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Late Authigenic Pyrite - An Indicator of Oil Migration and Entrapment in the Bonaparte Basin, Timor Sea, Australia

Abstract

Late authigenic pyrite cementation is common to abundant in the Middle Jurassic Laminaria and Plover Formation sandstone in numerous wells on the Laminaria High and in the Vulcan Sub-basin of the northern Bonaparte Basin. Pyrite cementation developed in these reservoirs by reduction of formation water sulphate in the presence of migrating and/or entrapped oil followed by the reaction of iron with the resultant hydrogen sulphide. Sulphate reduction is considered to be most likely by sulphate-reducing microorganisms, which utilise hydrocarbons for their metabolism. Development of pyrite cementation in the sandstone is therefore dependent on the presence of sulphate- and iron-rich formation water. An initial phase of oil entrapment in the Bonaparte Basin, indicated by oil-filled fluid inclusions, was not accompanied by pyrite cementation suggesting that the original formation water was sulphate-poor. However, the large volume of late authigenic pyrite cementation associated with a later phase of oil entrapment, as particularly evident in the Skua oil field in the Vulcan Sub-basin and the Jahal oil field on the Laminaria High, indicates that sulphate- and iron-rich formation water has been input into the reservoir at a later stage. This hydrocarbon-charged fluid is inferred to have migrated vertically via reactivated and dilated faults and fractures into the reservoir from below. This late authigenic pyrite is disseminated, in fractures and in solution fronts and provides a fingerprint of the mechanisms involved in oil-charged and sulphate- and iron-rich fluid migration into and through the reservoirs. Therefore late authigenic pyrite is considered to provide not only evidence of oil entrapment but a new methodology to understand and identify one mechanism of oil charged fluid flow into a structure.