Abstract

South Sulawesi is located in the centre of a complex tectonic region where Indo-Australian, Eurasian and Pacific plates interact and collide. The Tacipi Formation was deposited during the middle Miocene to early Pliocene in an intra-arc or forearc setting. Deposition, diagenesis and hydrocarbon potential of these carbonates was strongly influenced by the tectono-stratigraphic setting, faulting, subsidence and possibly eustatic sea-level variations.

Outcrops of the Tacipi Formation cover an area of 1,500 km² and the thickness of the formation varies between 300 - 700 m. Subcrops of these carbonates also occur in the Sengkang Basin and form economic gas reservoirs. Dominant lithologies within the formation include reef related facies (framestones, rudstones and floatstones) and packstones and wackestones. These contain a wide variety of shallow marine bioclasts, such as corals, coralline algae, molluscs and large benthic foraminifera, as well as planktonic foraminifera. Interpretation of seismic data, detailed facies mapping, logging and petrography has allowed differentiation of the carbonate deposits into three different regions. These have a markedly different arrangement of facies, depositional environments and corresponding hydrocarbon potential.

The northern outcrops of the Tacipi Formation (Northern Bone Region) consist of isolated knoll-reefs, mostly displaying a north-south trend, surrounded by deeper-water facies. These buildups are composed of deeper-water middle Miocene facies at the base and shallow upwards into late Miocene reef complexes with locally developed karstification. These outcrops are comparable with knoll reefs forming gas reservoirs in the subsurface. However, differential subsidence resulted in variations in the time of drowning of these reefs. Fine grained clastic sediments, including a high volcaniclastic component, cover the knoll reefs and act as seals. This study has implications for hydrocarbon exploration of other Tertiary SE Asian carbonates.