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Abstract

The South Brae reservoir sandstones were cemented by quartz late in their diagenetic history. The volume of quartz cement in the South Brae sandstones ranges from 0 (in early calcite cemented concretions) to approximately 11%, wifh a mean of 3.5%. In four studied wells, quartz overgrowth δ180 decreases with increasing depth and demonstrates a control by the sedimentological reservoir layering of the field. The data indicate that ali overgrowths in any one well cannot have precipitated from a single pore fluid. Overgrowths in shallow reservoir sandstones have precipitated from a higher δ18O fluid (basinal, δ18O + 5‰) than overgrowths in deeper reservoir sandstones (evolved meteoric, δ18O «= – 7 to +5‰). This long-term layering of diagenetic fluids also implies that quartz cernent can be formed from local sources: advective transport by warm fluids is not required. The greatest volume of quartz is inlerpreted to have precipitated at temperatures between 70°C and 110°C (2.3 to 3.7 km). Quartz cementation probably occurred most rapidly during periods of overpressure release. Volumes of quartz cement are not significamly different in shallower and deeper reservoir sandstones.

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