Microscopic characteristics analysis of tight sandstone reservoir: a case study from Chang 6 sandstones of Yanchang formation in Longdong area, Ordos basin

Abstract: In recent years, tight oil, an important unconventional oil and gas resource, has become a research hotspot of global oil and gas exploration. The typical representative of the tight oil reservoir is the Chang 6 reservoir of the Yanchang Formation in Ordos Basin. Its lithology is tight, and the pore throat is small and complex, making it difficult to describe the microscopic pore throat characteristics. Under the influence of pore throat structure, tight sandstone reservoir seepage characteristics are also more complex, which is an important factor affecting oil and gas exploration and development. Therefore, how to effectively characterize the microscopic pore throat characteristics of tight sandstone reservoirs is a key issue in the study of unconventional oil and gas resources. To clarify the characteristics and influencing factors of Chang 6 reservoir in this area, the rock mineral composition, diagenesis, and physical properties of Chang 6 tight sandstone in the Longdong area of Ordos Basin were studied utilizing rock core photos, casting thin sections, field emission scanning electron microscope, high-pressure mercury injection and constant rate mercury injection, and the influence of diagenesis on pore throat was qualitatively analyzed. The results show that the sandstone in the study area is mainly lithic feldspar sandstone and feldspar lithic sandstone, and the interstitial material is mainly clay minerals. The reservoir pore types are mainly residual intergranular pores, dissolution pores, and micropores. The pore throats are mainly distributed in the range of 0.004-100 μm, less distributed less than 0.1 μm, and more than 1 μm. The pore radius of each sample is concentrated between 60-348 μm. The throat radius of each sample is dispersed between 0.12-1.5 μm, and the roaring type is fine-micro roar type, showing strong heterogeneity. The throat mainly controls reservoir permeability, and the proportion of small throat increases with the decrease of permeability.