UNDERSTANDING PALEOGENE DEPOSITIONAL ENVIRONMENT IN EAST CEPU HIGH USING GEOCHEMISTRY AS AN APPROACH FOR EXPLORATION OPPORTUNITY IN NORTH EAST JAVA BASIN

摘要

East Cepu High is one of Paleogene High in East Java Basin that is an ideal place for the development of Oligo-Miocene carbonate build-ups, which serves as good hydrocarbon reservoir of several fields in Cepu Block. The Oligo-Miocene reservoir for all fields in Cepu Block is believed to be charged from Paleogene source rock. However, the different of hydrocarbon properties in several fields indicates the depositional environment of source rock in this area is still the biggest question. The purpose of this study is to understanding depositional environment of Paleogene deposits in order to gain explanation of hydrocarbon source in East Cepu High. Geochemical analysis (bulk properties, isotope stability, and biomarker) was conducted for analysing depositional environment of source rock using 6 exploration wells data. Oil Bulk Composition analysis of oil sample in East Cepu High for Field B, C and J are dominated by non-biodegraded saturated hydrocarbon with paraffmic liquid hydrocarbon as product and high pristane/phytane in chromatogram. Meanwhile, liquid extracted from rock sample in Field K, W, and E show low pristane/phytane. Qualitative interpretation of Triterpenes shows similar result in all data, C_(30) hopane > C_(29) hopane eliminates a carbonate origin for these hydrocarbon and component Oleanane is present in moderately high amount in all data (>0.2). Normal sterane distribution indicate a predominance of the C_(29) sterane over C_(27) sterane which implies a significant contribution from terrestial plant organic material to the parent source. The exception comes from sample in C Field which located in western area exhibit significant C_(27) presence suggesting the additional of algal component or more input of marine organic source in this area. Furthermore, analysis of carbon isotope shows southeast fields (J, W and E) show more terrestrial input. It was consistent with cross plot coming from streane ternary diagram. As a conclusion, hydrocarbon in reservoirs came from same source rock (Ngimbang Fm), which was deposited in different depositional environments during Early Tertiary. Hydrocarbon that filled reservoirs in southeast area came from fluvio-deltaic deposits while in northwest area came from deltaic-shelf deposits.