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Geological Society, London, Special Publications; 1992; v. 61; p. 263-287;
DOI: 10.1144/GSL.SP.1992.061.01.15
© 1992 Geological Society of London

Diagenetic processes in the Brent Group (Middle Jurassic) reservoirs of the North Sea: an overview

Knut Bjørlykke, Tor Nedkvitne, Mogens Ramm & Girish C. Saigal

Department of Geology, University of Oslo, PO Box 1047, Blindern, 0316 Oslo 3, Norway

The Brent Group is extensively cored in the northern Viking Graben and East Shetland Basin in the range between 1.8 and 4.5 km. It therefore offers the opportunity to study diagenetic processes arrested at different burial depths and temperatures. The sandstones of the Brent Group remain poorly indurated and contain only small amounts of quartz cement down to burial depths of about 2.5–3.5 km. The quartz cementation increases with depth reflecting progressive silica cementation sourced by pressure solution. Dissolved feldspar and authigenic kaolinite occur in the shallowest of the Brent Group reservoirs and secondary porosity shows no significant increase with depth. Petrographic analyses show a strong depletion of K-feldspar with depth. This can be explained by albitization of K-feldspar at intermediate burial depths (2.5–3.5 km) and dissolution of K-feldspar during illitization of kaolinite on deeper burial (below 3.7–4.0 km). Extensive quartz cementation and formation of illite are the main causes of an observed rapid permeability reduction below 4 km burial depth. K-Ar dates of authigenic illite indicate that illite formed mostly in Early Tertiary at rather shallow burial depth. Illite formed by alteration of kaolinite and K-feldspar seems, however, to increase very significantly at present burial depths of 3.5–4.0 km, suggesting a more recent formation. Emplacement of oil probably does not stop all diagenetic reactions in the remaining residual pore water, and precipitation of illite and quartz may continue in a closed system.





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