Change in the degree of catagenesis and hydrocarbon generation in the sedimentary rocks of the South Sumatra Basin,Indonesia

Temperature and catagenetic history of the South Sumatra Basin in Indonesia is considered by the example of the sedimentary sequence of the Limau graben during its subsidence from the Oligocene to the present time. GALO system for basin modeling was applied for numerical reconstruction of six sedimentary successions in the area of holes Pandan-81, Petanang-1, Tepus-2, Tepus-1, Gambir-1, and Lembak-8 located along the profile cutting across the Limau graben. Modeling suggests significant cooling of the basemen for the last 15–20 Ma from the high initial heat flow of 105 mW/m², which is typical of axial zones of continental rifting, and significant heating of the basin lithosphere during the last 2–5 Ma. Examination of variations in tectonic subsidence of the basin confirms the possible extension of the lithosphere in the Oligocene-Miocene with an amplitude β increasing from 1.12 on the flanks of the Limau graben (Hole Lembak-8) to 1.32 in the central part of the graben (Tepus-1 and 2), Tectonic analysis indicates also the notable thermal activation of the basin in the Pliocene, Pleistocene, and Holocene. This activation is consistent with the high temperature gradient typical of the present-day sedimentary cover of the basin. Numerical modeling of the evolution of the organic matter maturity and hydrocarbon generation by main source formations of the basin confirms good prospects of the inferred source formations (Lemat, Talang Akar, and Gumai) of the South Sumatra basin for the generation of liquid hydrocarbons (HC) in the Limau Graben. It is also demonstrated that the source rocks of the Lemat Formation are ore-generating rocks in the main part of the Limau graben and are gas-generating rocks only in the deepest portions of the graben. The rocks at the base and roof of the Talang Akar Formation could be considered as highly oil-generating rocks, probably except for the upper horizons of the formation in the shallowest portions of the graben (Hole Lembak-8). Oil generation reached peak in the last 5–10 Ma. Modeling showed that intense oil generation by the Gumai Formation may be significant in the most part of the Limau graben and negligible only in the distant flanks of the graben (Hole Lembak-8).