Abstract

Recent geological, geophysical, geochronological, geochemical, and remote sensing studies in the Kalosi PSC, South Sulawesi, have provided new insights into the tectonic and stratigraphic evolution of Sulawesi and the development of a Tertiary petroleum system which is responsible for several oil seeps in the area.

The most prospective hydrocarbon-bearing rocks in South Sulawesi are Cenozoic in age. Basement consists of indurated and metamorphosed Mesozoic sedimentary and ophiolitic rocks. These are unconformably overlain by Paleocene-Eocene volcanics (Van Bemmelens "Older Andesites" and associated rocks) and Eocene fluvial and lacustrine sedimentary rocks which pass conformably upwards through fluvial-deltaic clastics into widespread Upper Eocene to Middle Miocene platform carbonate deposits. The carbonates were buried during the Middle and Late Miocene to Pliocene by thick volcaniclastics and were intruded by Miocene & Pliocene plutonic complexes. These in turn are unconformably overlain by uppermost Miocene to lowermost Pliocene reef carbonates and Pliocene and younger synorogenic clastic deposits.

The rocks record a polyphase deformational history beginning in the Cretaceous with the development of an accretionary complex along the southeastern margin of Sundaland. A major Late Cretaceous to Early Eocene (mostly angular) unconformity separates the Mesozoic sequence from the overlying Tertiary section. Subsidence, possibly associated with extension and normal faulting during the Eocene, was followed by a period of stability and quiescence from the Late Eocene to Early Miocene. Subduction beneath South Sulawesi in the Early to Middle Miocene and the obduction of oceanic crust onto the micro-continent(s) of eastern Sulawesi were followed by collision and westward vergent partial subduction of continental plate(s) derived from the Australian craton during the Middle to Late Miocene. Voluminous lithospheric melting due to lithospheric imbrication and thickening produced a widespread north-south trending bimodal alkalic to calc-alkalic volcano – plutonic belt. Continued convergence through the Pliocene formed a westward-vergent orogen in South Sulawesi, with a thin-skinned thrust system in its western half expanding into a thick skinned, basement involved thrust system in its eastern half. This new tectonic scenario departs significantly from suggestions by previous workers of oceanic subduction-related Miocene magmatism or post collisional rift-related magmatism for the region, and documents for the first time continental rocks of probable Australian cratonic affinity beneath South Sulawesi.

The Neogene orogen provided the final element in creating a working petroleum system. Eocene oil-prone fluvio-deltaic coals and associated source rocks were depressed into the oil window by deposition of a thick sequence of andesitic pyroclastic deposits and sub-sequent tectonic loading associated with thickening of the orogenic wedge. Mature oils from seeps have been typed to these Eocene source rocks. Eocene siliciclastic carrier beds provided conduits from subthrust kitchen areas to potential Eocene siliciclastic and Mio-Pliocene carbonate and volcaniclastic reservoirs in compressional ramp anticlines.