Alluvial architecture and differential subsidence in the Statfjord Formation, North Sea; prediction of reservoir potential

Statfjord Formation sandstones (Rhaetian-Sinemurian) form important reservoirs in the North Viking Graben, and are still a target for exploration drilling. Sedimentological analyses from the Tampen Spur and Horda Platform show that the reservoir potential is controlled by the distribution, density and stacking pattern of multistorey/multilateral fluvial 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 proportion of channel deposits varies systematically with thickness, and indicate that the most sandstone-prone reservoirs associate with low to moderately high formation thickness. This suggests that differential subsidence influenced the architecture of the Statfjord Formation. Sandstone body stacking patterns derived from correlation panels show a sequential ordering of multilateral/multistorey sandstone sheets and intervals with a higher portion of mudrock and more isolated sandstone bodies. Sequence stratigraphic boundaries are recognized at the bases of multilateral sandstone sheets, and stacking patterns form a basis for detailed correlations within structural compartments with constant formation thickness. However, the number of recognizable sequences increases with increasing succession thickness, and the correct correlation of sequences across major fault systems pose major problems to stratigraphic studies.