Regional diagenetic porosity change in Palaeocene oilfield sandstones, U.K. North Sea
Palaeocene Montrose Group sandstones form a regionally extensive sequence of stacked sandbodies within the Central North Sea. The diagenetic sequence has been characterised as chlorite; micro-dolomite; pyrite; early carbonate concretions; dissolution of feldspars; steady precipitation of kaolinite during burial; quartz overgrowths increasing during deep burial; late calcite concretions; illite. Epigenetic chlorite, and pyrite precipitated within depositional marine porefluids (a180 -0.9%oSMOW). During the late Palaeocene-early Eocene, shortly after deposition of the Montrose Group, a dramatic sea-level fall and eastward delta progradation of the Moray group resulted in the regional meteoric flushing of the Montrose Group sands. This flushing is recorded in the isotopic signatures of early carbonate concretions, which indicate that aquifer waters had light meteoric a 180 values. Many of these concretions precipitated from bacterially-mediated reactions. This included fermentation and shallow anaerobic oxidation of hydrocarbons migrating 3km vertically from the underlying Kimmeridge Clay. Examination of 115 well logs shows that locations of vertical migration were structurally controlled, above faulted graben edges, or above thick shales along graben axes. Strontium ratios indicate that dissolution of detrital calcite supplied the calcium. Kaolinite volumes are usually 2-4%; anomalously high volumes of kaolinite (up to 12%) are found close to deltaic palaeo-shorelines and may represent precipitation during vigorous meteoric flushing of the sandstones. Kaolinite isotopic compositions throughout the Central North Sea indicate that precipitation took place within mixed meteoric-marine pore fluids, whilst surrounding marine shales compacted into a meteoric aquifer, over a temperature range of 30-8S·C. Deuterium values are unusually depleted -53 to -76, and suggest a combination of meteoric water and organic interaction. Quartz cement appears to be generally depth-controlled and forms about 4% at 8,SOOft burial. There is also a possibility of 8% quartz overgrowth adjacent to salt diapirs. Secondary porosity does not vary much with depth, always being 2-4%. This· indicates that any increase in porosity due to dissolution of feldspars has been thwarted by continued compaction and no net increase of porosity has occurred. During precipitation of late calcite concretions, pore-water a 180 was isotopically marine and C supplied by decarboxylation. This indicates that porewaters had become dominated by the introduction of evolved-marine compactional waters from overlying Palaeogene mudrocks. Late carbonate concretions contain up to 10% M nCO 3 and are enriched in radiogenic 87 Sr compared to Palaeocene shell ratios. This trend is similar to that noted in cements from the underlying Chalk. It is possible that strontium-rich fluids may have been transferred vertically into the Palaeocene from the deeper-buried Jurassic sequence. Porosity-depth profiles from conventional core analysis data in 42 wells show porosities of 22-36%, with permeabilities of 40-3,000mD at 5,700-9,200ft. The dominant controls are depositional facies, and compaction. Dewatering structures can reduce vertical permeability by lOx. Authigenic chlorite maintains high porosities, but with permeability reduced by lOx. Vertical gradients of porosity and of permeability with depth exhibit "bow curves", which· decrease at the top and base of each channel sand unit. Shorter core lengths give systematically higher rates of porosity decline, due to insufficient sampling of depositionally thick channels, whereas cores longer than 30m give gradients of 5-13%.km- 1. Porosity varies regionally, but no regional variation of decline-gradient was found.