The well-developed mudstone and gypsum mudstone in the Oligocene Linhe Formation (E₃l) in the Hetao Basin are the main source rocks for gypsum. However, the sedimentary environment and organic matter (OM) enrichment factors of E₃l are not clear, and this inhibits the prediction of hydrocarbon source rock distribution and resource calculation. Major and trace elements, total organic carbon (TOC), pyrolysis using the Rock-Eval II, and saturated hydrocarbon gas chromatography–mass spectrometry (GC–MS) analyses were performed in this study. The results show that E₃l was deposited in brackish water and saline–ultrasaline water, with weak oxidation reduction in an arid and hot environment. Terrestrial input inhibits OM enrichment, while the redox, paleosalinity, paleoclimate, and paleoproductivity play a catalytic role. The main controlling factors of the same lithologic source rocks are different: terrestrial input and paleoclimate have a greater impact on mudstone, and the redox and paleosalinity were more favorable to gypsum mudstone. Although the main controlling factors are different for different lithologies, their OM enrichment characteristics are still consistent. The E₃l water body was deep, and the contribution of nutrients from terrigenous debris to OM enrichment was less. In addition, the water retention environment changed significantly during the E₃l sedimentary period, resulting in fewer nutrients, which limited the improvement of surface water paleoproductivity. The arid climate increased water evaporation and salinity, which to some extent prevented consumption and decomposition. Weak oxidation-reduction fluctuations and the stratification of the water body were obvious, and this was not only conducive to the enrichment of OM but also to its preservation.

Dongdaohaizi area is an important hydrocarbon-rich depression in the Junggar Basin. Early resource evaluation has revealed that it has superior hydrocarbon generation conditions. No major exploration breakthrough has been observed in the hydrocarbon from the Permian Pingdiquan Formation source rocks, which are widely distributed and have a large sedimentary thickness. The unclear recognition of the genesis, the sources, and the hydrocarbon evolution history of the formation seriously restricted further exploration and development. Sixty-four samples were acquired during the study, consisting of 30 source rocks, 13 crude oil samples, and 21 natural gas samples. Studying the geochemical characteristics of the source rock extract and the surrounding structural crude oil in the Dongdaohaizi Depression, the differences in the stable carbon isotope, the biomarker compound, and the molecular relative composition of the three sets of main source rock products in the research fields are summarized. The results reflect that the drying coefficient of natural gas in the study area is generally low, and the fractional distillation value of methane and ethane is 0.32, which is most likely due to the loss of oil and gas migration and the mixing of different types of natural gases. The carbon isotope value is relatively low, with the Pr/Ph being generally less than 3.0. The content of sterane C₂₉ is the highest in the relative composition of steranes, followed by the content of sterane C₂₈, which together account for more than 80% of the total sterane content, and then followed by a lower content of C₂₇ sterane, accounting for only 5-20% of the total content, which generally conforms to the characteristics of Permian Pingdiquan Formation source rock products. The carbon isotope value of crude oil ranges from -30.94 to -28.31‰, which is different from the characteristics of typical Permian source rocks (values range from -34.49 to -28.21‰), while it is related to typical Carboniferous products (values range from -29.98 to -24.1‰), indicating that small amounts of Carboniferous source rock products were mixed in different degrees in the Dinan fault area. According to the distribution law of oil and gas, the geochemical characteristics and hydrocarbon sources were considered the oil source in the east of the Dongdaohaizi Depression, mainly from the source rocks of the Permian Pingdiquan Formation. The products of the peak period of hydrocarbon generation in the source rocks of the Pingdiquan Formation have not been transported to the high structural positions on a large scale to form reservoirs. They may still exist in the deep part of the Depression and the slope area. The low-amplitude structural and lithologic traps in the slope area of the Dongdahaizi Depression are promising targets for finding the products of the peak period of hydrocarbon generation. This is of great significance to reveal the Permain hydrocarbon evolution in the Junggar Basin and guide further research on the oil-source correlation of natural gas from the paleo-strata.

The geochemical study of natural gas has made a great contribution to oil and gas exploration. However, the diffusion of natural gas components will affect the determination accuracy of natural gas origin. This study supplemented the organic carbon content, rock pyrolysis, gas component composition, and component carbon isotope analysis and testing of the source rocks and natural gas samples. The results indicated that the methane and ethane carbon isotope values of the natural gas in the Shiqiantan area are heavy. It should be originated from Carboniferous source rocks. When the maturity and genesis index of natural gas are highly matched with source rocks, the abnormal drying coefficient of natural gas may be related to the methane diffusion.