Sedimentological evolution of the Statfjord Formation fluvial hydrocarbon reservoirs of the northern North Sea
The sedimentological evolution of the Statfjord Formation of the Viking Graben at the Triassic - Jurassic transition is detailed by integration of studies of mineral assemblages, isotope age data, sedimentological core analysis, palaeosol / mineralogical wireline analysis and regional correlation of subsurface wireline information, combined with theories on basin evolution in continental settings. The aim of this study is to enhance the description of, and correlation within, hydrocarbon reservoirs developed within intra-continental alluvial sediments which have been deposited in areas above the knickpoints of coastline attached incised valleys, where there is a paucity of biostratigraphical information. Sedimentological analysis of core, wireline suites and production data, allow a stratigraphic framework to be erected which can delineate reservoir flow units. In the absence of palynological datums, chronostratigraphic correlation is done by heavy mineral and geochemical analysis, distinguishing between individual flow units allowing a more genetic correlation between related sands to be made. These analytical studies also point the stratigrapher to a more accurate regional geomorphic interpretation of the core by defining units which have the same provenance, thus allowing comparison of differing sedimentological criteria within single fluvial or floodplain units. At a smaller scale, a short outcrop study of the Castisent Formation of the Spanish Pyrenees was done to illustrate the intra-alluvial sheet sand complexity present in subsurface hydrocarbon reservoirs of alluvial origin. Using data mainly from the Brent Field, Statfjord Field and Snorre Field a model for the regional evolution of the Statfjord Formation is developed. Regional correlation of the reservoir units within the Statfjord Formation, using the methods discussed above, demonstrates basic geomorphic principles which are specifically concerned with the regional development of aggradational and erosive alluvial suites in settings inland from coastal areas.