Abstract

Many carbonate successions in SE Asia are excellently placed with respect to source and seal lithologies and often from hydrocarbon reservoirs. This paper outlines the depositional history of the Eocene to middle Miocene Tonasa Limestone Formation of South Sulawesi and evaluates the implications for hydrocarbon exploration in the region.

The Tonasa Limestone Formation was deposited initially as part of a transgressive sequence, in the midst of an exceedingly complex tectonic area. Detailed facies mapping and logging reveal that by late Eocene times shallow-water carbonates were being deposited over much of South Sulawesi forming a widespread (100 km length) platform area, known as the Tonasa Carbonate Platform. Although shallow-water sedimentation was continuous until the middle Miocene on some areas of the platform, active normal faulting resulted in basinal graben formation and subaerial exposure in other areas.

Platform top lithologies are dominated by large benthic forminifera and consist of wackestones, packstones and grainstones. Platform top facies belts trend E-W and were relatively static through time, indicating subsidence and aggradation of the platform top. Tertiary exposure of shallow-water facies occurred only in areas close to those affected by block faulting. In fault bounded graben areas, basinal marls are interbedded with thick sequences of coarse, immature redeposited carbonate facies, which contain clasts from the underlying formations. These redeposited facies are inferred to have been derived from the footwall areas of syn-depositional faults.

The lack of abundant aragonitic bioclasts, together with only localized subaerial exposure result in little porosity and permeability development in platform top lithologies. In comparison, redeposited facies derived from block faulted areas are both porous and permeable, and comprise the lithologies most likely to form hydrocarbon reservoirs within this formation. This study has implications for hydrocarbon exploration in other Tertiary carbonate successions dominated by foraminifera, which occur widely throughout Southeast Asia.