Development of a Sequence Stratigraphic Framework for an Upper Carboniferous Basin - New Concepts using Spectral Gamma-Ray Response and Palynology

J. Sarah, Duncan McLean, David Oliver

The application of high resolution sequence stratigraphy in the offshore is largely dependent on the interpretation of well-log data tied to seismic data and limited core information with correlation between wells relying on detailed biostratigraphic analysis. The gamma-ray log is the most widely used of the available suite of wireline logs. Although spectral data are now standard, these data commonly remain unused owing to the perceived complexity of interpretation. Exposed successions can be directly compared with the subsurface by generating natural gamma-ray spectrrometry (NGS) logs at outcrop concomitantly with detailed sedimentological logging and palynological sampling.

The Upper Carboniferous delta systems from the Pennine Basin, U.K. are considered to be analogues for the Carboniferous strata in the offshore Southern North Sea basin. A high resolution sequence stratigraphic framework has been developed for the fluvial, slope and turbidite depositional systems of the R1c goniatite biozone (Kinderscoutian), representing a period of approximately 750,000 years. Within this framework, the expressions of base level change can be characterized in both depositional dip and strike and the variability of NGS response in a range of temporally-equivalent depositional settings assessed. Successive incised valley fills, which cannot be differentiated on the total gamma-ray response, are characterized by a set of distinctive NGS characteristics enabling differin mineralogies to be qualitatively interpreted. Individual systems tracts and significant surfaces are classified by combining both the NGS and palynological characteristics. The principles derived from this study are widely applicable, particularly to the understanding of offshore well-sections where core coverage is limited.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995