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Journal of Petroleum Geology, Vol.33, No.2, 183-196, 2010
HYDROCARBON-INDUCED DIAGENETIC DOLOMITE AND PYRITE FORMATION ASSOCIATED WITH THE HORMOZ ISLAND SALT DOME, OFFSHORE IRAN
The delta 13C values of black dolomite crystals range from -0.8 to -2.07 parts per thousand VPDB, indicating that little if any of their carbon is derived from hydrocarbon oxidation but that sea water has provided carbon and Mg for dolomite precipitation. The delta 18O values for these dolomites range from -9.2 to -15.3 parts per thousand VPDB, reflecting a temperature effect on isotopic fractionation. By contrast, delta 13C values for white to grey dolomites range from -17.81 to -35.68 parts per thousand VPDB, indicating that the carbon may be derived from the oxidation of hydrocarbons. Based on the delta 18O(dolomite) and temperatures obtained from fluid inclusion studies (215 degrees C), the calculated delta 18O(water) in equilibrium with these dolomites (+2 < delta 18O(fluid) < +12 parts per thousand) indicates the involvement of brines evolved via the interaction of seawater with the Hormoz Salt and associated sedimentary rocks. Some of the dolomite may have precipitated from post-Cambrian seawaters at lower temperatures (ca.100 degrees C). Thus, the dolomites may have begun to form during deep burial but have also formed during salt diapirism at more shallow depths. Pyrite and native sulphur are interpreted to have formed in reducing conditions where the source of sulphur was H(2)S produced by the thermochemical reduction of sulphate in the Hormoz Salt evaporites. Heavy delta 34S values for the anhydrites (ranging from 28.7 to 30.8 parts per thousand) and for sulphides (ranging from 17.2 to 23.4 parts per thousand) preclude a major contribution of sulphur from volcanic sources or from Early Cambrian shales. Pyrites, apatites and dolomites formed at depth within the diapir. It is envisaged that hydrocarbons leaked along the flanks of the Hormoz Island salt dome, resulting in reducing conditions which promoted the formation of diagenetic minerals.