Geochemical Characteristics of the Condensates and Their Evaporative Fractionation in Kuqa Depression of Tarim Basin, NW China

Abstract

After analyzing 21 samples in 12 oil and gas fields in Kuqa depression, two genetic types of condensates are identified. One type is the sapropelic condensate mainly derived from the Upper-Middle Triassic lacustrine source rocks which has a relatively higher content of C 5-7 n-alkanes (>35%) and lower methylcyclohexane content (<48%), pristane/phytane ratio (0.95~1.56) and carbon isotope values (-27.8‰ ~ -32.3‰); the other type is the coal-derived condensate derived from the Middle-Lower Jurassic terringenous source rocks which has a lower C5-7 n-alkanes content (28% in average) and relatively higher content of methylcyclohexane (>50%), pristane/phytane ratio (1.70 4.62) and carbon isotope values (-26.2‰ in average). Based on a comprehensive study on the n-alkanes, light hydrocarbons and carbon isotopes, it is found the condensates in Kuqa depression have widely experienced evaporative fractionation with a high aromaticity (>0.8) and their high molecules suffered loss. With the evaporative fractionation becoming more significant, the bulk carbon isotopic values of the condensates become heavier, indicating the multistages of the evaporative fractionation. The structural or lithologic traps in the Tertiary and the Jurassic tend to be favorable exploration targets for future oil exploration in Kuqa depression.

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Special Issue: Advances of petroleum exploration and geology research in West China's petroliferous basins

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