Optimization of crude oil production depends heavily on crude oil composition and its variation within individual reservoirs and across multiple reservoirs. In particular, asphaltene content has an enormous impact on crude oil viscosity and even the economic value of the fluids in the reservoir. Thus, it is highly desirable to understand the primary controls on crude oil composition and asphaltene distributions in reservoirs. Here, a complex oilfield in the North Sea containing six separate reservoirs is addressed. The crude oil is believed to have spilled out of deeper reservoirs into shallower reservoirs during the overall reservoir charging process. Asphaltene content is measured in-situ through downhole fluid analysis and is generally consistent with a spill-fill sequence in reservoir charging. Detailed compositional analysis of crude oil samples by comprehensive two-dimensional gas chromatography (GC×GC) is used to determine the extent and variation among the reservoirs of water washing, biodegradation and thermal maturity. Increased biodegradation and water washing in the shallower reservoirs is consistent with a spillfill sequence. The water washing is evidently assisted by biodegradation. Moreover, analyses of four thermal maturity biomarkers show that shallower reservoirs contain less mature oil, again consistent with a spill-fill sequence. The combination of DFA for bulk compositional analysis and GC×GC for detailed compositional analysis with geochemical interpretation is an effective tool for unraveling complex oilfield scenarios.

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.

The Catcher area fields – Catcher, Varadero and Burgman – were discovered in the Central North Sea between 2010 and 2011. The three fields are found in Block 28/9a. Oil is produced from Eocene sandstones stratigraphically equivalent to the Cromarty and Tay Sandstone members of the Sele and Horda formations, respectively. The reservoir for the Catcher area fields was formed by the large-scale injection of sand from the Eocene Cromarty turbidite system into shallower Sele and Horda Formation mudstones to form the Greater Catcher area injectite complex. The Catcher area development is a floating production, storage and offloading (FPSO) based development, with 18 production and injection wells drilled from two drilling templates per field, tied back to the centrally located BW Offshore Catcher FPSO. A further development well will be drilled in 2020 to complete the base development. A phased approach to development drilling, with focused data acquisition, allowed the well layout and count to be optimized as the fields were being developed. Excellent well results have meant that the well count has been reduced relative to the development plans at sanction while delivering an increase in predicted reserves. Further infill wells and satellite field development drilling is planned for the future.