High-Resolution Biostratigraphy Applied to the Assessment of Source Rock Potential in the English Kimmeridge Clay

GALLOIS, R. W., British Geological Survey, South Devon, United Kingdom

The Kimmeridge Clay is the richest source rock in the United Kingdom stratigraphical sequence, and is the source of much of the oil in the North Sea. It has an extensive outcrop and subcrop in the adjacent land area, but there are few onshore areas where it has been sufficiently deeply buried to have generated significant quantities of oil. Most of the organic content in the onshore sequence is concentrated in a large number of relatively thin (<20 cm thick) seams of oil shale, the richer of which have organic contents >40%. Because the formation contains a rich marine fauna, is structurally undeformed, and has suffered little diagenetic alteration, it is eminently suitable for high-resolution biostratigraphical studies.

By using an iterative process involving biostratigraphy, lithostratigraphy, and wireline logs, it is possible to divide the sequence (50-550 m thick) into about 50 stratigraphical units which can be recognized with confidence throughout the outcrop and subcrop areas. Continuous cores through the full thickness of the formation have been broadly zoned on the basis of ammonites to enable the larger scale lithological variations and the major sedimentary events to be correlated. Detailed analyses of the abundances of the ammonites and the non-ammonite faunas, notably long-ranging bivalves, crinoids, and brachiopods and rare flood occurrences of coccoliths, have enabled the minor sedimentary events to be correlated. Comparison of the sedimentary sequences with the wireline logs, notably g mma-ray logs, has then enabled individual widespread marker bands (some only 1 cm thick) to be recognized throughout the outcrop area. This, in turn, has led to refinements in the biostratigraphical sequence which have enabled individual oil shale seams to be correlated.

The ratio of organic-rich to calcium-carbonate-rich muds in the Kimmeridge Clay, which markedly affects its source-rock potential, is largely controlled by syndepositional faults which separate potentially productive basins from thinner sequences on the intervening structural highs. The use of high-resolution biostratigraphy has been an essential tool in unravelling the patterns of sedimentation and in predicting the areas of high potential oil yield.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)