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

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 (R_o,eq) ranging from 0.55 % to 0.88 % (average: 0.72 %), indicating low-to-moderate maturity. In well SB71X, the R_o,eq ranges from 0.53 % to 0.70 % (average: 0.62 %), suggesting low-maturity source rocks. Meanwhile, source rocks from well SB5 display R_o,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.