EMERGENCE OF NEW PETROLEUM SYSTEM IN THE MATURE SALAWATI BASIN : KEYS FROM GEOCHEMICAL BIOMARKERS

摘要

Salawati Basin, West Papua is a mature basin producing oil since 1930s. Single proven petroleum system is Miocene Kais/Klasafet : Kais (!). Exploration success using this system is getting smaller and more difficult. New petroleum system is required for further significant discoveries. The Salawati Basin has potential for presence of new petroleum systems. Methods used in this study are applicable for other mature basins.Salawati Basin is an asymmetric foreland basin with foredeep kitchen bordered by deformed zone of the Sorong Fault. The increasing maturities of oils from up dip to down dip areas are shown by vitrinite reflectances of oils calculated from aromatic biomarker of methylphenanthrene (MP) (m/z 192). The maturities of oils increase from Re (calculated vitrinite reflectance) of 1.06 to 1.2 %. However, overlying the Sorong foredeep kitchen, there are a number of oil seeps that do not follow the general trend of maturity. The oils were generated from Re averagely 0.7 %. These oils should not be sourced by the Kais/Klasafet marly carbonaceous shales. There will be no oil generated by the Kais/Klasafet sources in this area since they have been in overmaturing window. Instead, the oil seeps must be sourced by younger sources that are still in the oil window. Based on the geochemical data, the source rocks for the oil seeps are the early Pliocene Lower Klasaman shales.Oils from the Lower Klasaman shales have been identified to charge the Intra-Klasaman sand reservoirs. Therefore, a new petroleum system of the Lower Klasaman - Intra-Klasaman (!) is identified. The Klasaman Formation composes 80 % of the basin's stratigraphy. Accordingly, generative capacity of the Lower Klasaman sources is high. Intensive deformation since the Pliocene by the Sorong tectonism has generated structural traps of the Klasaman Formation.Challenges to this new system are presence and quality of the Intra-Klasaman reservoirs.