Characteristics and controlling factors of shale oil reservoirs in the seventh member of Triassic Yanchang Formation, western Mahuangshan area, Ordos Basin

The seventh member of the Triassic Yanchang Formation (Chang 7) in the Ordos Basin is abundant in shale oil resources. However, there are significant differences in reservoir characteristics between the basin margin and the basin center. Currently, research on shale oil reservoirs in the western Mahuangshan area at the basin margin remains insufficient, which has constrained shale oil exploration in this area. In this study, a detailed characterization of the shale oil reservoirs of Chang 7 member in this area was conducted using scanning electron microscopy (SEM), high-pressure mercury injection, low-field nuclear magnetic resonance (NMR), nitrogen adsorption, and micro-CT, combined with core observations and thin-section identification. Additionally, carbon and oxygen isotope and major and trace element analyses were conducted to investigate the development mechanism of high-quality reservoirs. The results showed that: (1) The shale oil reservoir in the Chang 7 member of the western Mahuangshan area is mainly composed of fine sandstone, siltstone, and mud shale. The mineral composition is characterized by high contents of felsic and clay minerals, with felsic minerals generally exceeding 50%. (2) The reservoir exhibits diverse space types, and the pore development differs among lithologies. In fine sandstone, intergranular pores and clay mineral intercrystalline pores are dominant, with pore sizes mainly distributed in the 1 to 2 μm range. In siltstone (mainly argillaceous siltstone), both inorganic pores and organic matter pores are developed, with dominant pore sizes near 3 nm and 500 nm. Mud shale is mainly composed of pores with organic-clay composites matter and microfractures, developed with pores below 10 nm and those in the range of tens of nanometers. (3) Analysis of reservoir physical properties and pore structure indicated that interbedded-type fine sandstone reservoirs represent the high-quality reservoir type. During compaction diagenesis, the porosity was reduced by approximately 26% to 33%. Hydrocarbon-generating fluids significantly modified the central parts of the sandstone bodies of reservoirs, and the current porosity generally exceeds 5%. In contrast, the edges of sandstone bodies and laminated-type reservoirs experienced substantial cement sedimentation, resulting in relatively poor physical properties.