Some Geological Controls on Oil Composition in the U.K. North Sea

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Abstract
Summary: The U.K. North Sea contains a wide variety of reservoired oils, ranging from the biodegraded heavy oils, via mid-gravity crudes to light oils and condensates. Statistical data from a suite of U.K. North Sea oils gives an average API gravity of 36°, an average asphaltene content of 1.4% and an average saturate/aromatic ratio of 2.02. Averages for pristane/phytane, pristane/n-C 17 and phytane/n-C 18 are 1.24, 0.63 and 0.56 respectively. It is possible to explain the ocurrence of the vast majority of the U.K. North Sea crudes in terms of generation over a range of maturities from single source rock horizons (the Kimmeridge Clay Formation hot shales), followed by alteration during migration or within the reservoir. The source rocks are shown to be immature below 175°F (80°C); early mature between 175–225°F (80–107°C); mid-mature between 225–265°F (107–130°C) and late mature above 265°F (130°C). Early mature, full maturity spectrum and late mature oils are identified using maturity-controlled sterane and triterpane ratios established from the source rock studies, and are related to bulk properties such as API gravity and saturate/aromatic ratios. Bacterial degradation in shallow cool reservoirs (less than 6000 ft; below 140°F (60°C)) is responsible for the majority of the heavy oil in the U.K. North Sea, while asphaltene enrichment is another mechanism for producing heavy oils. Though the Kimmeridge Clay Formation hot shales are fairly homogeneous over the U.K. North Sea, molecular ratios such as the pristane/phytane ratio, the abundance of a C 28 hopane and a C 27 moretane can be used to correlate oils with specific facies of the generally homogeneous source rock.