Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China: Part 1. Oil family classification

Abstract

A series of Palaeozoic marine oils ranging from condensates to very heavy oils from the cratonic region of the Tarim Basin, NW China have been examined to delineate oil families. Crude oils from the Mesozoic foreland Kuqa Depression were also analyzed for comparison. The genetic family was assessed using bulk hydrocarbon isotopic composition, alkylation pattern of polycyclic aromatic hydrocarbons (PAHs) and biomarker distributions. Detailed oil–oil correlation suggests two distinct families occur in the cratonic region. Despite the diversity of the reservoir rocks and some variability in oil physical properties, the Tabei and Tazhong oils are classified, in general, into a single family, with a possible indication for secondary subdivision. These oils are characterized by low pristane/phytane (Pr/Ph) ratio, high dibenzothiophene/phenanthrene (DBT/P) ratio, light bulk isotopic composition, high degree of PAH alkylation, high proportion of C 23 and C 24 tricyclic terpanes and low abundance of C 24 tetracyclic terpane and triaromatic dinosteroids. This family is inferred to have a Middle–Upper Ordovician source rock. The Tadong family, distinctly different from the Tabei and Tazhong families, is characterized by relatively high Pr/Ph ratio, low DBT/P ratio, slightly heavier bulk isotopic compositions and low degree of PAH alkylation. It also has a unique biomarker signature, with a high proportion of triaromatic dinosteroids, possibly indicating Cambrian–Lower Ordovician source rocks. The oils from the Kuqa Depression are easily distinguished from the other two families due to their terrigenous origin. The major characteristics of the Kuqa oils are high Pr/Ph ratio, very low DBT/P ratio, heavy bulk isotopic composition, low PAH alkylation degree, high proportion of C 20 and C 21 tricyclic terpanes and C 24 tetracylic terpane, and abundance of triaromatic dinosteranes and 4-methyl-24-ethyltriaromatic cholestanes. These characteristics are consistent with terrestrial organic source rocks deposited under sub-oxic conditions.