Geochemical characteristics of oils from Taiwan

Abstract

Geochemical studies on a number of oils from Taiwan have shown that they are sourced predominantly from terrigenous organic matter. Biomarker characteristics include the presence of high amounts of long-chain regular isoprenoids, bicyclic sesquiterpanes, diterpanes, low concentrations or absence of tricyclic terpanes, presence of several C 24 tetracyclic terpanes and some source-specific pentacyclic terpanes and a predominance of C 29 steranes. For most of the oils, the n-alkanes extended to C 30 or above and have CPI values of approximately 1, indicating the mature nature of these oils. The pristane/phytane ratios, generally in excess of 5, suggest the likely prevalence of a highly oxic, nearshore, marine-open peat swamp depositional environment which incorporated large amounts of terrigenous organic matter. The long-chain isoprenoids extending to C 42, and possible higher, are proposed to be regular head-to-tail isoprenoids possibly derived from polyprenols in higher plants. The sesquiterpanes present in the oils range from C 15 and C 17 and are dominated by 8β(H)-drimane and 8β (H)-homodrimane. The diterpanes include C 17-C 20 tricyclic and C 19 and C 20 tetracyclic diterpanes. The presence of diterpanes in these oils suggests that resins derived from conifers contributed to the source of these oils. At least nine tetracyclic terpanes with carbon numbers ranging 24 to 27 were detected in Taiwan oils. Two series of tetracyclic terpanes are apparent, one is the de-A-oleanane/ursane series and the other is the 17,21-secohopane series. The former is more source-specific than the latter and is probably related to the presence of oleanane. A number of pentacyclic terpanes, including oleanane, have been detected and many of these compounds may be of terrigenous origin and source-specific. Oleanane, de-A-oleanane/ursane and de-A-lupane can be used as indicators for angiosperms which have evolved since the late Cretaceous. Sterane distributions in the Taiwan oils are dominated by the C 29 steranes and include regular and rearranged steranes, indicating the terrigenous nature of the organic matter in the source rocks. The significance of the variations observed in the biomarker distributions of the oils from Taiwan is discussed in some detail in this paper.