Abstract

Diagenetic minerals in a water-filled borehole from the Jurassic Fulmar Formation, United Kingdom central North Sea, record two phases of hydrocarbon filling and emptying. Initial oil charge was during the Late Cretaceous, at shallow burial depths of about 1.5 km (0.9 mi). As we consider that hydrocarbon has preserved porosity during burial, this has implications for the understanding of the porosity evolution of the Fulmar Formation, which, in other locations, is an important hydrocarbon reservoir. The early oil charge, as recorded by illite K-Ar ages, progressively filled the structure from 84 to 59 Ma, and possibly precipitated bitumen because of biodegradation. The first oil predated many of the burial diagenetic reactions within the sandstone. After leak-off at ca. 60 Ma, diagenetic reactions continued in an open geochemical system, with possible import of CO2. Products of these reactions include ankerite and quartz overgrowth cements. Hydrocarbon staining postdates these phases and provides evidence of a hydrocarbon charge, probably gas condensate. The second hydrocarbon charge also leaked off, and the sandstone is now water bearing. Previous work on the Fulmar Formation has incorrectly placed all the diagenetic reactions as predating the first arrival of hydrocarbon. The present-day pore fluids are high-salinity, high-δ18O fluids derived from the underlying Permian Zechstein evaporates. These fluids entered the reservoir during a phase of overpressure release that caused fracturing of the framework quartz grains, possibly coincident with the second phase of hydrocarbon leak-off at 2–5 Ma.

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