BIOMARKER GEOCHEMISTRY OF CRUDE OILS FROM THE QAIDAM BASIN, NW CHINA

Yi Duan*+, Chaoyang Zheng*, Zhiping Wang**, Baoxiang Wu*, Chuanyuan Wang*, Hui Zhang*, Yaorong Qian***, Guodong Zheng#

*Lanzhou Institute of Geology, Chinese Academy of Sciences, Lanzhou, Gansu Province 730000, People's Republic of China.

+ author for correspondence, email: duany@ns.lzb.ac.cn

**Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, People's Republic of China.

***Analytical Chemistry Branch BEAD/Office of Pesticide Programs, US EPA 701 Mapes Rd, Fort Meade, MD 20755-5350, USA.

#W.M. Keck Geoenvironmental Laboratory Center for Environmental Systems, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030, USA

A suite of 16 crude oil samples from 13 oilfields in the Qaidam Basin were analyzed using techniques including gas chromatography and gas chromatography - mass spectrometry. Biomarker compositions and parameters were used to investigate the palaeoenvironmental and depositional conditions and to correlate the oils with each other. Oils from the western Qaidam Basin have pristane/phytane (Pr/Ph) ratios of less than 0.7, and contain abundant gammacerane, C₂₇ steranes, 4-methyl steranes and long chain tricyclic terpanes. The C₂₉ sterane 20S/(20S+20R) and ββ/(ββ+αα) ratios show that the western Qaidam oils have variable maturities ranging from immature to mature. Oils from the northern Qaidam Basin, by contrast, have Pr/Ph ratios greater than 3, low gammacerane contents, and relatively abundant C₂₉ steranes, bicyclic terpanes and alkylcyclohexanes. C₂₉ sterane 20S/(20S+20R) and ββ/(ββ+αα) ratios indicate that the northern Qaidam oils are mature.

δ¹³C values, which range from –25.4‰ to –28.3‰ with the exception of one oil from the north (–31.6‰), are similar for oils from both the northern and western parts of the Qaidam Basin. The oils’ carbon isotope compositions are similar to those of the organic matter in potential source rocks.

The western Qaidam oils are inferred to have originated from Tertiary source rocks deposited under anoxic and saline-hypersaline lacustrine conditions with dominant algal organic matter. The northern Qaidam oils are interpreted to be derived from Jurassic source rocks which were deposited in a freshwater lacustrine environment and which are dominated by terrigenous organic matter.

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