Abstract

The Chalk Group (Cretaceous/Tertiary) in the Machar Field (British North Sea) contains both fracture-filling and microcrystalline calcite cements. Modeling of fluid-rock interaction using data on light stable isotopes obtained by whole rock analyses and laser ablution analyses of calcite cements reveal that the fracture and matrix diagenetic systems were largely decoupled. The calcium and carbonate of the fracture-filling calcite cements were derived largely from the adjacent chalk matrix. The fracture diagenetic system had a high water-rock ratio, which maintained a relatively stable water delta 18 O ratio during calcite dissolution and precipitation. The chalk matrix on the contrary, had a low molar water-rock ratio during recrystallization, which resulted in increases in the pore-water delta 18 O value during recrystallization at elevated temperatures. This evolution of the pore-water delta 18 O value is manifested by highly variable cement delta 18 O values. The present-day formation waters of the Machar Field have 87 Sr/ 86 Sr ratios significantly higher than the whole rock and fracture-filling cement calcite values, evidence that the chemical composition of the formation waters is not representative of that of the pore waters during chalk recrystallization. Little diagenesis is therefore now occurring in the Machar Field. The diagenetic systems of the chalk matrix and fractures both had a high degree of openness with respect to carbon, because of the introduction of organically derived bicarbonate rather than advection of water through the chalk. The hulk of calcite cementation in fractures and the recrystallization and cementation of the chalk matrix occurred at temperatures in the 80-100 degrees C range, at or just below the present-day reservoir temperature of 97 degrees C.

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