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1 School of Geosciences, The Queen’s University of Belfast, Belfast BT7 1NN, UK
2 BP Exploration Operating Company Limited, Uxbridge One, 1 Harefield Road, Uxbridge, Middlesex UB8 1PD, UK
3 BP Exploration Operating Company Limited, Sherwood House, Blackhill Road, Holton Heath Trading Park, Poole, Dorset BH16 6LS, UK
Tertiary sandstones in the Gulf of Thailand, Malay and South China Sea basins have been supplied with sediment rich in pelitic metamorphic rock fragments, weathered basic igneous rock fragments and micaceous rock fragments by similar drainage systems. We have analysed data from the Gulf of Thailand and Malay basins to predict reservoir quality in the South China Sea basin. These Tertiary sandstones contain 20–40% lithic rock fragments that are rich in clay minerals. There are good correlations between both porosity and permeability and the total clay mineral content. They undergo rapid loss of porosity with depth at a rate commensurate with sandstones bearing 20–40% ductile grains. The relationship between porosity and permeability is also strongly influenced by the presence of ductile grains. The rapid loss of permeability with porosity is a result of ductile grains being squeezed between the rigid grains, blocking pore-throats and leaving isolated, ineffective porosity. Mineral cements seem to be unimportant modifiers of porosity or permeability. The rapid loss of porosity with depth and the decrease in permeability with porosity leads to low permeabilities at shallow depths relative to many other hydrocarbon provinces (e.g. North Sea, Gulf of Mexico, etc.) which have lower ductile grain contents. We have used modelled compaction curves for sandstones with 20 and 40% ductile grains to predict a porosity value for any given depth. We have also used sandstones of 100 and 300 µm grain size to predict permeability for a given porosity. The combination of these predictions can be used to predict permeability as a function of depth of burial. The choice of ductile content and grain size will depend upon the conditions of deposition. The main positive reservoir quality anomaly may result from overpressure generation before or during the final phase of burial.
