The eighth member of the Permian Shihezi Formation is one of the main tight sandstone gas layers in the Longdong Area of Ordos Basin, and the source rocks are dark mudstones and shales located in the Shanxi Formation and Taiyuan Formation of the Permian. The tight muddy sandstone at the top provides shielding conditions and constitutes traps. The lithology is mainly lithic quartz sandstone, followed by lithic sandstone. The reservoir space is mainly dissolved pores, inter crystalline pores, intergranular pores and so on. Clay minerals are the main interstitial materials, and chlorite has the highest content in it, a product of alkaline, moderate- to high-temperature, reducing conditions, effectively inhibited quartz cementation and enhanced secondary porosity development during mesodiagenesis. The average porosity of the reservoir is about 4.01%, and the average permeability is about 0.5 × 10⁻³ μm³, which is a typical low porosity and ultra-low permeability tight reservoir. The thickness of the sandstone reservoir in the study area is from 5 m to more than 25 m, mainly in the NE direction. The sand bodies are distributed in lenses on the plane.

Unconventional oil and gas resources are important energy sources for alleviating energy crises and maintaining sustainable social development. Therefore, the subsequent exploration and development of the basin summarizes and analyzes the reasons for the difference in natural gas enrichment between the southern and northern parts of the Ordos Basin. The tight sandstone gas of the Upper Paleozoic in the Ordos Basin has become a key field for increasing reserves and production of natural gas in the basin. As a high-quality tight sandstone gas reservoir, the eighth member of the Shihezi Formation only has good gas production in the south of the basin. For this reason, the types of natural gas source rocks in the north and south of the Upper Paleozoic were summarized, and the main types of gas source rocks in the north and south were clarified. The sedimentary facies of the eighth member of the Shihezi Formation and the Shanxi Formation were analyzed. The results show that the main gas-producing source rock in the northern part of the basin is coal rock, and in the southern part, it is dark mudstone. The Permian Shanxi Formation in the northern part of the basin mainly develops the delta front, which provides good conditions for the development of coal and rock. Compared with the northern Shanxi Formation, the Permian Shanxi Formation in the southern Ordos Basin mainly develops marine facies and marine-continental transitional facies, and the coal seam is not created. Therefore, the natural gas produced by the coal of the Shanxi Formation can easily migrate to the good reservoir in the eighth member of the box, which is also the reason for the difference in the enrichment of tight sandstone gas in the north and south of the Ordos Basin.

This study focuses on the tight sandstone reservoirs of the He 8 Member (Lower Permian Shihezi Formation) in the western Sulige area, Ordos Basin. Multiple analytical methods were integrated, including core observation, thin-section analysis, X-ray diffraction (XRD), and rock mechanics experiments, to systematically evaluate the reservoir’s petrology, pore microstructure, physical properties, and fracture formation mechanisms. Results indicate that the reservoir is primarily composed of quartz arenite (78%), characterized by low porosity (avg. 5.5%) and permeability (avg. 0.15 mD). The pore system comprises dissolution pores, lithic dissolution pores, intergranular pores, and intercrystalline pores. Depositional microfacies significantly influence reservoir quality. Subaqueous distributary channel sands exhibit the best properties (porosity > 5%), followed by mouth bar deposits. The reservoir experienced intense compaction and siliceous cementation, which considerably reduced primary porosity. In contrast, dissolution and tectonic fracturing processes significantly enhanced reservoir quality. Rock mechanics tests reveal that highly heterogeneous rocks are more prone to fracturing under differential stress (σ₁–σ₃). These fractures considerably improve the flow capacity of tight reservoirs.

Reservoir evaluation in multi-source systems is challenging because studies generally follow single-source principles. This limitation has substantially hindered the understanding of reservoir and hydrocarbon accumulation processes in source–reservoir systems. This study examines the Dingbian area of the Ordos Basin, China, and investigates the key factors controlling hydrocarbon accumulation in the Chang 9 reservoir of the Triassic Yanchang Formation within a multi-source system. The study area spans approximately 0.9 × 10⁴ km². First, by comparing the biological markers in Chang 9 crude oil with those of potential source rocks, the oil source of the Chang 9 reservoir was identified. The study area was subsequently divided into three provenance zones—northeast, northwest, and central mixed source areas—based on heavy mineral content and the orientation of sedimentary sand bodies. Additionally, well logging data, oil production data, petrographic thin sections, scanning electron microscopy (SEM), and mercury injection porosimetry were used to investigate the reservoir characteristics, oil reservoir features, and crude oil properties across different source areas. The results indicate that the oil source of the Chang 9 reservoir in the Dingbian area is the Upper Chang 7 source rock. The northwest source area exhibits superior reservoir properties compared to the other two zones. In the northwest source area, lithology-structure oil reservoirs are predominant, whereas the central mixed source area is characterized by structural-lithology oil reservoirs, and the northeast source area predominantly features lithology-controlled reservoirs. From the northwest to the central mixed source areas, and finally to the northeast source area, crude oil density and viscosity increase gradually, while the degree of oil–water separation decreases correspondingly. Based on these findings, the study concludes that the distribution of structures, lithology, and source rocks significantly influences the Chang 9 reservoirs in the Dingbian area. The controlling factors of oil reservoirs differ across the various source zones. In multi-source systems, evaluating oil reservoirs based on source zones provides more precise insights into the characteristics of reservoirs in each area. This approach provides more accurate guidance for exploration and development in multi-source regions, as well as for subsequent “reserve enhancement and production increase” strategies.