Reservoir Implications of a Spill-Fill Sequence of Reservoir Charge Coupled with Viscosity and Asphaltene Gradients from a Combination of Water Washing and Biodegradation

Abstract

Abstract Reservoirs with multiple processes that impact the quality and distribution of crude oils can be complex. Here, a series of seven reservoirs in the North Sea contain a viscosity profile that is strongly affected by a spill-fill sequence of reservoir charging, biodegradation, water washing, and variation in thermal maturity. Mapping the viscosity gradient and its origins is critical to understand how each factor contributes to the complexity of these reservoirs. Additionally, evaluation of reservoir connectivity is key. Combined studies of downhole fluid analysis (DFA) and high-resolution compositional analysis by comprehensive two-dimensional gas chromatography (GC×GC) can be used to unravel the contributions of various effects and provide an improved understanding of the reservoir. The seven reservoirs examined are consistent with a simple description of the multiple processes that contribute to the viscosity gradient and connectivity analysis. The maturity of charge in this spill-fill sequence has continued to increase with time; consequently, crude oil in deepest reservoir is most mature and the shallowest, the least mature. The oil spills from the oil-water contact (in these injectite reservoirs); consequently, the deepest reservoir is the least biodegraded and the shallowest reservoir the most biodegraded. Observations here are consistent with both biodegradation and water washing being dominated by in-reservoir confinement of the oil as opposed to during migration. The deepest reservoir exhibits mild biodegradation and mild water washing, while the nearby shallowest reservoir exhibits severe biodegradation and severe water washing. Finally, in this spill-fill process, the replacement of new oil for old, especially in the deepest reservoirs, indicates excellent reservoir connectivity.