Formation of bitumen in the Elgin/Franklin complex, Central Graben, North Sea: Implications for hydrocarbon charging

The Elgin/Franklin complex contains gas condensates in Upper Jurassic reservoirs in the North Sea Central Graben. Upper parts of the reservoirs contain bitumens, which previous studies have suggested were formed by the thermal cracking of oil as the reservoirs experienced temperatures >150°C during rapid Plio-Pleistocene subsidence. Bitumen-stained cores contaminated by oil-based drilling muds have been analysed by hydropyrolysis. Asphaltene-bound aliphatic hydrocarbon fractions were dominated by n-hexadecane and n-octadecane originating from fatty acid additives in the muds. Uncontaminated asphaltene-bound aromatic hydrocarbon fractions however contained a PAH distribution very similar to normal North Sea oils, suggesting that the bitumens may not have been derived from oil cracking.1-D basin models of well 29/5b-6 and a pseudowell east of the Elgin/Franklin complex utilise a thermal history derived from the basin’s rifting and subsidence histories, combined with the conservation of energy currently not contained in the thermal histories. Vitrinite reflectance values predicted by the conventional kinetic models do not match the measured data. Using the pressure-dependent PresRo® model, however, a good match was achieved between observed and measured data. The predicted petroleum generation is combined with published diagenetic cement data from Elgin/Franklin to produce a composite model for petroleum generation, diagenetic-cement and bitumen formation