INFLUENCE OF MICROSCOPIC PORE STRUCTURE DIFFERENCES ON RESERVOIR MOVABLE FLUID OCCURRENCE IN LOW PERMEABILITY SANDSTONE RESERVOIRS——TAKING XI 2TH MEMBER RESERVOIR OF THE NIUQUANHU AREA IN THE SANTANGHU BASIN AS AN EXAMPLE

Abstract: In order to study the microscopic pore structure differences on reservoir movable fluid occurrence in low permeability sandstone, this paper takes Xi 2th member of the Xishanyao Formation in the north, east and south of the Niuquanhu area in the Santanghu Basin as an example to conduct a comprehensive study on pore structure and its affection on movable fluid distribution. Based on core observation and microscopic sheet identification, cast thin sections and scanning electron microscopy have been used to analyze the microscopic pore structure types. Combining with results from constant velocity mercury intrusion experiment, microscopic pore structure characteristics have been quantitatively described. The occurrence characteristics of movable fluid in samples have been fully studied by nuclear magnetic resonance experiment. Based on the comparison of the pore structure and the characteristics of the movable fluid in samples from the north, east and south area, the influence of pore structure has on the occurrence of the movable fluid have been analyzed and summarized. The results show that there are various types of pores in the study area which mainly including residual intergranular pores, tuff dissolved pores and feldspar dissolved pores. Although the pore radius in three area is not much different, the throat radius, especially the mainstream throat radius, is significantly different. The throat of the north area is not well developed, and the radius of the throat is smaller than that of the south and east area. In terms of quantity, the number of pores per unit volume and the number of throats per unit volume are not much different in the north and east area, and less in the south area; The types of movable fluid saturation in the east and south areas are mainly II and III. The north area have a more complex fluid types including II, III and IV which shows a strong heterogeneity; Microscopic pore structure is an important factor that affects the occurrence of movable fluid in the reservoir; The development of the throat radius is the main reason for the difference of movable fluids occurrence; The existence of movable fluid in the reservoir also has a very important relationship with the heterogeneity of the pore structure. When the heterogeneity of the pore structure is strong, the difference between the pore and the throat is large, which is more detrimental to the occurrence of the movable fluid of the reservoir.