Using Spatial Analysis of Hydrocarbon Compositions to Better Understand the Petroleum Systems of the South Sumatra Basin, Indonesia

Abstract

The South Sumatra Basin has been a focus for hydrocarbon exploration since the earliest oil discoveries in the late 1890s. Despite production of over 2500MMbbls of oil and 9.5TCF of gas our regional understanding of the basin’s petroleum systems is still evolving. Most discoveries occur along a series of Late Neogene NNW-SSE elongated anticlines. The most prolific reservoirs are fluvial – shallow marine sandstones of the Upper Oligocene – Lower Miocene Talang Akar Formation but hydrocarbons have also been discovered in numerous sandstone and carbonate reservoirs ranging in age from Middle – Late Miocene to Eocene. Pre-Tertiary fractured Basement reservoirs are also important gas producers. A geochemical database for produced, tested and seep oils and gases has been compiled from the analytical reports, produced by different service companies over a 40-year period, to understand the spatial distribution of hydrocarbon types and relate this to source type, source maturity and migration patterns. Integration with published palaeoenvironmental reconstructions for the time intervals associated with source rock deposition has enabled a better understanding of migration directions and migration limits. The database of over 100 oils and 40 gases has revealed a wider variation in geochemical character than previously thought, indicating the presence of numerous fluvio-deltaic and lacustrine types suggesting subtle variations in the character of the effective source rocks within the basin, related to both organic matter type and depositional environment. Seven major oil families, often with several sub-groups, have been identified, while the presence of both biogenic and thermogenic gases of varying maturities are also noted. Spatial analysis of these hydrocarbons, integrated with source rock indications, palaeoenvironmental reconstructions and structural maps have allowed definition of kitchen areas and drainage areas for these hydrocarbon accumulations and a better understanding of the charge risk and likely hydrocarbon type in undrilled areas.