Publications

Understanding the nature, style and geometry of fractured and dolomitised carbonate reservoirs is of interest in oil and gas exploration and development since either or both processes can not only enhance porosity but also boost the permeability. Production from Permian carbonates is unusual across onshore and offshore Thailand, but does occur in the Nang Nuan oil field in offshore Southern Thailand and the Sin Phu Horm, also Nam Pong gas fields in NE Thailand. Dolomitization and karstification (hydrothermal) in both regions were part of the reservoir diagenetic evolution in both producing area. In order to better understand the influence that the dolomitisation process can exert on rock properties in fractured Ratburi Limestone, a dolomite outcrop was studied in detail in an active quarry near Krabi in Southern Peninsular Thailand, in the Sibumasu tectonic domain. Detail fieldwork revealed three distinct dolomite textures, dolomite-e (euhedral) and dolomite-s (subhedral) and dolomite-nps (non planar sutured), all of which formed during burial in response to ongoing rock-fluid interaction in a milieu of evolving mesogenetic fluids. Diagenetic alteration in the original platform carbonates began with physical compaction, followed by fracturing and faulting that was oriented in a N-S direction and allowed entry of the dolomitising fluids. It was overprinted by younger E-W fractures. The whole fracture system has now been uplifted and overprinted by meteoric dissolution and karstic alteration features, including calcite speleothems and infiltrated clays. These zones of meteoricspeleothem overprint in the dolomite are tied to matrix porosity, from 2% to 5% within limestone * FBRRG, University of Chulalongkorn, Thailand-Bangkok ** University of Monash -Melbourne, Australia NNE and NNW fracture/fault sets that crosscut the dolomite. Dolomitisation in this region was driven by ongoing fault-focused hydrothermal fluid flushing. The process had continued in the mesogenetic realm to where this intensely deformed Permian carbonate terrane is totally overprinted by dolomite. All the former intercrystalline porosity, which once existed in the earlier stages of dolomitisation, is now gone, the rock is clearly over dolomitised. In this situation dolomitisation has degraded, not enhanced poroperm quality of the matrix. It means that either: 1) Dolomitization and hydrothermal emplacement are not a significant factor in porosity development in the Permian reservoirs in the surrounding region. Or, 2) That there is a different set of fluid processes associated with dolomitisation in less intensely altered carbonates where open intercrystalline porosity enhances, not destroys fracture and matrix porosity. The type of pervasive porosity-occluding dolomitization and hydrothermal brecciation that occurs in the study area gives a strong contrast to the open intercrystalline porosity that typifies localized burial dolomite and hydrothermally karstified reservoir occurrences in faults, fractures and joints in Oligo-Miocene carbonates elsewhere to the east in SE Asia and in the Nang Nuan field to the north, where the productive interval is hosted in fractured Permian carbonates. Keywords: Carbonate, Permian, dolomite, fractures, hydrothermal, stable isotope