Geochemical Characteristics and Origin of Crude Oil and Natural Gas in the Southern Slope Zone, Kuqa Foreland Basin, NW China

Abstract

Continuous exploration has triggered a heated debate on hydrocarbon resource potential in the southern slope zone of the Kuqa foreland basin, and sources of the Mesozoic-Cenozoic oil and gas have become a key problem to be solved in this region. Composition and organic geochemical parameters of crude oil and natural gas from the southern slope zone of the Kuqa foreland basin were illustrated in order to reveal their origin by using a combination of gas chromatograph (GC), gas chromatogram-mass spectrum (GC-MS) and carbon isotope analyses. The characteristics of crude oil, such as low density, viscosity, solidification point and sulfur content, and high wax content, indicate that source of the crude oil is continental. The biomarker compositions of crude oil are characterized by low to medium molecular weight compounds (n-C₁₂ to n-C₂₀), high Pr/Ph ratios (>1.0), low phytane/n-C₁₈ ratios (0.06–0.54), and predominant regular sterane C₂₉. All biomarker parameters clearly indicate that the crude oil was derived mainly from algae and aquatic plankton and deposited under weak reduction-oxidation environment, and has the characteristics of mixed kerogens. The Cretaceous crude oil was mainly derived from the Triassic lacustrine source rocks, which also contributed to the Paleogene crude oil together with Jurassic coal source rocks. Natural gas is characterized by moderate methane content, high heavy hydrocarbon and nitrogen content, and no hydrogen sulfide. The methane and ethane in Paleogene natural gas are relatively rich in ¹³C with δ¹³C₁ and δ¹³C₂ values ranging from −37.3‰ to −31.2‰ (mean = −34.25‰) and from −25‰ to −21.3‰ (mean = −23.09‰), respectively, indicating the coal-derived gas from the Middle and Lower Jurassic strata. Hydrocarbon products in the southern slope zone of the Kuqa foreland basin are primarily generated from source rocks in the mature stage. The low-amplitude structural and lithologic traps with the updip pinch-out sand bodies or plugging secondary fault at relatively high tectonic positions are the most favorable areas for discovery and breakthrough in the study area. Results of this study will provide useful information for controlling factors of reservoirs and oil and gas exploration deployment in the southern slope zone of the Kuqa foreland basin.

Keywords

crude oil / natural gas / oil and gas source / organic geochemistry / Kuqa foreland basin

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