Temperatures of oil expulsion from source rocks: evidence from oil inclusions in diagenetic cements in North Sea sandstones

ABSTRACT The temperature of oil expulsion from source rocks is one of the central questions of petroleum geology. Many differing opinions regarding what this temperature or temperature range may be are presented in the literature, and a consensus has not been reached. The present study addresses the topic through detailed petrographic investigation of oil inclusions trapped in carbonate cements, in the dust rims beneath quartz overgrowths, and in albitized plagioclase in the quartzose reservoir sandstones of the Norwegian sector of the North Sea. Previous studies show that the temperature of albitization in the area is approximately 88°C, and that quartz overgrowths start forming at around 80°C. Moreover, pervasive carbonate cement enclosing quartz grains without quartz overgrowths must have formed at temperatures less than the temperature of initial quartz cementation. Regardless of burial depth, most of the studied reservoirs currently filled by oil or wet gas contain oil inclusions in one, two, or all three of the mentioned diagenetic cements where formation temperature is high enough for such cements to be present. This clearly suggests that the temperature of oil filling in North Sea reservoirs is normally less than 80–90°C. Reservoir temperatures of less than 80–90°C were commonly reached in Cretaceous times when the study area’s Upper Jurassic source rocks were not deeply buried, implying temperatures of oil expulsion less than 100°C, possibly considerably less. Also, some of the studied oil-filled sandstones are encased in the Upper Jurassic source rocks, indicating little difference between temperatures of oil expulsion and temperatures of oil filling. The presence of oil inclusions in pre-quartz overgrowth calcite and dolomite cements or in the dust rims beneath quartz overgrowths in almost all these intra-source-rock reservoirs therefore suggests oil expulsion at not more than 80°C. The oil-inclusion data from the North Sea thus point to oil expulsion from source rocks being essentially complete at temperatures of 80–100°C, and the available evidence does not exclude lower expulsion temperatures. This further suggests that oil expulsion has taken place earlier and over a much larger area than normally assumed, and that the need for long-distance oil migration has been exaggerated. It seems reasonable to infer that this will also apply to many other basins.