STUDY ON PORE STRUCTURE AND ITS CONTROLLING FACTORS OF HIGH-QUALITY SHALE RESERVOIR

15 marine shale samples (TOC>2%) of Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation are chosen from high yielding well A (20 million m³/d) and well B which only has gas testing reaction in Sichuan Basin, and 3 samples of Lower Cambrian Niutitang Formation are chosen from low yielding well C (410 m³/d) around Sichuan Basin. Pore structure and its controlling factors of high-quality shale reservoir are studied by testing TOC, R_o, XRD, low pressure N₂ adsorption analysis, high pressure CH₄ adsorption analysis (30°, 0~15MPa, dry basis) and FE-SEM microscopy and applying Image J software to count the face porosity of organic pores and classifying lithofacies. Samples are all over-mature organic shale (TOC>2%, Ro>2.5%, Ⅰ type of kerogen). By comparison, the following four essential elements should be acquired for high yielding shale reservoir. ①Sufficient adsorption surface area (27.1~41.0 m²/g, 34.8 m²/g in average), enough pore volume (2.68×10^-2cm³/g in average) and proper pore size distribution. The ratios of BET surface and pore volume for micropores (0~2nm), mesopores (2~50nm) and macropores (>50nm) are 6.5: 3.5: / and 3.6: 5.7: 0.7. ②Intensive heterogeneity or high fractal dimension(D≈2.88). The pore size distribution has been divided into three sections.Adsorption pores are smaller than 6~9nm, Percolation pores are larger than 47.6nm, and the intermediate section is the transition pores. ③High face porosity of organic pores (18.2%~32.5%, 25.7% in average).The frequency distributions of pore size counting by Image J software are consistent with the result of pore distributions by low pressure N₂ adsorption. ④Large CH₄ adsorption capacity (V_L=3.64~4.66 m³/g, 4.1 m³/g in average) and great adsorption potential (P_L=1.98~2.74MPa, 2.28MPa in average). The adsorption potential is greater and the CH₄ adsorption capacity under equal pressure is larger when the value of P_L is lower. The porous type pores (type H4) and the ink-bottle pores (type H2) have greater potential for methane adsorption (IUPAC).The TOC content is the main controlling factor for the development of shale pores in the vertical sequence. Lithofacies controls the characteristics of shale reservoirs in the transverse distribution. The mud-rich silica shale (type S-3) is the optimization of lithofacies for high yielding reservoir