Print this pageABSTRACT: Diagenesis, Porosity, and Fluid Evolution in the Brent Sandstone: United Kingdom North Sea
R. Stuart Haszeldine, John F. Brint, Anthony E. Fallick, P. Jo Hamilton, Stewart Brown
The Middle Jurassic Brent Sandstone formation is the major reservoir of the northern North Sea, hosting several giant oil fields and containing about 12 × 109 bbl of oil. These sediments were deposited on the margin of a north-south-rifting basin by deltaic and marine shelf sediments prograding from the west and southwest.
The diagenetic sequence from 3500 km2 of the basin can be simplified to early kaolinite and quartz overgrowth with some feldspar dissolution; growth of carbonate concretions; and late growth of kaolinite, quartz, and illite. These cements typically reduce porosity to 10-25% and permeability to 10-1000 md. The major diagenetic minerals have been separated and analyzed isotopically and quartz paleotemperatures measured by fluid inclusion microthermometry. These results show that the sandstones experienced open-system, topographically driven, flow of Jurassic low-salinity rainwater (^dgr18O -7^pmil) during the first 1.1 km of burial. Deeper than this, thick Lower Cretaceous mudstones sealed the hydrologic system, and the isolated pore water changed progressively to ecome richer in ^dgr18O with burial. This fossilized trend is identical to that seen from present-day waters in many world basins. Some feldspar dissolution occurred during this period. Quartz cement, which often forms 10% of the rock, grew from meteoric water at 1.8 km depth and at anomolously warm temperatures (73-131°C) from isotopically heavy fluids (^dgr18O +2^pmil) about 55 Ma, shortly before trap filling. We infer that such fluids must have risen from deeper in the basin, carrying heat and quartz, during three spasms of open-system circulation. However, no silica veins have been found in core. These events were a similar age to the final rupture of the North Atlantic 300 km to the northwest, which may have induced fracturing, breaching the diagenetically closed system.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990