KEATING, JOHN, Univ. Wales, Aberystwyth, Dyfed, Wales, UK; * JOHN HOWELL, STEPHEN FLINT, and CATHERINE HUNT, Univ. of Liverpool, Liverpool, UK; and BRUCE TOCHER, Univ. Wales, Aberystwyth, Dyfed, Wales, UK

ABSTRACT: The Role of Biostratigraphy in a Sequence Stratigraphic Study of the Middle and Upper Jurassic, U. K., North Sea South Central Graben

Problems presented by the Upper Jurassic reservoir sands of the North Sea Central Graben include a predominance of monotonous, highly bioturbated "shelf sands", complex structuration on a number of scales including localised halokinetic interaction with rapid regional extension, and relatively poor well coverage. Existing lithostratigraphic schemes imply that the sands are regionally connected and isochronous.

High biostratigraphic sample densities and extended, careful preparation yield palynological assemblages despite the high thermal alteration. Comparison with the very high resolution palynostratigraphy generally attainable for the Upper Jurassic provides a key stratigraphic framework for the study and is particularly applicable to the recognition and dating of major flooding surfaces. Multivariate statistics, graphic correlation, and ranking and scaling techniques have been applied in the analysis andintegration of data sets.

Integration of biostratigraphy with high resolution facies analysis has resulted in the recognition of several regionally correlatable flooding surfaces and sequence boundaries which has enabled the development of a sequence stratigraphic model. This model demonstrates the presence of a series of isolated, discrete sandbodies which show a general south and westward onlap related to Graben expansion. An incised valley fill complex has also been identified with important implications for sediment derivation. Studies of sedimentation rates within biostratigraphic zones hasrevealed the intricate effects of localised tectonics.

In addition to prediction of potential exploration targets, recognition of several individual, separated sandbodies of different ages is important in both diagenetic modelling and reservoir simulation and development planning.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.