--> Carboniferous-Permian Carbonate Platform Development, Loei-Phetchabun Foldbelt, NE Thailand

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Carboniferous-Permian Carbonate Platform Development, Loei-Phetchabun Foldbelt, NE Thailand

Abstract

Carboniferous (Pennsylvanian)–Middle Permian carbonate platforms (Saraburi Group) are major petroleum source rocks and reservoirs in the Khorat region of northeastern Thailand. A variety of reservoir styles are recognized in the Pha Nok Khao Formation with production linked to porosity and permeability formed through dolomitisation, dissolution, and/or fracturing associated with Triassic and younger collisional tectonics. Predicting the distribution of porous and permeable zones has proven difficult. Analogues of the Pha Nok Khao carbonate reservoirs crop out in the Loei-Phetchabun Foldbelt (LPF) on the western side of the Khorat Plateau and, although deformed, provide an opportunity to reconstruct morphology and 3D stratal architecture of the platforms. Previous sedimentological work on these platforms in outcrop, and core in various fields, has highlighted the dominance of platform interior (subtidal to intertidal) facies and platform margin (shoal) facies, with minor boundstone facies typically associated with the platform interior (e.g. phylloid algal mounds). In the Loei Syncline, we recognise several phases of platform evolution. Siliciclastic sediments deposited during early platform history were derived from erosion of Mississippian rocks, probably local granites, based on their detrital zircon ages. Turbiditic sandstones that overlie the drowned platform contain zircons derived from Proterozoic, Late Paleozoic and Mississippian parent rocks. Diverse platform-margin facies, including boundstones, are represented all through the platform history. Microbial components in these boundstones are more widespread and important in reefal construction than previously recognized. Microbialite, sponge-microbialite and large phylloid algal-microbial mounds (at least 10 m high) occupied platform margin positions (off-platform/outer ramp). Skeletal–intraclastic facies with diverse encrusting organisms, or displaying clotted or stromatolitic fabrics, suggest local binding and stabilization of sandy shoals. Boundstone/reefal facies are important reservoirs in Permian basins worldwide and recognition of more widespread boundstone facies in the LPF has important implications for the distribution and quality of reservoir facies in the region. The distribution of bound facies and ‘clean’ skeletal facies suggests that higher energy oceanographic conditions prevailed along the western and northwestern–northern margins of the platform (modern-day orientation).