Diagenesis, burial history, and hydrocarbon potential of Cambrian sandstone in the northern continental margin of Gondwana: A case study of the Lalun Formation of central Iran

Undeformed Cambrian-Ordovician sandstones on the northern margin of Gondwana are in places important hydrocarbon reservoirs. The factors that favoured diagenetic preservation of porosity in such sandstones are reviewed and compared to the Lower Cambrian Lalun Formation of central Iran, in order to assess what aspects of geological history have controlled the cementation and porosity of the sandstones. The Lalun Formation is up to 640 m thick and comprises braided-fluvial sandstones with minor shales that are overlain by marine shales, sandstones and carbonates. In four sections along a 650 km traverse, the sandstones are predominantly quartzarenites, locally arkosic and lithic, and contain minimal detrital matrix. Provenance analysis and paleo-flow suggest derivation from the Arabian Shield. Diagenesis was dominated by quartz overgrowths and abundant infra-grain microfractures healed by silica cement, with modest amounts of poikilotopic and mosaic calcite, authigenic clay, and iron oxide as early grain rims and late-stage fracture fills. Minor phases include dolomite, ankerite, siderite, magnetite and barite. Marine strata contain more carbonate cement, with silica cement prominent in topmost nearshore sandstones. Burial-history analysis suggests relatively rapid burial in a cratonic setting to more than 4 km by the end of the Silurian and a maximum temperature of similar to 150 degrees C during burial diagenesis, sufficient for hydrocarbon generation by the end of the Paleozoic. Dissolution of K-feldspar, ferro-magnesian grains and, locally, calcite cement generated secondary porosity, but porosity and permeability values are low, averaging 1.5% and < 0.1 mD, respectively. Rapid subsidence of thick Cambrian fluvial successions at the northeastern Gondwanan margin may have limited early dissolution and promoted occlusion of primary porosity by quartz overgrowths, limiting permeability pathways for fluids that might have generated secondary porosity later in the basinal history. In contrast with Cambrian and Ordovician sandstone reservoirs across North Africa and the Middle East, the existence of Paleozoic source rocks may be problematic in central Iran.