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Hydrocarbon filling and leakage history of a deep geopressured sandstone, Fulmar Formation, United Kingdom North Sea

¹ School of GeoSciences, Grant Institute, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh, Scotland EH9 3JW, United Kingdom; mwi{at}glg.ed.ac.uk
² School of GeoSciences, Grant Institute, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh, Scotland EH9 3JW, United Kingdom
³ Scottish Universities Environmental Research Center, East Kilbride, G75 0QF, Scotland, United Kingdom

Mark Wilkinson studied sandstone-hosted calcite concretions for his Ph.D. and has subsequently tried to take a more holistic view of the diagenesis of sedimentary basins. His career has been funded by a variety of oil companies, although he is currently involved in geological disposal of some of the CO₂ that he was partly responsible for producing.

Stuart Haszeldine is a professor at Edinburgh University. He received his Ph.D. on clastic and coal sedimentology in 1981 from the University of Strathclyde. After an attempted reeducation in the oil industry, he has collaborated with many companies to research on hydrocarbon reservoir quality, radioactive waste disposal, and coal resources. He currently heads the United Kingdom's largest group investigating carbon dioxide sequestration.

Tony Fallick was educated at the University of Glasgow, Scotland, in physics (B.Sc.) and chemistry (Ph.D.). He did research at McMaster University, Ontario, Canada, and Cambridge, England, before moving in 1980 to the Scottish Universities Research and Reactor Center, where he is currently the director.

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-¹⁸O 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.