Pore-Throat Structure in the Chang 7 Subformation Influencing the Differential Accumulation of Tight Sandstone Oil: Insight from SEM, Rate-Controlled Mercury, and High-Pressure Mercury Results

Abstract

The complicated pore-throat structure of unconventional resource formations such as tight sandstones influences petroleum accumulation and even exploitation, because the pore-throat structure is difficult to mark. A pore-throat system was distinguished quantitatively utilizing high-pressure mercury (HPM), scanning electron microscopy (SEM), casting film, and rate-controlled mercury (RCM) methods. According to the analysis of the reservoir, the factors of differential accumulation of tight oil in Longdong and Northern Shaanxi were assessed. The results showed that lamellar throats and tube bundle throats were the primary types of throats in the study area. Tight sandstones from Longdong have a higher content of large throats than those from Northern Shaanxi, both with pore radii of 100–200 μm, whereas Northern Shaanxi has better pore-throat connectivity and better homogeneity of pores and throats. The fractal dimension of fine pores (D2) is significantly correlated with the pore-throat structure, including the median pressure and mercury saturation of macropore throats. Therefore, the pore-throat structure can be characterized by the fractal dimensions of small pores and throats. The continuous distribution of tight oil in Northern Shaanxi, with a higher degree of tight oil than Longdong, is positively related to the good pore-throat connectivity of the reservoir and the relatively small fractal dimension of the small throat. Thus, the differential accumulation of tight oil was influenced by the pore-throat connectivity and heterogeneity of the reservoir structure. The results of this study can be helpful in evaluating reservoir characteristics and selecting exploration zones.