R. Stuart Haszeldine

Oil emplacement retarded the rate of quartz cementation in the Brae Formation deep-water sandstone reservoirs of the Miller and Kingfisher fields (United Kingdom North Sea), thus preserving porosity despite the rocks' being buried to depths of 4 km and 120°C. Quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs. Important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time (>15 m.y.). The earlier the hydrocarbon fill, the greater is the degree of porosity preservation. Failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg, potentially leading in turn to poor decisions about the need for and placement of downflank water injectors. During exploration, the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement.

Sandstones of the Palaeocene Montrose Group were deposited in a deepwater fan environment, and form a major oil reservoir in the North Sea. Calcite concretions occur commonly within thick-bedded and structureless sandstones. These concretions have been identified by sonic logs and well reports, and were cross-checked with available core data. Regionally, 101 wells have been examined and carbonate concretions form 0.6–7.2% of the core. Concretions are most abundant along the flank of the Fladen Ground Spur, the north Witch Ground Graben (WGG), the east south Viking Graben and East Central Graben (ECG). Concretions of the ECG formed at deep burial, with C from decarboxylation. Geochemical inheritance of Mn and Sr from Cretaceous chalk clasts may occur. Concretion growth may also have been influenced by vertical expulsion of fluids (leak-off) localized above salt tectonics. Isotopic and petrographic evidence indicates that much carbonate C in the WGG was derived from biodegradation of migrating oil in meteoric water at shallow depth. The locations of abundant carbonate with characteristic negative C isotope signatures can be used as shallow exploration guides to leak-off points located above deep overpressured structures.

The Central Graben of the North Sea is an actively subsiding, highly overpressured, basin. At the present day, hydrocarbons are migrating both vertically and laterally through the stratigraphy to fill reservoirs from Permian through to Eocene. The late Jurassic Fulmar Sandstone approaches rock fracture pressure in many Central Graben areas. Petrographic studies show that sandstones originally contained as much as 35–40% potassium feldspar. This has dissolved rapidly below 3 km, so that by 4.5 km sandstones may contain only 5% feldspar. This loss is not matched by growth of authigenic clays or feldspars; aluminium is interpreted to have been exported from the sandstones by an unknown mechanism. In some cases, excess kaolin may form in overlying Palaeogene sandstones. Improved stratabound secondary porosity (an additional 5–15%) is created along the top of reservoir sandstones and at structurally high leak-off points. Leak-off points maintain high porosities, partly because overpressure reduces the effective compactional stress. This secondary porosity forms zones of good reservoir quality in predictable positions. Oils migrating vertically and laterally into Palaeogene sandstones have been bacterially oxidized, to form calcites with distinctive carbon isotope signatures. The distribution of such cements can be a guide to deeper leak-off positions. Modelling of modern temperatures across the graben shows that spikes of elevated temperature occur above leak-off positions, demonstrating that vertical fluid-flow is still active. The Central Graben, and any actively subsiding deep basin, can be viewed as one interconnected system, where localized deep porosity is created, and shallow porosity is destroyed. The organic, inorganic, temperature and overpressure fluid systems are all interactive.