Shiyou shiyan dizhi (Jul 2023)

Geochemical characteristics and source of crude oil from the eastern Shawan Sag, Junggar Basin

  • Xiaoqi WU,
  • Xincheng REN,
  • Dezhi LIU,
  • Yingbin CHEN,
  • Yuqing MU,
  • Bin WANG,
  • Fengkai SHANG,
  • Zhongliang SUN,
  • Yansheng QU,
  • Zhenxiang SONG,
  • Qi QIU

DOI
https://doi.org/10.11781/sysydz202304646
Journal volume & issue
Vol. 45, no. 4
pp. 646 – 655

Abstract

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The Shawan Sag is one of the important exploration areas in the hinterland of the Junggar Basin. In order to better clarify the source of crude oil from the Permian, Jurassic and Cretaceous reservoirs in the sag and identify hydrocarbon accumulation processes and enrichment pattern, the authors conducted a carbon isotope and biomarker analysis on the typical crude oil samples from the eastern Shawan Sag in this study, and further carried out crude oil classification and oil-source correlation based on the analysis of geochemical characteristics of crude oil. The results indicate that the crude oil from different strata in the eastern Shawan Sag can be divided into three types. The first type mainly occurs in the Permian and Jurassic reservoirs, and its whole oil δ13C value and Pr/Ph ratio range from -31.0‰ to -29.0‰ and from 1.0 to 2.0, respectively. The relative contents of αααR regular sterane display the characteristics of C27 C23 TT and C24TeT/C26TT C21>C23TT. The distribution fraction of methyl phenanthrene reflects that the crude oil is in the stage of high-maturity evolution. All these indicate the mixing of crude oil generated in the Lower Permian source rocks. The second type occurs in the Middle Jurassic reservoirs. The whole oil δ13C value and individual n-alkanes δ13C value are higher than -29.0‰. The Pr/Ph ratio is relatively higher, ranging from 2.0 to 2.5, with C24TeT/C26TT>1, which indicate that this type of crude oil is mainly derived from the Jurassic source rocks. The third type occurs in the Lower Cretaceous reservoirs, and its whole oil δ13C value and Pr/Ph ratio are lower than -31.0‰ and 1.0, respectively. The αααR regular sterane contents display the characteristics of C27≈C28 <C29, with the gammacerane index higher than 0.50. The individual n-alkanes δ13C value gradually decreases with the increase of carbon number and is commonly lower than -31.0‰. This type of oil is demonstrated to be mainly derived from the Lower Cretaceous source rocks.

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