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The tight oil reservoir of the Chang 7 member, Yanchang Formation in the Yan'an area of the Ordos Basin has the characteristics of oil and gas "co-existence" and strong heterogeneity. Unclear understanding of special geological characteristics and reservoir characteristics hinders the exploration and development of continental shale oil and gas. In this paper, a large number of core observations, thin sections, scanning electron microscopy, conventional mercury injection, and nuclear magnetic resonance experiments were used to characterize the reservoir properties of the Chang 7 oil shale. The experimental results show that the main mineral types of shale in the study area are clay minerals, quartz and feldspar, in addition to a small amount of carbonate and pyrite. The average size of rigid particles in argillaceous shale is 5.4 μm, and there are no sand-grade particles. Mud particles (≥8) can account for more than 50% of the medium particles. The TOC difference of shale, laminar shale and laminar shale is obvious, indicating that the TOC of shale has strong heterogeneity. The gas-bearing porosity occupied by shale gas in silty laminae is larger than that of shale. Moreover, the pore development characteristics are mainly intergranular pores and intra granular pores. Before oil washing, the porosity ranges from 0.27% to 8.06%, with an average of 2.35%. The Chang 7 tight oil reservoir contains oil in the pores from macropores to nano-pores, and the pores from micropores to nano-pores are full of oil. The micro-cosmic understanding of the reservoir characteristics of the Chang 7 tight oil reservoir is of great significance to the exploration and development of unconventional oil and gas resources in the Ordos Basin. [ABSTRACT FROM AUTHOR]

The microscopic pore-throat structure characteristics of tight oil reservoirs are the focus and difficulty of tight oil reservoir research, as well as the key to restrict the development of tight oil. After years of development, there are currently many problems such as large differences in horizontal well production and severe production stabilization. Therefore, it is necessary to do in-depth research of reservoir characteristics and comprehensive evaluation of the reservoir. The result of comprehensive reservoir evaluation will directly affect the effect of reservoir development. Taking the Chang 7 Member tight oil reservoir in the northern Shaanxi area of the Ordos Basin as an example, this research was guided by the theory of reservoir geology, sedimentology and logging geology, and basic geological data, logging data, production data and fluids data were used. According to the data, the microscopic pore throat structure of the reservoir is analyzed through observation under microscope and mercury intrusion experiment. According to the observation of casting thin section and scanning electron microscope, the tight sandstone reservoir of Chang 7 Member in the research area mainly developed primary remaining intergranular pores, intragranular dissolution pores, intergranular dissolution pores, inter crystalline micropores and microcracks, among them, the intergranular dissolution pores and intragranular dissolution pores are the main ones. The high-pressure mercury intrusion analysis identified four types of Chang 7 Member tight sandstone reservoir pore structures, of which type I and type II pore throat radii were larger, and the maximum connected pore throat radius was 0.593µm-0.756µm, which showed good connectivity. [ABSTRACT FROM AUTHOR]

The effective stress of tight sandstone reservoirs increases due to the attenuation from the formation energy. The physical properties of reservoirs change correspondingly, which then adjusts the electrical parameters. It is difficult to comprehensively evaluate reservoirs because the electrical parameters obtained from petro-electricity experiments at normal temperatures and pressures cannot accurately reflect the reservoir properties. A total of 45 tight sandstone samples were measured under the effective covering pressure of 23 MPa, pore pressure of 15.6 MPa, and temperature of 60 °C. The following results are shown. (1) Under reservoir conditions, the lithologic coefficient (a) increases, the formation cementation index (m) decreases, and the cementation index (m) is 1.05~1.21 times higher under an effective reservoir stress (30 MPa) than under normal pressures (5 MPa). The cementation index (m) at 80 °C is 0.94~0.98 times higher than that at 30 °C. (2) Under reservoir conditions, the reservoir porosity has a certain stress sensitivity, and lower porosities cause more obvious stress sensitivities. The formation factor (F) increases to some extent and the saturation index decreases significantly. (3) The petroelectrical parameters are closely related to the pore structure and pore size of the rocks as well as the anti-compaction ability of rock particles. [ABSTRACT FROM AUTHOR]