Eogenetic dolomite cementation in lower Permian reservoir sandstones, southern Zagros, Iran

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Abstract

In spite of the increasing importance to hydrocarbon exploration of the Palaeozoic succession in southwest Iran (Zagros Belt), few comprehensive diagenetic studies of this interval have been carried out. This is the first study presented on the diagenetic characteristics of the Early Permian Faraghan Formation in its most representative exposures in the Zagros region. This formation is dominated by cross-stratified sandstones at outcrop, along with stromatolitic dolomite, dolomitized limestones and subordinate siltstone/sandy shales. The formation grades upward into the thick-bedded carbonates of the Late Permian Dalan Formation. Both formations are major hydrocarbon reservoirs in SW Iran and are time equivalents of the Unayzah and Khuff formations, respectively, in Saudi Arabia. The constituent facies of the Faraghan Formation represent sedimentation in a siliciclastic shallow-marine environment with foreshore (including tidal flat and numerous tidal channels), estuary, sabkha, shoreface and offshore sub-environments. In this study, the development of ferroan dolomite cements in the Faraghan Formation is investigated. Geometrically, ferroan dolomite cements occur in laterally extensive, cemented beds in conglomerates and sandstones, stratabound layers and as nodules in sandstones. Their various petrographic characteristics (such as microcrystalline nature, preservation of porosity, formation as the first diagenetic phase on the grains and their high volume) all indicate an eodiagenetic origin. Oxygen isotopic signatures of the cement (−2.10 to −6.42‰) are close to the proposed content for Lower Permian seawater. Development of this cement is attributed to the eodiagenetic environment marked by fresh and seawater mixing. In this setting, sabkha-type dolomitic grains/lithics acted as nucleation sites facilitating dolomite cement precipitation. Copyright © 2009 John Wiley & Sons, Ltd.

This article was published online on 13 May 2009. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected 17 August 2009.

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