Origin of Beatrice Oil by Co-Sourcing from Devonian and Middle Jurassic Source Rocks, Inner Moray Firth, United Kingdom

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Commercial hydrocarbons in the Moray Firth area of the North Sea occur in reservoirs ranging in age from Devonian to Tertiary, with the bulk of the reserves located in Upper Jurassic and Lower Cretaceous clastic rocks. Most hydrocarbon discoveries in this area have been made in the Outer Moray Firth, primarily in the Witch Ground graben. A single oil field, Beatrice, has been developed in the Inner Moray Firth area. The unusual location and chemical composition of Beatrice, compared to Piper and other North Sea oils generated from the Kimmeridge Clay, have contributed to the longstanding controversy on its origin. Oil source potential in the Inner Moray Firth has been recognized in lacustrine Devonian dolomitic siltstones, Middle Jurassic paralic coals, Upper Jurassic marine shales, and other rocks. However, prior to this study, only the Upper Jurassic Kimmeridge Clay was recognized as a significant effective source rock in the main graben area of the North Sea. Biomarker and stable carbon isotope analyses conclusively show that Beatrice oil could not have been generated from the classic Kimmeridge Clay source rock, but is a mixture of products derived from effective Devonian and Middle Jurassic source rocks. This work demonstrates the power of a multiparameter geochemical approach in solving this difficult oil to source rock correlation problem. Use of this approach can lead to the recognition of new exploration plays in previously well-explored basins.