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Unocal Science & Technology, 376 South Valencia Avenue, Brea, California 92621, USA
Feldspar dissolution is an important process affecting the reservoir quality of Brent Group sandstones in the northern North Sea and shows a strong relationship to reservoir temperature and pore water chemistry. In low temperature, brackish, shallow to moderately buried reservoirs (2400–3700 m, reservoir temperature less than 100–120°C) feldspar is generally a major detrital component whose abundance is affected by local variations in facies, extent of early meteoric leaching, and kaolinitization. In deeply buried Brent reservoirs (3700–4700 m) where present burial temperatures exceed 130–140°C detrital K-feldspar is absent and sandstones are characterized by major illitization of early formed kaolinite, precipitation of fibrous illite, extensive quartz cementation, and variable albitization. K-feldspar dissolution apparently supplied elements necessary for illitization, which, according to K-Ar evidence, occurred during early Tertiary burial, associated with initial stages of hydrocarbon accumulation and brine migration. Illite K-Ar ages are unaffected by feldspar contamination in deep reservoirs and the narrow range of measured ages for widely ranging illite size-separates indicates that the major dissolution/illitization reaction was a geologically rapid process. In contrast, illite K-Ar ages from shallow and intermediate-depth reservoirs range from Palaeocene to Early Miocene and often have considerable size dependency due to small amounts of feldspar contamination in coarser fractions. Variable contamination and its effect on measured K-Ar ages often precludes establishing the history of illitization and a well-defined relationship to hydrocarbon accumulation in intermediate-depth reservoirs. Although some feldspar is dissolved during early burial history by low temperature circulation of meteoric water, only deeply buried Brent reservoirs are characterized by the complete diagenetic removal of feldspar; however, the secondary porosity generated by late dissolution is of little benefit to reservoir quality because accompanying cementation results in major loss of permeability.
