Depositional environment, age determination and source diagnose of oils from the western Chepaizi Uplift (Junggar Basin) constrained by molecular biomarkers

Abstract

The western Chepaizi Uplift of petroleum-enriched Junggar Basin hosted multiple oil-bearing intervals, i.e., Neogene Shawan Formation (N1s), Lower Cretaceous Tugulu Group (K1tg), and Upper Carboniferous Xibeikulasi Formation (C2x). However, their origins had always been debated, due to the occurrence of Jurassic, Cretaceous and Paleogene source rocks in the neighboring Sikeshu Sag. In this paper, two oil groups, i.e., group A (N1s and K1tg oils from the west margin of western Chepazi Uplift), and group B (oils from the east margin of the western Chepaizi Uplift). The latter can further subdivided into family B1(Upper Jurassic Qigu Formation (J3q) oil and N1s oils from east margin of western Chepaizi Uplift close to the Sikeshu Sag, and oil sands from the Sikeshu Sag) and family B2 (N1s and C2x oils from eastern margin of the western Chepaizi Uplift away from the Sikeshu Sag) can be defined. Age-specified biomarker ratios (24-norcholestane, Oleanane index and extended tricyclic terpane ratio) implied Jurassic and Paleogene source ages with minor Cretaceous contributions. Depositional environment-diagnosed biomarkers (Pr/Ph, Pr/nC17, Ph/nC18, methyldibenzothiophene/methylphenanthrene, C22/C21 tricyclic terpanes, C24/C23 tricyclic terpanes, C29αβ/C30αβ hopane, C31 hopane (22R)/C30αβ hopane, and gammacerane/C30 hopane) showed lacustrine source input with high hypersalinity for family B1 but freshwater lacustrine facies for group A and family B2. C27–C28–C29 regular steranes and triaromatic steroids distributions indicated ascending order of high plant contribution from family B1 to family B2 to group A. Combined with stable carbon isotope, the uplift-scale petroleum contribution to multiple oil-bearing intervals by multiple source intervals were lined out. Group A oils implied origins from source rocks deposited in freshwater lacustrine facies under weak oxidation conditions with predominant organic input of high plants, and genetically matched with the Lower Jurassic Badaowang Formation (J1b) mudstone source rock. Family B1 oils showed provenance from source rocks occurred in reducing source facies with high hypersalinity and great contribution of lower organisms, and well corelated with both J1b and Paleogene Anjihahe Formation (E2-3a) source rocks. However, where away from the generating Sikeshu Sag, especially for family B2 oils, the hydrocarbon contribution of E2-3a source rocks was decreased, mainly controlled by their variable distribution and thermal evolution.