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The reservoir properties of Late Oligocene - Miocene sandstones of the Malay Basin depend on primary depositional facies, mineralogical contents and burial diagenesis. Petrographic studies and classIfication show significant differences in the texture and mineralogical content of sandstones from different seismically defined groups, namely E, I, J and K. The oldest sandstones, group K, are braided stream deposits and contain maiDlymedium­ grained sands. The group J sandstones deposited in a brackish- to shallow­ marine environment, contain well sorted, fme- to medium-grained sands. The group E and I sandstones, which were deposited in estuarine environments, are fine- to very fine-grained and matrix rich. The original mineral content of the sandstones has influenced the trend of post-depositional diagenetic changes. Higher primary porosity is generally present in the mature or clean sandstones. The loss of porosity in these sandstones commonly is due to quartz cementation and precipitation of authigenic clays. Mechanical compaction caused a major loss of initial primary porosity during shallow burial. Pressure dissolution of sandstone grains was mild, as deduced from cathodoluminescence studies, where the area of framework grain core contacts is minimal and interlocking grain boundaries are due to quartz overgrowths. The immature sandstones contain either high percentages of detrital clays or unstable rock fragments or both. Mechanical compaction caused the main diagenetic damages resulting in a major loss ofintergranular porosity through deformation of ductile grains and subsequent formation of pseudo-matrix. Early loss of porosity and permeability retarded or inhibited further diagenesis. Thus, quartz overgrowths and cements are rare in these sandstones. Secondary porosity generated by the dissolution of grains, especially feldspar grains, played an important role in contributing to total porosity. Investigations of grain and pore morphologies using the scanning electron microscope revealed a high percentage of micro-porosity preserved between the clay matrix and newly formed authigenic clays, especially kaolinite. Other diagenetic products are calcite and siderite cements with glauconite, chlorite and smectite being less common.

Petrographic and diagenetic studies of the reservoir sandstone of the Malay Basin