Abstract: Palynostratigraphic Calibration of the Brent Group in the East Shetland Basin, North Sea

Nicholas Butler

The Middle Jurassic Brent Group forms the most important reservoir in Northwest Europe. To facilitate exploration and development strategies, a well defined and accurate stratigraphy focusing on the relationships between the major formational boundaries is essential. This has been achieved through a major regional study utilising high-resolution palynostratigraphy. Current literature is in general agreement that the Broom Formation Is Aalenian, the Rannoch, Etive and Ness Formations are Bajocian, and the Tarbert Formation is early Bathonian in age. However, closer examination has revealed insubstantial evidence for these interpretations. This paper outlines an alternative interpretation using palynological datums calibrated with macrofossil dated onshore sections in the UK and Greenla d. The Broom, Rannoch, Etive and Ness Formations are interpreted as Aalenian to the earliest Bajocian H.discites Ammonite Zone age. This is based on the first downhole appearance of the microplankton species Nannoceratopsis triceras in saline facies in the uppermost Ness Formation and substantiated by common occurrences of Nannoceratopsis gracilis, N.senex and Chasmatosporites spp. The Tarbert Formation is latest Bajocian to early Bathonian in age indicated by abundant Dissiliodinium willei, Sentusidinium verrucosum and locally abundant Phallocysta thomasi and indicates a major stratigraphic break between Ness and Tarbert Formations with most of the Bajocian truncated. The degree of top Ness Formation truncation can be documented across the study area using repeatable and facies independ nt palynological abundance events. This analysis has formed the basis for a major sequence stratigraphic model across the entire Brent Province and more focused sand body studies at the field scale.

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