Abstract

Three different organic facies for source rocks are recognized in the NW Java Basin based on biomarker composition of oils. The largest group of oils, derived from a deltaic source rock, typically has a high concentration of oleanane, C₂₇/C₂₉ sterane > 1, hopane/sterane > 4, and a predominance of medium weight n-alkanes. A second group of oils, probably of lacustrine origin, has an abundance of botryococcane, hopane/sterane > 8, an unusual sterane distribution (C₂₈ > C₂₉ > C₂₇), and a slight odd-carbon n-alkane predominance in the C₂₁ - C₃₃ range. A third type, represented by two oils, is characterized by a predominance of low molecular weight n-alkanes (n-C₁₁ to n-C₁₇), low diasterane/sterane, hopane/sterane > 2, C₂₃ tricyclic terpane higher than C₂₄ tetracyclic terpane, C₂₉/C₃₀ hopane > 1, a high homohopane index, C₃₅/C₃₄ homohopane > 1, and an intermediate to high sulfur content. These features suggest a marine carbonate depositional setting for the source rock, although high pristane/phytane (4 to 5) and high concentrations of C₁₉ and C₂₀ tricyclic terpanes conflict with this interpretation. Accordingly, these two oils may have been mixed with oil from a noncarbonate source rock. This is the first indication that a carbonate source rock, probably the Baturaja Formation, is present in the NW Java Basin.