The high- quality source rocks of the semi- deep lake—deep lake facies in the 7th Member of the Yanchang Formation of Triassic （Chang- 7 Member) in Ordos Basin are the important sources of oil and gas, including black shale and dark mudstone, which are the important source rocks of the basin. Most of the previous studies have ignored the contribution of dark mudstone to oil and gas. Therefore, this paper analyzes the geochemical characteristics of tight oil and source rock of Chang- 7 oil formation in Qingcheng area, evaluates the effectiveness of Chang- 7 source rock, and analyzes the oil and gas source of Chang- 7 oil formation, providing scientific basis for the study of oil and gas accumulation in the study area, and providing guidance for the next step of oil and gas exploration. Methods: The tight sandstone reservoir was evaluated by using the whole rock X- ray diffraction, porosity and permeability data. The physical properties of tight sandstone oil in the 7th Member of the Triassic Yanchang Formation were analyzed by using the physical properties and group composition of crude oil. The maturity, parent material source and environment of crude oil were analyzed according to the saturated hydrocarbon chromatography-mass spectrometry data. The lower limit of organic carbon of effective source rock was determined by using organic geochemical data, combined with the geological data of oil and gas distribution in single well, the source and distribution origin of oil and gas are comprehensively evaluated. Results: The research shows that the black shale and dark mudstone of Chang- 7 Member in Qingcheng area both contribute to hydrocarbon accumulation, of which black shale has the largest contribution, followed by Chang- 73 dark mudstone, and the dark mudstone of Chang- 72 and Chang- 71 has the least contribution. The crude oil has the characteristics of low density, low viscosity and high saturation hydrocarbon. According to the characteristics of biomarkers, crude oil is divided into three types. Class A crude oil has the characteristics of low C30 rearranged hopane/C29 hopane, low C29Ts/C29 hopane, and low Ts/Tm, contributed by black shale, mainly distributed in Chang- 71 Member; Class C crude oil has the characteristics of high C30 rearranged hopane/C29 hopane, high C29Ts/C29 hopane, and high Ts/Tm, mainly contributed by dark mudstone, with a small quantity and limited distribution; The parameters of the biomarkers of Class B oil are all in the middle, which is a mixture of Class A and Class C crude oil, mainly distributed in the Chang- 72 Member. Conclusions: As the black shale was earlier than the dark mudstone in hydrocarbon generation, and the viscosity and density of crude oil were lower, the low viscosity Class A crude oil from the black shale is easy to migrate to the Chang- 72 and Chang- 71 members, while the Class C crude oil generated from the dark mudstone is mainly concentrated in the near source and mixed with Class A oil distributed in the Chang- 72 Member, so the Class A oil formed is mainly distributed in the Chang- 71 Member, the Class B oil is mainly distributed in the Chang- 72 Member, and the distribution of Class C oil is limited. This is of great significance for the next exploration and development and resource evaluation.