Publications

The Cenozoic Sumatra Accretionary Prism outcrops exposed in the chain of Simelue, Nias, Mentawai and Enggano Islands offshore Sumatra are not, at first glance, of interest to the explorationist. These areas represent different accretionary styles off the Sunda Margin. Since the Middle Eocene, subduction along the Sumatra margin began with an oblique subduction. It was dominated by the subduction accretion of the Indo-Australian basaltic crust, biogenic and non-biogenic sediments, in addition to quartz rich-sediments of Bengal-Nicobar Fan and volcanic material derived from Sunda Arc. The purpose of this study is to identify the structural character, tectonostratigraphy and occurrence of petroleum system elements using the integration of geological fieldwork in Pagai-Sipora Island, along with the integration of seismic and the results of the Ocean Drilling Project Well Legs 213 and 211. The proposed stratigraphic complexity within this region is divided into pre, syn and post-accretion sequences. The Pre-accretion sequence consists of the Wharton Basin (oceanward) and sediment overlying the Woyla Terrane (continentalward). The Syn-accretion sequence (Tanahbala - Sipika equivalent) consists of tectonic melange, gravity-mass transport and turbidites deposited in piggy-back and trench basins. The Post-accretion sequence (Rapa-rapa equivalent) consists of pelagic-mud drapes and carbonates. However, the geological complexities provide implications for petroleum system elements. Source rock from marine plantonic mudstones are generated due to the upwelling in slope during accretionary growth. Potential reservoirs are turbiditic sandstones, and fractured shale and basalts. Seal rocks are intra-formational shales, pelagic-mud drapes and carbonate rocks. Potential migration and trap formation is related to anticlines in the imbricate structures that prograde westward. The Mentawai Strike-Slip Fault Zone can be considered an additional potential trap formed between the accretionary prism and the forearc border. Continous subduction provides thickening older accretionary prism to build outer highs, and this structure is perhaps the most prospective for hydrocarbon accumulation. Maturation occurred in the methamorphic zone outboard of the outer high. As a source rock mainly from marine planktonic, potential hydrocarbon is gas instead of oil. It is proposed that this hypothesis should be tested to prove the Cenozoic Sumatra Accretionary Prisms play fairway, albeit with high risk!