ALLUVIAL ARCHITECTURE AND DIFFERENTIAL SUBSIDENCE IN THE STATFJORD FORMATION, NORTH-SEA - PREDICTION OF RESERVOIR POTENTIAL

Statfjord Formation sandstones (Rhaetian-Sinemurian) form important re servoirs in the North Viking Graben, and are still a target for explor ation drilling. Sedimentological analyses from the Tampen Spur and Hor da Platform show that the reservoir potential,iq controlled by the dis tribution, density and stacking pattern of multistorey/multilateral fl uvial channel sandstones within successions of interbedded fluvial and interfluvial deposits. Formation thicknesses vary from about 50 m to more than 500 m within the study area. Empirical relationships linking the sandstone content to succession thickness, show that the proporti on of channel deposits varies systematically with thickness, and indic ate that the most sandstone-prone reservoirs associate with low to mod erately high formation thickness. This suggests that differential subs idence influenced the architecture of the Statfjord Formation. Sandsto ne body stacking patterns derived from correlation panels show a seque ntial ordering of multilateral/multistorey sandstone sheets and interv als with a higher portion of mudrock and more isolated sandstone bodie s. Sequence stratigraphic boundaries are recognized at the bases of mu ltilateral sandstone sheets, and stacking patterns form a basis for de tailed correlations within structural compartments with constant forma tion thickness. However, the number of recognizable sequences increase s with increasing succession thickness, and the correct correlation of sequences across major fault systems pose major problems to stratigra phic studies.