Pore structure and physical properties of sandy conglomerate reservoirs in the Xujiaweizi depression, northern Songliao Basin, China

摘要

Pore structure analysis is the basis of reservoir classification and exploitation, especially for tight reservoirs with complex pore throat systems. The sandy conglomerates in the Lower Cretaceous strata in northeastern China have attracted little attention from explorers in the past 50 years, but prospecting of promising tight gas reservoirs has recently begun in this area. Here, we report on the pore structure and physical properties of samples from the Shahezi Formation in the Xujiaweizi depression. Pore throat structure was investigated using thin section observations, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR). Regression analysis was used to analyze the correlation between the various pore structure parameters and the reservoir's physical properties. The results show that the pore types in the Shahezi sandy conglomerates are primarily dissolution pores with few inter-particle pores and micro-cracks. The pores are mostly less than 1 mu m in radius, and the large pores are generally connected by tiny throats with radii of less than 0.1 mu m. The Por(apex) and Por(em) values, which reveal the connected pore volume, were derived from the HPMI capillary pressure curve and the NMR T-2 spectrum, respectively. The Por(apex) and Por(em) values are more strongly correlated with the reservoir's permeability than the other pore structure parameters, including the max throat radius (R-max), the median radius (R-50), the homogeneity coefficient (phi), the sorting parameter (S-p), the displacement pressure (P-d), and the mercury removal efficiency (E-w). Por(apex) and Por(em) account for only a small portion of the total pore volume (average of 37.1%) but contribute (average of 81.7%) to the permeability. By studying the microscopic and macroscopic characteristics, three typical pore structures were identified mainly based on Por(apex) and Por(em). Type I reservoirs are characterized by relatively well connected inter-dissolution pores and high Por(apex)(>2.0), Por(em)(>2.0), and total porosity(>5.5%). Type III reservoirs show poor physical properties with some intercrystalline pores and small Por(apex)(<1.0), Por(em)(<1.0), and total porosity(<3.5%). Type II reservoirs are between type I and type III reservoirs. Favorable reservoirs (Type I and Type II) are located in the strata shallower than 4200 m.