GEOLOGY AND TECTONIC EVOLUTION OF FORE-ARC BASINS IN THE WESTERN INDONESIA

The fore-arc region of western Indonesia is distributed in length from Sumatra to Java Islands for more than 3000 km. It can be divided into four different areas based on bathymetry and geology. These are North Sumatra, West Sumatra, Sunda Strait and Southern East Java. This paper will discuss geological comparison as well as their hydrocarbon potential based on integrated surface geology and newly acquired 2D seismic data supported by the current knowledge of regional tectonic.

Based on current tectonic model, it shows that the western Indonesia fore-arc region was formed during Late Eocene to Early Miocene and continues to the present time. The sediments within the fore-arc basin are mostly very thick consisting of shale, sandstone and limestone deposited in various depositional setting including deltas, shallow marine, slope and deep marine turbidities. It was very clear evidence from wells and seismic data that this region was developed as fore-arc since Early Miocene. The basement composition is considered as important parameters in determining the prospectivities of the region. Most of the areas are interpreted underlain by transitional crust consisting of previous subduction mélanges and older fore-arc sediments except for North Sumatra and Southern East Java, which are show possibilities of underlain by older rift basin consisting of lacustrine deposit underlain by continental crust. Deformation in the fore-arc region is relatively low except for Sunda area, which shows several evidence of series young complex thurst, normal as well as strike-slip faulting related to crosscut by tips of the Sumatra Fault zone.

Traditionally, the interpretation of hydrocarbon prospectivities of the region are low particularly related to the present of source rocks and their maturity because of low gradient geothermal. Our study indicates moderate hydrocarbon potential due to several parameters including evidence of hydrocarbon seeps in the field indicating petroleum system occurrences in west Sumatra fore-arc. However, because of uncertainty in tectonic and depositional model, hydrocarbon maturity and migration, deep-water setting as well as high frequent earthquakes considering as very high risk as well as frontier area.