MICROPORE STRUCTURE CHARACTERISTICS OF TIGHT OIL RESERVOIRS IN THE YANCHANG FORMATION IN THE ORDOS BASIN, CHINA

Pore structure is an important factor affecting the reserve and recovery of tight reservoir. In this paper, taking shale samples from the Triassic Chang 7 Member in the Ordos Basin as examples, the porosity, pore type, and pore size distribution of tight oil reservoirs were studied using scanning electron microscopy, high-pressure mercury tests, and CO2 and N-2 adsorption methods. The key factors affecting the pore structure of shale were discussed. The results show that the porosity of the Late Triassic shale in the Ordos region is 1.30-13.37%, with an average of 4.56%. The types of shale micropores are intergranular pores, organic pores and intragranular pores. Organic pores are the main type of pores. The shale mainly has micropores (<2 nm) and mesopores (2-50 nm). The total shale surface area accounts for 94.3% of the total specific surface area, and its pore size is mainly distributed in three intervals: 0.4-0.7 nm, 3-10 nm, and > 50 nm. Shale pore structure is affected by shale organic carbon content, organic matter maturity, and mineral content. As shale porosity and pore volume increase, its maturity R-o increases and clay mineral content increases, but shale porosity and pore volume gradually decrease.