Characteristics of oil shale development and paleoenvironment restoration of Jurassic Yan’an Formation in southwestern Ordos Basin

[Objective]There are many sets of oil shale associated with coal and non-associated oil shale in the Jurassic Yan’an formation in the southwest of Ordos Basin, but the study on the characteristics and sedimentary environment of oil shale in this area has been relatively lacking for a long time. [Methods]Based on the oil shale samples from wells in the study area, the industrial index, geochemistry characteristics and sedimentary paleoenvironment of oil shale were studied by low-temperature retorting, rock pyrolysis, major and trace elements measurement and gas chromatography. [Results]The oil shale in study area oil-bearing ratio is 3.8% ~ 6.7%, the ash is 49.46% ~ 75.58%, the total sulfur is 0.31% ~ 2.29%, and the calorific value is 11.95 kJ/g (average value). The logging curves show the characteristics of high gamma, high resistivity and low natural potential, which is obviously different from coal bed and mudstone. The main type of organic matter was II2. The abundance of organic matter is high, and the whole is in the stage of immature-low maturity. It belongs to middle quality, siliceous ash and extra low sulfur oil shale. The main elements in the oil shale are SiO2 and Al2O3, which are 48.26% (average) and 17.57% (average) and the micronutrient Sr, Th, and U were enriched, and Ni, V, and Ba were depleted, indicating that there were more potassium-containing minerals and higher stable components in the study area. [Conclusion]Based on the analysis of paleoenvironmental element markers and saturated hydrocarbon chromatography, it is considered that the organic matter mainly comes from the mixed organic matter, ancient productivity is medium and the sedimentary environment is unique, which belongs to the warm and humid continental fresh water depression environment. The analysis of environmental factors shows that the redox condition is the main influencing factor of organic matter enrichment and the sedimentary model of oil shale evolution in Yan’an formation in different depositional periods is established. The research results provide important theoretical guidance and technical support for the oil shale exploration and development of the Jurassic Yan’an formation in the southwestern margin of Ordos Basin.