is not thermally mature under most of Dorset. In addition to the central Weald of Sussex and Kent, the offshore area of the Portland - Isle of Wight Basin, is most promising in terms of maturity. The topic is discussed further below.

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Simplified geological map of part of the Central English Channel showing potential areas for oil exploration and for possible production of Shale Gas (fracking etc)

A map showing depth to Pre-Permian Basement in the Wessex and English Channel Basins, modified after DECC, and in relation to shale gas

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An isopach map showing the thickness of the Hettangian to Barremian, Blue Lias to top Wealden, in the central English Channel, in relation to source rock heating

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Licence Block map, Offshore Oil and Gas Licensing, Central English Channel, 26th Seaward Round, May 2011

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The Kimmeridge Oil Shales are the most famous of the oil shales in Britain, and are important as the source rock for North Sea Oil. At present there is a general interest, and also concern, about possible gas-fracking, or fracturing oil shales under high pressure underground and extracting large quantities of gas. The Kimmeridge oil shale in the south of England might seem an obvious target.

However, it should be noted that there may be good reasons for not using it for gas fracking off the Dorset coast. The main problem is here that it is close to the surface. Almost everywhere in the English Channel close to the coast the Kimmeridge Oil Shale is not buried under a great thickness of strata. It is overlain in places by Portland and Purbeck strata but these are not thick, and to the east (towards the Isle of Wight) by Wealden. Faults in the Kimmeridge Clay are probably not sealing in most cases. Gas escape is quite likely unless those faults end at the unconformity at the base of the Gault (the Late Cimmerian or Sub-Albian Unconformity). If boreholes penetrate from above this unconformity for fracking purposes then the safety factor is greater. However, Upper Cretaceous strata (Chalk etc) are only present as an extensive, low-dipping cover at about 20 or 30km south of the Dorset and Hampshire coast. Only the Isle of Wight may have site where an ERD borehole could, at present, reach Kimmeridge strata to the south beneath an Upper Cretaceous cover. Complications arise because of lateral facies changes and it is not clear whether the Kimmeridge oil shale is well-developed very far south.

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Hydraulic Fracturing (Fracking) for Kimmeridge Clay, Shale Gas

Fractures and Joints

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A rectilinear fracture pattern, possibly useful for hydraulic fracturing, seen in the Kimmeridge Blackstone or oil shale in the cliffs between Cuddle and Clavell's Hard, east of  Kimmeridge Bay, Dorset

As can be seen in the photograph above, the Kimmeridge oil shale or Blackstone has a particular rectilinear fracture pattern. Filling the thin fractures is a pure white calcite, as a thin layer. This is only seen in the richly bituminous shales. It is most conspicuous in the Blackstone and it is also in the Bubbicum, a slightly lower and thinner oil shale. It is not conspicuously seen elsewhere, although no thorough search has been made. It does not seem that so far any petrographic or detailed study of the white veining has taken place.

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A very simplified model of hydraulic fracturing or fracking of Kimmeridge Clay oil shales for shale gas

A very simplified diagram above shows the basis for exploration for unconventional gas in the Kimmeridge Clay. Some notes follow here on the visible fractures that can be seen in the cliff section at Kimmeridge. However, this cliff exposure is above the water table and the strata are not under pressure. Thus it is does not give a true indication of conditions at depth; it is just concerned with an opportunity to look at fractures and joints in the field, for educational purposes. The subject is not considered seriously here.

Desiccation or drying out fractures above the water table in the Kimmeridge oil shale and adjacent strata, east of Clavell's Hard,  Kimmeridge, Dorset, June 2013

One early aspect of hydraulic fracturing for unconventional gas is to study the natural fracture system. Natural fractures in the rock can be used to improve fracture stimulation ( (Khair, 2013) and new fractures may also initiate and propagate when stimulated. To improve the effectiveness of fracture stimulation and ultimately the efficiency of gas collection, it is vital to have as much information as possible about the natural fracture network within the resource and the magnitude and orientation of the stresses that it experiences.

Detailed information on the natural fracture system of the Kimmeridge Clay is not yet easily available but, no doubt, will be sooner or later. In the Cooper Basin of South Australia, studied by Khair et al. 2013), fracture density was found to be highest close to faults and on top of tight antiforms. In the local Kimmeridge Clay of Dorset there are no tight antiforms visible in cliff sections. However, increase in fractures in proximity to faults is probably common in the Kimmeridge Clay of Dorset.

Abul Khair's research also suggests that there is a good correlation between high fracture density and high gamma ray values - an indicator of the shale volume. The correlation between high fracture density and shale content is somewhat counterintuitive as shale is expected to have a higher tensile and compressive strength (less prone to fracturing) than sandstone.
Normally shales are the hardest to fracture but in the Cooper Basin the shales are stiff and so they are easy to frack or fracture. The horizon with the highest fracture density would be the first target for exploration as it would respond well to hydraulic stimulation. Open fractures are also good corridors for gas in the shale.

[With regard to the Kimmeridge Clay of southern England, this subject matter is only in early stage of preparation and more notes on this subject will be added progressively]

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Hydraulic Fracturing of Kimmeridge Clay

Problems of Fault Leakage

The complication of extensional faults cutting the Kimmeridge Blackstone or oil shale and implications for oil shale fracking, Kimmeridge, Dorset

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Faulting at Clavell's Hard has caused additional fracturing to the Kimmeridge Blackstone or oil shale, east of Kimmeridge Bay, Dorset, June 2013.

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COMPARISON:

Kimmeridgian Shale, the Gas-Fracking, Haynesville Formation of the USA

Kimmeridgian to Early Tithonian (similar to the Dorset Kimmeridge Clay)

Comparison of Kimmeridgian Haynesville Shale of the USA, a major gas-fracking unit, and the Kimmeridge Clay of Dorset

The Kimmeridgian Haynesville Shale of the southern USA, with unconventional gas resources, search for this webpage

The Kimmeridgian is notable in the USA as an extremely important unit for shale-gas fracking. The Haynesville Shale is an extremely important Kimmeridgian unit of the southern States of America. In many respects it is like the Kimmeridge Clay of Dorset; it has shales with crushed ammonites, dolostone beds, coccolith beds etc.
According to Hammes et al. (2011), the Upper Jurassic Haynesville Shale, Kimmeridgian to early Tithonian, is one of the most prolific emerging shale-gas plays in the continental United States. It has estimated play resources of several hundred trillion cubic feet and per-well reserves estimated as much as 7.5 bcf. The reservoir spans more than 16 counties along the boundary of eastern Texas and western Louisiana.

The Haynesville Shale is an organic- and carbonate-rich mudrock that was deposited in a deep partly euxinic and anoxic basin during the Kimmeridgian to the early Tithonian, related to a second-order transgression that deposited organic-rich black shales worldwide, as at Kimmeridge, Dorset.

See also\: Bresch and Carpenter (2009). For more general information here is a small extract from Wikipedia:

"Haynesville shale is an important shale-gas resource play in East Texas and Louisiana. Estimated recoverable reserves are as much as 60 Tcf, with each well producing on the average of 6.5 Bcf. The Haynesville Shale came into prominence in 2008 as a potentially major shale gas resource. Producing natural gas from the Haynesville Shale involves drilling wells from 10,000 feet (3,000 m) and to 13,000 feet (4,000 m) deep. The formation is deeper in areas nearer the Gulf of Mexico. The Haynesville Shale has recently been estimated to be the largest natural gas field in the contiguous 48 states with an estimated 250 trillion cubic feet (7.1×1012 m3) of recoverable gas. Production has boomed since late March 2008, creating a number of new millionaires in the Shreveport, Louisiana region."

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See particularly:
Hammes et al. (2011). Geologic Analysis of the Upper Jurassic Haynesville Shale.. Because of the imporance of these strata in relation to those of Dorset, the abstract is given below. It a convenient summary regarding the Haynesville Shale, a formation which is not generally a well-known subject in Britain.

Abstract: "The Upper Jurassic Haynesville Shale is currently regarded as one of the most prolific emerging shale-gas plays in the continental United States. It has estimated play resources of several hundred trillion cubic feet and per-well reserves estimated as much as 7.5 bcf. The reservoir spans more than 16 counties along the boundary of eastern Texas and western Louisiana. Although this basin has a long history of exploration and analysis of its Mesozoic section, a comprehensive subsurface study characterizing the Haynesville Shale has not been conducted. This article is the first to address the structural setting, stratigraphy, depositional environment and facies, fracturing, and production challenges of the Haynesville shale-gas play.
Basement structures and salt movement influenced carbonate and siliciclastic sedimentation associated with the opening of the Gulf of Mexico. The Haynesville Shale is an organic- and carbonate-rich mudrock that was deposited in a deep partly euxinic and anoxic basin during the Kimmeridgian to the early Tithonian, related to a second-order transgression that deposited organic-rich black shales worldwide [as at Kimmeridge, Dorset, England]. The Haynesville Basin was surrounded by carbonate shelves of the Smackover and Haynesville lime Louark sequence in the north and west. Several rivers supplied sand and mud from the northwest, north, and northeast into the basin. Haynesville mudrocks contain a spectrum of facies ranging from bioturbated calcareous mudstone, laminated calcareous mudstone, and silty peloidal siliceous mudstone, to unlaminated siliceous organic-rich mudstone. Framboidal to colloidal pyrite is variably present in the form of concretions, laminae, and individual framboids and replaces calcite cement and mollusk shells. Haynesville reservoirs are characterized by overpressuring, porosity averaging 8 to 12%, Sw of 20 to 30%, nanodarcy permeabilities, reservoir thickness of 200 to 300 ft (70 to100 m), and initial production of as much as 30 mmcf/day. Reservoir depth ranges from 9000 to 14,000 ft (3000 to 4700 m), and lateral drilling distances are 3000 to 5000 ft (1000 to 1700 m). Typical Haynesville wells exhibit a steeper decline curve (sim80% in the first year) than other shale-gas plays, which is attributed to a very high overpressure."

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BIBLIOGRAPHY AND REFERENCES - Kimmeridge

Please see separate Bibliography and References

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For information and photographs of the Blackstone west of Kimmeridge Bay at Brandy Bay, please go to:

Oil Shale or Blackstone at Brandy Bay - West of Kimmeridge Bay

ACKNOWLEDGEMENTS

There is much reliance in this webpage on the detailed work on the Kimmeridge Clay of Dorset by Cox and Gallois (1981). Dr John Bellamy who wrote a thesis and a paper on the dolomite beds has provided much valuable information. This excellent thesis is particularly good with regard to the Rope Lake Head area. I am much obliged to Dr Geof. Townson for pointing out the Rhizocorallium trace fossils in the Rope Lake Head Dolomite Bed. The late Keith Abineri kindly contributed photomicrographs of details of the Freshwater Steps Stone Band. I am very grateful to the Channel Coastal Observatory for permission to use their impressive aerial photographs of the Kimmeridge coast. Denise Noel has kindly discussed the White Stone Band in the field and afterwards and I much appreciate this. Pari White and Michael Bauer have accompanied me on field work and helpfully provided photographs. I much appreciate the opportunity to reproduce a low-resolution version of one the coastal photographs taken from the sea by Richard Edmonds. I thank John Moylan and the television camera photographer of the BBC for acompanying me in on the cliffs in June 2013. I thank the expert and well-known fossil collector and palaeontologist of the Kimmeridge cliff, Steve Etches for help on several occasions. Alan Holiday has been particularly helpful in providing some new photographs of the oil shale and of the Rope Lake Head Stone Band. I much appreciate the advice and help of my daughter, Tonya Loades of Bartley West, Chartered Surveyors.
I am very much obliged to the Dean and Staff of the Faculty of Natural and Environmental Sciences of Southampton University for kindly supporting this website. iSolutions of Southampton University are thanked for hosting the website.

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COPYRIGHT REGULATIONS REGARDING THIS WEBPAGE AND THIS WEBSITE.

Copyright © 2014 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a private, academic website intended to be useful for research, reference and educational purposes. Images and text may not be copied for publication or for use on other webpages such as MOOCs or for any commercial activity. A reasonable number of images and some text may be used for non-commercial, non-charged, non-online and non- published academic purposes, including field trip handouts, student projects, dissertations etc, providing the source is acknowledged. All images so used must contain the original caption, including the copyright statement. Some images are not those of the author and the copyright is that of the original photographer and these are not for any use without specific permission from the source photographer. This particularly applies to aerial photographs, but also to some sets of field photographs.

Disclaimer: Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea or other causes. Not all places need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancell part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.

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Dr Ian West, author of these webpages

Webpage - written and produced by:


Ian West, M.Sc. Ph.D. F.G.S.

.

at his private address, Romsey, Hampshire. It is kindly supported by Southampton University, and web-hosted by courtesy of iSolutions of Southampton University. The website is an unfunded, private activity, and does not represent the views of Southampton University. Any field activities shown are not necessarily those of any specific organisation and mostly represent private field work. [2013 version] .