Publications

Sedimentary rocks formed in lacustrine systems are common throughout much of the geologic record. Large accumulations of hydrocarbons sourced from these ancient lake systems occur in the western part of the United States (Uinta Basin, Utah and Colorado), in several basins in China, Indonesia, Africa (Cabinda, Sudan), India, and Brazil, and on a small scale in Australia. These rocks yield high pour point, waxy oils.Most of the oil and gas in the Central Sumatra Basin of Indonesia has been generated from organic-rich shales of lacustrine origin within the Paleogene Pematang Group. During Paleogene time large freshwater lake systems developed within structurally controlled rift basins. Palynology indicates freshwater conditions prevailed. However, occurrences of slightly saline conditions suggest minor climatic changes, chemical stratification, or occasional marine incursions.Tropical climatic conditions with no annual lake turnover favored lacustrine anoxia and deposition of nonmarine hydrocarbon source beds. Models are presented to explain the structural evolution and depositonal history of the Pematang Basins.The paragenetic sequence of diagenetic events and theix effects on reservoir properties is presented.Geochemical analyses have been used in the identification of source beds, interpretation of their depositional environments, oil to source rock correlations, and determining the maturation history of the source rock. Systematic variations in organic matter type reflect depositional environments and largely determine the type of hydrocarbon generated.