ABSTRACT

Samples of calcite-filled fractures from the chalk reservoir of the Skjold oil field were studied by fluorescence microscopy, cathodoluminescence microscopy, fluid inclusion microthermometry, and oxygen and carbon stable isotope and trace element analyses. Two generations of calcite infilling are tentatively related to two major phases of faulting caused by salt movements. Owing to recrystallization of the first generation of infillings, the chemical, isotopic, and fluid inclusion data only apply to the later generation. Results of the fluid inclusion studies suggest a precipitation temperature of the infilling calcite significantly above what it should be, applying today’s geothermal gradient (52°C/km or 2.85°F/100 ft) to the past. This higher temperature indicates that hot water ascending from deeper strata may have flushed the reservoir, apparently during a late Miocene to Pliocene faulting phase when ring faults guided the water to the reservoir. This feature may be typical of most chalk reservoirs in the Danish Central graben where diapirs have penetrated the chalk.

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