Dolomitization and H2S generation in the Permian Khuff Formation, Offshore Dubai, U.A.E.
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Dolomite in the Permian Khuff Formation from the T-1 well (Fateh field, offshore Dubai, United Arab Emirates) formed during the Miocene from a dense brine at a depth of approximately 15,500 ft (4,724 m), as indicated by fluid inclusion data. As the majority of the porosity is intercrystalline, most of the porosity formed at the same time. Fluid inclusion data, together with petrography, luminescence, and oxygen and carbon stable isotopes, indicate the diagenetic history was extremely complex with multiple episodes of precipitation of cements (calcite, saddle dolomite, anhydrite, fluorite), but fluid inclusion geothermometry data indicate that the majority of cementation occurred from the Eocene to the Miocene at depths of at least 12,300 ft (3,749 m). The matrix dolomite, anhydrite nodules, and calcite and fluorite cements have higher87Sr/86Sr ratios (0.70780 to 0.71084) than expected for late Permian seawater, which supports a non marine (burial) origin. Anhydrite with higher than expected δ34S (16.5 to 20.2o/oo CDT) for late Permian seawater gives supporting evidence for this interpretation.
Gas from the T-1 well contains 38% H2S. As suggested by the great depth (almost 15,000 ft or 4,572 m) and calculated temperature of the Khuff reservoir, the H2S probably formed by a thermochemical reaction involving the reduction of some of the abundant anhydrite by methane to various products including H2S and calcite. δ13S of calcite cements as light as_-28.5o/oo PDB support this interpretation.
KeywordsCalcite Dolomite Fluid Inclusion Anhydrite 87Sr
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