CHARACTERIZATION OF MICROSCOPIC PORE AND THROAT STRUCTURE OF TIGHT SANDSTONE RESERVOIR BY MULTI-METHODS - A CASE STUDY OF CHANG 9 RESERVOIR OF YANCHANG FORMATION IN BAIBAO AREA OF ORDOS BASIN

The pore-throat structure of tight sandstone reservoirs restricts the accumulation and development potential of oil and gas. Chang 9 reservoir of Yanchang Formation in Baibao area of Ordos Basin has complex pore-throat configuration relationship. In order to further explore the micro pore-throat structure characteristics of Chang 9 sandstone reservoir in the study area, 17 core tight sandstone samples were conducted by nuclear magnetic resonance (NMR) test, casting thin sections, scanning electron microscopy (SEM), clay mineral X-ray diffraction, high-pressure mercury injection and rate-controlled porosimetry (RCP). The effective porosity of movable fluids can be obtained by calculating the saturation of movable fluids. This parameter can reflect the reservoir performance and seepage capacity, and quantitatively characterize the effective pore space of compact reservoirs, which makes up for the deficiency of existing parameters such as porosity in evaluating the effective pore throat space of reservoirs. According to the effective porosity of movable fluids and other parameters, Chang 9 reservoir in the study area is divided into three types. The results show that the effective porosity of movable fluid in type I reservoir is more than 5%, that of type II reservoir is between 3% and 5%, and that of type III reservoir is less than 3%. From type I to type III reservoirs, movable fluid occurrence ability, the proportion of large pore-throat development and homogeneity of pore-throat become worse in turn. The content and occurrence form of clay minerals affect the connectivity of pores. According to the classification criteria of pore-throat structure, it provides a theoretical basis for oil and gas development of tight sandstone reservoirs in the study area.