Abstract: High Resolution Biostratigraphy in the Middle Jurassic Brent Reservoir

Nicholas Butler

The Brent Group sandstones from the most important hydrocarbon reservoir in North West Europe. Greater attention is now given to maximizing recovery from fluvial channel sands in the Ness Formation delta plain facies. The first step in constraining channel geometries and relationships is a detailed chronostratigraphic framework. This has been achieved through high resolution biostratigraphy utilizing organic walled microfossils or palynomorphs. The technique is differentiated from palynofacies analysis which focuses on correlating facies types and lithologies independent of chronostratigrphy. Selective sampling from core (at on average 5 foot intervals), targeting argillaceous sediments, has maximized recovery of palynomorph taxa. Two hundred specimens have been counted from each prep ration contributing to a quantitative data base. Evolutionary datums give an Aalenian to earliest Bajocian age for the Broom, Rannoch, Etive and Ness Formation and a latest Bajocian to early Bathonian age the Tarbert Formation indicating a significant unconformity and most of the Bajocian absent. The short time interval for Ness Formation deposition (less than 4 ammonite zones) precludes an evolutionary based zonation to subdivide the formation. Consequently the datums used are based on quantitative changes in palynomorph abundances; top increases, base increases and acme events of taxa. These datums are considered isochronous since they cross out sedimentary facies boundaries and exhibit high levels of repeatability. Flooding surfaces are indicated by influxes of marine microplankton as ociated with brackish water phases. Some are localised at field scale reflecting a combination of local tectonics and decreased sediment supply. In contrast, the 'mid Ness shale' has a highly distinctive palynofloral signature and is more regional in its distribution.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994