The Effect of Differing Hydrogeological Regimes on Sandstone Diagenesis: Brent Group Oilfields, U.K. North Sea

Osborne, Mark James (1993) The Effect of Differing Hydrogeological Regimes on Sandstone Diagenesis: Brent Group Oilfields, U.K. North Sea. PhD thesis, University of Glasgow.

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Deltaic sandstones of the Middle Jurassic Brent Group, northern North Sea, have a complex diagenetic history. The paragenetic sequence can be simplified to; siderite - feldspar dissolution - vermiform kaolinite - calcite - feldspar dissolution - blocky kaolinite - quartz - ankerite - illite. Petrographic and stable isotopic studies indicate that Fe-rich siderite precipitated from meteoric water which flushed through the Brent Group during the northward progradation of the delta system. Later Ca and Mg rich siderites precipitated from sea water during the subsequent marine transgression which eventually drowned the delta. Later diagenesis is related to depth of burial and hence depends upon the subsidence history of individual oilfields. In shallow buried oilfields (<1.0km) vermiform kaolinite precipitated, at temperatures of 25-47°C, while in more deeply buried oilfields blocky kaolinite precipitated (1.0-2.0km; 50-80°C). Kaolinite mainly precipitated during the late Cretaceous to early Eocene, following the dissolution of feldspar. Oxygen and hydrogen isotope studies indicate that both kaolinite types precipitated from pore-waters with delta18O= -6.5 to -3.5‰. These fluids are interpreted to be mixtures of meteoric water and sea water. Influx of meteoric water is inferred to have occured during the Palaeocene. Meteoric waters probably entered the Brent Group via the East Shetland Platform landmass in the west, fluid flow being driven by a hydrostatic head above sea level. These surface-derived fluids displaced and mixed with marine waters which were being expelled from compacting sediments. Oil migrating from the deeply buried source rocks into the shallower reservoirs became biodegraded as it interacted with the meteoric water flowing into the basin. In very deeply buried Brent sequences (>3.0km; >80°C), continued subsidence meant that the reservoirs were sealed-off from the influx of surface-derived meteoric fluids. Illite cement, and greater volumes of quartz cement (>5%) then precipitated in a closed geochemical system. The unusually high "palaeotemperatures" of the fluid inclusions in the quartz overgrowths are false and are a product of resetting during continued subsidence, and do not indicate that influx of hot fluids occured. Finally pore-waters evolved to present day isotopic compositions (delta18O= 0 to +2‰) due to water-rock interaction.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Geology, Sedimentary geology, Petroleum geology
Date of Award: 1993
Depositing User: Enlighten Team
Unique ID: glathesis:1993-75835
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Dec 2019 09:15
Last Modified: 19 Dec 2019 09:15

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