Differentiation of oils from the NW Java basin into three oil types based on biomarker composition

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.