HOWELL, JOHN, and STEPHEN FLINT, University of Liverpool, Liverpool, England

ABSTRACT: Sequence Stratigraphic Signatures in a Complex Shelf Sand System: Implications For Reservoir Modeling

The Upper Jurassic of the United Kingdom South Central Graben, is a poorly understood shallow marine succession. Problems associated with working in these sequences arise on a number of scales. Regionally, the Fulmar Formation comprises at least six individual sandbodies of varying ages; their distribution is controlled by a combination of late Jurassic rifting and halokenetic structuration. Biostratigraphic dating of marine flooding surfaces has revealed a southward onlap through time related to graben expansion.

On a subregional and field scale, problems in predicting reservoir anatomy arise due to the predominance of thick, apparently aggradational successions, deposited within a poorly defined paleogeographic setting with complex localized salt structuration. These factors, combined with the high degree of bioturbation, the scarcity of siltstone/claystone horizons, the uniformity of the grain size, and rare evidence for subaerial exposure have led to the general belief that conventional facies based on sequence stratigraphic modeling may not be applicable to this succession.

Detailed relogging of l500 m of core combined with ichnofabric analysis have revealed the presence of parasequences and parasequence sets subtly defined by changes in sand/silt ratio, and the presence of shell hashes and early diagenetic fabrics. Stacking patterns within these reservoir elements have led to the recognition of both type l and, most significantly, type 2 sequence boundaries related to localized salt movement.

Field scale, three-dimensional computer modeling of these newly defined correlative surfaces has resulted in a greatly enhanced understanding of reservoir anatomy with direct application for drilling strategy in a developing field.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.