Mounds and Boundstone Facies in the Late Carboniferous Early Permian Pha Nok Khao Formation-equivalent of the Loei Syncline, Loei-Phetchabun Foldbelt: Implications for Reservoir Quality

Abstract

Abstract Late Carboniferous-Middle Permian carbonate platforms are major petroleum reservoirs in the Khorat region of northeastern Thailand. Previous sedimentological work on these platforms in outcrop and core 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). Reservoir quality reflects depositional and diagenetic controls on porosity and permeability development, notably dissolution of skeletal components and dolomitisation of platform-margin bioclastic facies, and/or faulting and fracturing associated with post-depositional (Triassic and younger) collisional tectonics. An outcrop analog study of the Pha Nok Khao-equivalent platform in the Loei Syncline (northern part of the Loei-Phetchabun Foldbelt, LPF) shows that diverse platform-margin boundstones were locally developed. Three features of the study are highlighted:sandy shoal facies include diverse encrusting organisms (such as Tubiphytes) suggest local binding and stabilisation;phylloid algal mounds, including large mounds up to 10 m high and 30 m long grew in outer platform to upper slope settings and are associated with off-mound grainstones-rudstones that also contain clotted fabrics and in situ growths of calcimicrobes and/or calcareous algae; andcalcimicrobes are a notable component in boundstones and other features such as abundant peloids in phylloid algal mounds also attest to microbial activity. The importance of boundstone/reefal facies as reservoirs is well known 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 ongoing production in nearby fields and an active exploration program. A potential subsurface analogue may be recorded in the Dong Mun Field. The distribution of bound fabrics/mounds is consistent with the distribution of 'clean' skeletal facies and both indicate that higher energy oceanographic conditions prevailed along the western and northern/northwestern margins of the platform (modern-day orientation). Large mounds with stylolite-associated porosity and adjacent microbialite/grainstone facies represent an alternative exploration target to dolomitised grainstone-packstone facies with vuggy and intercrystalline porosity.