Source rock distribution and evaluation in the Talang Akar formation, onshore northwest Jawa Basin, Indonesia

Tl\u27.- Talapg Akar Formation, which produces cominercial hydrocarbons in the onshore Northwest Jawa Basin, was investigated to determine the relationships between organic matter, maturity and hydrocarbon source rock potential. This study was mostly based on organic petrological methods vvhich permit the identification the type, abundance and maturity of organic matter. These three parameters play an important role in determining which sedimentary sequences are sources for hydrocarbons. The biomarker distribution in crude oil from, the postulated source rocks was studied using gas chromatography (GO and gas chromatography/mass spectometry (GC/MS). In the Talang Akar Formation organic matter is most abundant in coal and shaly coal beds but is also present as dispersed organic matter in shale. Vitrinite is the dominant maceral, followed by liptinite and minor inertinite. Vitrinite consists mainly of detrovitrinite and some tslovitrinite. Liptinite comprises sporinite, cutinite and resinite as major components, whereas liptodetrinite, suberinite and exsudatinite are present as minor components. Semi-quantitative evaluation of organic matter v/as attempted. Vitrinite reflectance provides a relatively accurate assessment of source rock maturity. Comparisons were made between vitrinite reflectance and m.aturation modelling based on the Lopatin method. Major differences for some wells probably reflect the choice of a high TTI value. Coke structures in vitrinite in samples studied from the SIN-3 well indicate the presence of local intrusions. Hydrocarbons have probably been generated from liptinite and vitrinite macérais in the Talang Akar Formation. Intervals with an abundance of these macérais generally lie within_ the oil generation window {ranging from 0.45% to 1.02% R max) except in the PWK-1 and SIN-3 wells where the -^ank ranges from 0.71% to 1.58% and 0.66% to 3.47% R^^max, respectively. Oil cuts, oil droplets and bitumen observed during fluorescence-mode microscopy also suggest that hydrocarbons have been generated in this formation. Seiniquantitative assessment of the hydrocarbon generation potential indicates that the Talang Akar Formation has, in general, poor to fair potential although some interval have good potential. Shales with dispersed organic matter have poor potential, whereas coal generally has a poor to good potential with some coals having a very good potential to generate hydrocarbons