Hydrocarbon filling history from diagenetic evidence: Brent Group, UK North Sea

Reconstruction of the hydrocarbon filling history of a reservoir is important for prediction of field-scale porosity and permeability, and for the calibration of basin models. Published histories of the Brent Group show only a single phase of hydrocarbon filling, which occurred after the diagenetic reactions had run their course. In contrast, diagenetic minerals preserve evidence of multiple episodes of hydrocarbon charging. In the Cormorant Field, UK North Sea, authigenic blocky kaolin shows a systematic change in oxygen isotopic composition with depth, and the trend displays a conspicuous shift at the Mid-Ness Shale, a regionally extensive permeability barrier. The isotopic data are most readily explained if kaolin recrystallisation (from earlier vermiform kaolin) was synchronous with hydrocarbon charging, with two hydrocarbon pools, one above and one below the Mid-Ness Shale. The filling history begins with a relatively early, slow, filling phase (45–70 °C; 80–50 Ma) that formed an oil column with the oil–water contact substantially below the present-day position. This first hydrocarbon subsequently leaked off, allowing renewed diagenetic activity, including the formation of quartz overgrowths with aqueous inclusions. Emplacement of the present-day hydrocarbon charge was the last event in the history of the reservoir (90–100 °C; 10–0 Ma).