Publications

Lacustrine mudstones are a key source rock in the petroleum systems of Western Indonesian basins but as yet are little studied and consequently poorly understood. Recent geological studies on a 200m thick spectacularly exposed outcrop in the Karbindo Coal Mine of the Kiliran sub-basin, West Sumatra have significantly increased our knowledge and understanding of lacustrine source rocks.The base of the Karbindo Coal Mine section is a 25 metre thick palaeosol which is interpreted to represent continuous subsidence in a low relief fluvial setting. The palaeosol is conformably overlain by a thick (up to 18m) black vitreous coal. This coal is an excellent gas prone source rock (TOC 63.55%, HI 235) and was deposited in a slowly subsiding reed swamp. Upwards the coal contains brown algal rich coal interbeds which pass laterally into freshwater carbonates. These deposits are interpreted to represent ephemeral lakes developed as the rate of subsidence changed prior to the onset of continuous lacustrine conditions. The coal passes conformably upw'ards into a 90m thick Brown Shale interval comprising a heterogeneous facies assemblage of seasonally laminated paper shales, grey shales, red weathering shales, turbidites and gastropod coquinas. The Brown Shale is an excellent algal-rich, oil prone source rock (TOC 2.58% to 8.91%: HI up to 743). Facies variations are interpreted to reflect subtle variations in the hydrological regime rather than reflecting large scale modifications to water depth or lake morphology.The interpretation is of lacustrine source rock deposition and accumulation within a slowly subsiding shallow lake on the flexural margin of a small rift basin. This is markedly different from previous interpretations which have tended to favour deposition in a deep lacustrine basin. The preservation of organic matter in a shallow lacustrine setting as demonstrated here has profound implications for hydrocarbon exploration in syn-rift sections and may open new play fairways in areas where deep syn-rift grabens are not present.