Amy Gough and Robert Hall
The Middle Eocene to Oligocene sediments of the South Sumatra Basin are relatively poorly exposed. Limited outcrops occur around the Tigapuluh Hills and the Barisan Mountains in the South Palembang and Jambi Sub-basins. As a result of this, the formations are mainly described through seismic profiles and borehole data. Data are collected from these limited exposures to help better understand the depositional styles, sediment routing profiles, and the effect these sediments have upon hydrocarbon generation and charge. Additionally, this project uses a well-exposed analogue from the Paradox Basin, western U.S.A. to counteract the lack of clear exposure.
The South Sumatra Basin formed in the Late Cretaceous as a result of east-west extension. Continued tectonic activity throughout the Eocene led to the further subdivision of the basin into four segregated sub-basins. Towards the central and southerly extent of the basin the deposits of the Lahat and Lemat Formations are interpreted as initial basin margin alluvial fans, which passed laterally into contemporaneous fluvial fans, fresh water deltas, and lacustrine environments. At the most southerly limits of the basin there is evidence of sub-marine deltaic environments.
The observed deposits include lacustrine derived organic shales and marginal coals, which have been interpreted as potential sourcerocks within the local hydrocarbon play. A gradational marine transgression led to the subsequent deposition of the late Oligocene to the Early Miocene Talag Aker Formation, with is formed from fluvial, lacustrine, lagoonal, shallow marine, and turbiditic sediments. The Talag Aker Formation is one of the main reservoir lithologies in the basin. In order to better understand the deposits of the Lahat and Lemat Formations a well exposed analogue from the Cutler Group, western U.S.A. has been developed. The Paradox Basin is an intracratonic foreland basin, defined by the limits of evaporitic strata. This strata was highly mobile prior to the deposition of the Cutler Group, leading to the formation of a highly complex halokinetic topography, not dissimilar to the complex topography of the South Sumatra Basin during the Late Eocene. The Cutler Group evolves laterally across the basin from the deposits of undifferentiated basin margin fans, through to contemporaneous lacustrine and aeolian deposits in the basin centre. The overall Cutler Group closely resembles the deposits of the Lahat and Lemat Formations.