Oil & Gas Geology ›› 2025, Vol. 46 ›› Issue (1): 246-260.doi: 10.11743/ogg20250117

• Petroleum Geology • Previous Articles     Next Articles

Organic geochemical characteristics and exploration significance of the Upper Ordovician Qrebake Formation source rocks, Shunbei area, Tarim Basin

Shuai HE1,2,3(), Anlai MA1(), Lu YUN4, Zicheng CAO4, Xianqing LI2,3, Cheng HUANG4, Guosong ZHANG1, Song HU1, Tieyi WANG1, Weilong PENG1, Zhili ZHU1, Futian CUI1,2,3   

  1. 1.Petroleum Exploration and Production Research Institute,SINOPEC,Beijing 102260,China
    2.State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources,China University of Mining and Technology (Beijing),Beijing 100083,China
    3.College of Geoscience and Surveying Engineering,China University of Mining and Technology (Beijing),Beijing 100083,China
    4.Northwest Oilfield Company,SINOPEC,Urumqi,Xinjiang 830011,China
  • Received:2024-10-22 Revised:2024-12-27 Online:2025-02-28 Published:2025-03-03
  • Contact: Anlai MA E-mail:2592724745@qq.com;maal.syky@sinopec.com

Abstract:

Source rocks in the Upper Ordovician Qrebake Formation in the Shunbei area of the Tarim Basin have been newly discovered in recent exploration. Investigating their geochemical characteristics holds great significance for future hydrocarbon exploration. Using comprehensive geochemical experiments, analyses, and tests on source rock samples from six wells in the Shunbei area, we explore their organic matter abundance, types, and thermal maturity in the Upper Ordovician Qrebake Formation. The results indicate that the selected source rocks consist primarily of grayish-black mudstones, dark gray calcareous mudstones, and dark gray argillaceous limestones. The grayish-black mudstones, among others, exhibit total organic carbon (TOC) content ranging from 0.20 % to 2.81 % (average: 1.59 %) and Rock-Eval pyrolysis-based hydrocarbon generative potential (S1+S2) from 0.43 to 12.05 mg/g (average: 5.59 mg/g). The dark gray calcareous mudstones show TOC content varying from 0.46 % to 2.24 % (average: 1.47 %) and S1+S2 from 0.18 to 1.97 mg/g (average: 0.91 mg/g). In contrast, the dark gray argillaceous limestones manifest TOC content ranging from 0.22 % to 3.41 % (average: 1.72 %) and S1+S2 from 1.37 to 18.56 mg/g (average: 9.04 mg/g). Notably, wells SB5, SB7, and SB71X present source rocks of the highest organic matter abundance. Source rocks in the Qrebake Formation are formed in a weakly reducing, brackish marine sedimentary environment, with parent materials originating primarily from low aquatic algae. Most samples exhibit hydrogen index (HI) values ranging between 289 and 512 mg/g (average: 416 mg/g), suggesting humic-sapropelic organic matter (Type Ⅱ1) with high hydrocarbon generative potential. Source rocks from well SB7 present vitrinite equivalent reflectance (Ro, eq) ranging from 0.55 % to 0.88 % (average: 0.72 %), indicating low-to-moderate maturity. In well SB71X, the Ro, eq ranges from 0.53 % to 0.70 % (average: 0.62 %), suggesting low-maturity source rocks. Meanwhile, source rocks from well SB5 display Ro,eq between 0.85 % and 0.95 % (average: 0.90 %) of moderate maturity. Source rocks in the formation measure from 20 to 25 m in thickness, distributed primarily in the intra-platform depressions in the Nos. 5 and 7 fault zones in the Shunbei area.

Key words: biomarker, molecular geochemistry, source rock, Qrebake Formation, Ordovician, Shunbei area, Tarim Basin

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