Characteristics of Cepu Block Oil, Wonocolo Formation, East Java Indonesia: Study of aliphatic biomarkers

The research of organic geochemistry has been carried out through the report of aliphatic hydrocarbon fraction biomarkers in Cepu Block oil, Wonocolo Formation, East Java. Organic geochemistry studies were carried out through biomarker analysis such as the depositional environment, the origin of organic compounds, and the maturity of Cepu Block crude oil samples. The biomarker was analyzed using extraction, fractionation, and structure identification methods by Gas Chromatography-Mass Spectrometer (GC-MS). The odd-to-even carbon number predominance of n-alkanes with a CPI value of 1.04 indicates that the organic material sample had low maturity. The indicator of maturity is also supported by the Ts/Tm ratio of 0.52, and the abundance of 17α(H),21β(H)-hopane which is lower than 17β(H),21a(H)-hopane, and the high abundance of 5α(H)-7-cholestene. The ratio of pristane/phytane (Pr/Ph) of 3.25 and Ts/(Tm+Ts) ratio of 0.34 indicates that the oil sample came from an oxidative depositional environment. The bimodal distribution of n-alkane biomarkers with a ratio of long-chain to short chains n-alkanes (Σ2/Σ1) of 1.25 indicates that the source of oil organic compounds comes from higher plants, bacteria, and algae. The presence of hopane and homohopane, 8β(H)-drimane and homodrimane compounds, and also the sterane group indicates the presence of bacterial input with bacteriohopanetetrol precursors produced by Archaebacteria. The results that have been obtained indicate that the Cepu Block oil sample, Wonocolo Formation has various organic sources with low maturity.