Evolution of Heavy Mineral Suites in the Statfjord and Nansen Formations of the Brent Field, North Sea: An Alternative Approach for Hydrocarbon Reservoir Subdivision and Correlation

Andrew C. Morton, Cato Berge

Heavy mineral data provide an independent basis for subdivision and correlation of the Late Triassic-Early Jurassic Statfjord reservoir sequence in the Brent Field, UK North Sea. Because this sequence dominantly comprises continental fluvial sandstones and floodplain shales, it lacks a well-defined biostratigraphically-based correlation framework. Variations in a number of key heavy mineral parameters have established a major three-fold subdivision into heavy mineral zones A, B and C. These have been further divided into 6, 4 and 2 subzones, respectively. The zones and subzones can be confidently correlated in cored sections across the Brent Field. The correlations made using the heavy mineral scheme generally parallel the boundaries between reservoir units, giving confid nce to the current reservoir zonation and indicating that it has a sound geological basis.

Zones A and B are dominated by sandstones rich in low-grossular, almandine-pyrope garnets, but there are low-garnet intervals in the uppermost part of each zone. These are associated with major mudstone units that act, in part, as pressure barriers, and coincide with changes in the overall facies distribution. They may therefore represent important sequence boundaries. The base of Zone C marks a major change in sediment source, and is closely followed by the marine transgression which terminated Statfjord Formation deposition. This suggests that there may be a tectonic link between termination of sand supply and the transgressive event.

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