Proceedings, Indonesian Petroleum Association
Thirty-First Annual Convention and Exhibition, May 2007
Central Java, Indonesia – A “Terra Incognita” in Petroleum Exploration: New Considerations on the tectonic Evolution and Petroleum Implications
Awang H. Satyana*
Central Java, in the middle part of Java Island, Indonesia, shows a conspicuous re-entrant or indentation of its coastline compared to those of the western and eastern parts of Java Island. This indentation is considered to express wrench segmentation. Two major Paleogene strike-slip faults with opposing trends and slips are responsible for the indentation. The faults are called (1) the Muria-Kebumen Fault, left-lateral, trending southwest-northeast; and (2) the Pamanukan-Cilacap Fault, right-lateral, trending northwest-southeast. The two faults cross Central Java, separated in the northern area but meeting in the southern area.
The two faults are considered to cause significant geologic changes in Central Java. The faults caused: uplift of the Serayu Range and exposure of the pre-Tertiary Luk Ulo mélange complex, subsidence of the northern part of Central Java and indentation of the northern coastline, subsidence of the Southern Mountains of Java in southern Central Java and indentation of the southern coastline, and northward shifting of the Quaternary volcanic arc in Central Java.
The petroleum potential of Central Java is not yet fully understood, in contrast with its counterparts in the West Java and East Java basins which have been known to be very prolific since the late 1800s. Central Java has seen a definite lack of exploration making it a “terra incognita” in petroleum exploration. The tectonics of Central Java are unique relative to West and East Java and this is considered to affect its petroleum geology. The presence of two opposite regional strike-slip faults crossing each other in southern Central Java has configured the petroleum geology of Central Java. Southern Central Java was uplifted (the Serayu Range); the uplift was compensated isostatically by the subsidence of two areas to the north and south of the uplifted area: a northern area (North Serayu) and a southern area (South Serayu/Banyumas). The subsided area became basins within which petroleum system elements and processes are operating as manifested by numerous oil and gas seepages within the basins. Gravity tectonics of toe-thrusting, inverted anticlines composed by turbidites, and reefal carbonates on horst blocks may become exploration targets in these areas. Some potential is present but we have to consider the tectonic uniqueness of Central Java to explore this region.