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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.

South Sumatra is considered a mature exploration area, with over 2500MMbbls of oil and 9.5TCF of gas produced. However a recent large gas discovery in the Kali Berau Dalam-2 well in this basin, highlights that significant new reserve additions can still be made in these areas by the re-evaluation of the regional petroleum systems, both by identification of new plays or extension of plays to unexplored areas. In many mature areas the exploration and concession award history often results in successively more focused exploration programmes in smaller areas. This can lead to an increased emphasis on reservoir and trap delineation without further evaluation of the regional petroleum systems and, in particular, the hydrocarbon charge component. The Tungkal PSC area is a good example of an area that has undergone a long exploration history involving numerous operators with successive focus on block scale petroleum geology at the expense of the more regional controls on hydrocarbon prospectivity. An improved understanding of hydrocarbon accumulation in the Tungkal PSC required both using regional petroleum systems analysis and hydrocarbon charge modelling. While the Tungkal PSC operators had acquired high quality seismic data and drilled a number of wells, these were mainly focused on improving production from the existing field (Mengoepeh). More recent exploration-driven work highlighted the need for a new look at the hydrocarbon charge history but it was clear that little work had been done in the past few year to better understand exploration risk. This paper summarises the methodology employed and the results obtained, from a study, carried out in 2014-15, to better understand hydrocarbon accumulation within the current Tungkal PSC area. It has involved integration of available well and seismic data from the current and historical PSC area with published regional paleogeographic models, regional surface geology and structure maps, together with a regional oil generation model. This approach has allowed a better understanding of the genesis of the discovered hydrocarbons and identification of areas for future exploration interest.

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