Heterogeneity of Organic Matter Distribution in Relation to a Transgressive Systems Tract: Kimmeridge Clay (Jurassic), England

HERBIN, JEAN-PAUL, Institut Francais du Petrole, France, JEANNINE GEYSSANT and FREDERIC MELIERES, Universite P. M. Curie, France, CARLA MULLER, Institut Francais du Petrole, France, IAN PENN, British Geological Survey, U.K., and the YORKIM GROUP, Institut Francais du Petrole

The Kimmeridge clay has been drilled in four continuously cored boreholes put down to sample the organic carbon content of the formation. Three of them sited in the Cleveland basin (Yorkshire) prove over 200 m of strata ranging from Mutabilis to Pectinatus zones; the fourth, completing a 35 km transect, proves the lower part of the Kimmeridge clay of the thinner Eastern England shelf.

The results show the total organic carbon content (TOC) increasing by 50% when traced from shelf into the basin where deeper bathymetry and more rapid sedimentation have favored preservation of organic matter.

Stratigraphical variation of % TOC shows a hierarchical cyclic pattern of abundance. Over 200 basic cycles grouped into 23 larger scale cycles may be recognized. These in turn are part of an overall upward increase in % TOC at the top of the Eudoxus Zone and at the bases of the Hudlestoni and Pectinatus zones which both correspond to the periods of higher sea level during Upper Jurassic times.

Assuming the sequence represents some 6.5 Ma (-144 Ma, Cymodoce Zone to -137.5 Ma Pectinatus Zone), then the basic sedimentary cycles represent a periodicity of 30,000 years and the longer-term cycles, 280,000 years. Although these values depend on the absolute time scale chosen, it is felt they do give the correct order of magnitude.

The study of the stratigraphical variation of % TOC enables evaluation of fundamental problems of these cycles in which organic matter is one of the major constituents.

Such cycles exist throughout the Jurassic and Lower-Middle Cretaceous sequences but a transgressive systems tract such as that of the Kimmeridgian Stage enables the phenomenon to be studied in detail.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)