Influence of Relative Sea Level on Facies, Stacking Patterns and Reservoir Heterogeneity: Middle Jurassic Brent Group, UK North Sea
David C. Jennette
The lower Brent Group, above the genetically distinct lowstand deposits of the Broom Formation, is a progradational sequence set made up of the lower-shoreface Rannoch, the upper-shoreface and beach-plain Etive, and the back-barrier Lower Ness Formations. Much of the reservoir heterogeneity in this sandstone-rich succession can be attributed to facies and stacking-pattern variations within higher frequency systems tracts. Highstand systems tracts have strongly offlapping progradational geometries. Sweep efficiency is influenced by: (1) seaward-dipping, mica-rich bed-sets; (2) high-quality ebb tidal-delta sandstone bodies; and (3) the sharp increase in reservoir quality at the sub-horizontal base of the upper shoreface. The increased permeability leads water in the Etive t overrun oil in the Rannoch. Horizontal wells are targeting unswept reserves in the lower-permeability facies.
Relative falls in sea level punctuated highstand progradation and led to the subaerial erosion of the Etive and Rannoch formations. Lowstand deposits are contained within incised valleys and consist of fluvial sandstones, coastal-plain mudstones and coal. Well-performance anomalies and abrupt variations in sandstone thickness are associated with the heterogeneous valley-fills.
Shoreface sedimentation resumed during transgressive systems tracts. High sediment flux enabled the nearshore system to stack aggradationally. Diachronous lithostratigraphy, aggradation of the heterolithic back-barrier Ness Formation, interbedding of Rannoch facies in the lower Etive and increased carbonate cement in the lower shoreface Rannoch mark this systems tract.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California