Geochemical Characteristics and Origin of Hydrocarbons from Lower Ordovician in Tazhong Area, Tarim Basin

Abstract

Abstract: Multiple approaches are applied to analysis of the characteristics and origin of the oils from the Lower Ordovician in Tazhong area, Tarim basin. The results show that the oils are primarily condensate oil featured by dominant fractions with low molecular weight and much low content of biomarkers such as steroids and terpenoids, suggesting high-overmature crude oils. It is distinguished from that of the overlying Middle-Upper Ordovician by particularly high-super high dibenzothiophene in aromatics. It is suggested that most of the Lower Ordovician oils are mixed-source oils from both the Cambrian-Lower Ordovician and the Middle-Upper Ordovician with a little higher contribution of the former based on chemical compositions of hydrocarbons and compound-specific carbon isotope of n-alkanes in the oils. It is detected that the Lower Ordovician oils are enriched in mercaptan and long chain alkyl thiolanes. These features together with high H2S content in associated gas as well as high dibenzothiophene in the oils indicate that hydrocarbons from the Lower Ordovician should have suffered some alterations such as thermochemical sulphate reduction (TSR) after accumulation to a certain degree. These might have played an essential role in hydrocarbons alteration and reservoir preservation in this area.

Key words: Tarim basin, Tazhong uplift, Lower Ordovician, biomarker, isotope, oil source, TSR

CLC Number:

TE112.111

LI Su-mei, XIAO Zhong-yao, LV Xiu-xiang, ZHANG Bao-shou, ZHANG Hai-zu. Geochemical Characteristics and Origin of Hydrocarbons from Lower Ordovician in Tazhong Area, Tarim Basin[J]. Xinjiang Petroleum Geology, 2011, 32(3): 272-276.

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