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The shale from the Cretaceous Taizhou Formation is an important source rock in the Gaoyou Sag, Northern Jiangsu Basin. Fifty-three shale core samples analyzed for total organic carbon (TOC) and Rock-Eval pyrolysis analyses were used to evaluate the geochemistry of this shale. Twenty-one samples of them were analyzed using solvent extraction, extract fractionation, Gas Chromatography-Mass Spectrometer (GC-MS) of the saturated fraction, and organic petrology analyses. Based on the test results, the shale of the Taizhou Formation contains mainly Type II-III organic matter and is chiefly oil-prone and oil-gas-prone, as a fair to good oil-source rock with a small quantity of gas-source rock. Vitrinite reflectance, Rock-Eval pyrolysis, and biomarker suggest that in this sag, the shale of the Taizhou Formation is mainly in the early mature stage and within the oil window, so the shale generates chiefly liquid oil. The Gaoyou Sag is an oil-generating sag. The saturated fraction and biomarker composition of the shale provide evidence that the Taizhou Formation shale was formed mainly in a reductive sedimentary environment with anoxic fresh to low salinity water. The organic matter came chiefly from plankton and partly from land plants and bacterial organisms. [ABSTRACT FROM AUTHOR]

The origin of crude oils and their varied geochemical features along the western and northern slope of the Mahu depression in the Junggar Basin has been controversial. Based on the molecular and isotopic geochemistry of hydrocarbons from 46 drill-stem test crude oil samples and 36 core samples, three oil groups (I, II, III) and five subgroups (I 1 , I 2 , II, III 1 and III 2 ) have been recognized and oil-source correlations have been established. The subgroup I 1 oils originated mainly from lower Permian Fengcheng (P 1 f) carbonate source rocks, subgroup I 2 from P 1 f mudstone source rock and group II oils from Middle Permian Wuerhe (P 2 w) mudstone source rock whose potential was underestimated in the past. Group III consists of mixed oils, i.e., subgroup III 1 received contributions from P 1 f carbonates and P 2 w mudstones and subgroup III 2 is a mixture of oils from P 1 f mudstones and P 2 w-generated oils. In order to determine the main source rocks, the laboratory oil mixing experiments were carried out. Tricyclic terpane biomarker parameters (TTs) were used to quantify the source contributions. Whole oil carbon isotope ratios were also used to verify the proportions of different oils in each mixture. The results show that subgroup III 1 oils have diverse mixing ratios in different reservoir, subgroup III 2 oils are mainly from P 1 f mudstones and P 2 w source rock contributes little. Finally, the accumulation process of oil fields along the northern slope of the Mahu depression was analyzed and the contributions from different sources were revealed. This study provides an effective quantitative method to identify contributions from different lacustrine source rocks to mixed oils in the Mahu depression, which could be helpful in predicting location and composition of undiscovered oils. [ABSTRACT FROM AUTHOR]