Abstract

The geochemical data in Bunyu Island show a gentle rise on the top of the oil window from Bunyu field in southeastern part of the tapa Field in nortwestern part of the island. Crude oil composition is based on a range of geochemical parameters including GC-MS that indicate the water-wash alteration occured in Bunyu field, the largest oil field at Bunyu. While at Tapa, the major gas field, the altered hydrocarbon was caused by fractionation process. However, samples with sterane distribution analyses were collected from both fields. It is concluded that the Bunyu hydrocarbon is derived from fluviodeltaic source rocks, while Tapa hydrocarbon is generated from shallow lacustrine to estuarine source. The differences of the hydrocarbon type in this island are largely controlled by the thermal gradient, which in the northwestern part is relatively higher than in the southeast. The Tertiary deposition is relatively uniform from southeast to northwest, where the sand cyclics have a regular pattern and well correlated. The restored profiling through intra Miocene, Mio-Pliocene and Plio-Pleistocene concluded that during the deposition of Santul Formation some parts of the Tabul Formation were uplifted and the basement high blocks were constructed. The hydrocarbon migration pattern is largely controlled by tectonic framework and its position during Mio-Pliocene tectonism. The uplifted blocks, which developed during Mio-Pliocene tectonism, have a good inclination of hydrocarbon potential. This study is a part of PERTAMINA EP Kalimantan's effort to increase it production output.