Pore structural characteristics and pore-fracture combination modes of reservoirs in Chang 6 tight sandstone oil reservoirs

In order to research the relationships between the pore structural characteristics and the oil flowing in tight oil reservoirs, F200 field scanning electron microscope (SEM) was used to microscopically analyze the tight oil samples of Ganguyi Oilfield in the Ordos Basin, and the microscopic pore-fracture system of the sample reservoir was established. Then, the distribution of pore throats and the contribution rate of pores and fractures of different sizes to the permeability were analyzed by conducting mercury injection experiments. Finally, combined with the SEM results of the samples, the effectiveness of pores and fractures to hydrocarbon storage and flow was inferred. It is indicated that the pores in the tight oil reservoirs in this area are mainly divided into four types, i.e., primary pore, intragranular dissolved pore, intergranular (matrix) dissolved pore and intercrystalline pore. Intragranular and intergranular dissolved pores are the main reservoir space of oil and gas, primary intergranular pore is merely the secondary reservoir space of oil, and a great number of nano-scale pores are the main reservoir space of oil. Micro-fractures are dominant and they are mainly composed of calcite cleavage fissure, dissolved fracture and diagenetic contraction fracture, among which the former two are the main pathway of flow and migration. Micro-fractures and hydraulic fractures constitute fracture networks. Micro-pores (< 2 nm) are effective for oil and gas storage, but ineffective for oil and gas flowing. Thus, it is concluded that the oil storage and flowing pores of tight oil reservoirs in this area are in the mode of "nano-scale dissolved pores acting as oil reservoir space, and micro-fractures and hydraulic fractures acting as oil flowing pathway". Accordingly, pore-fracture combinations are divided into three modes, i.e., dissolved pore-fracture combination mode, calcite cleavage fissure+dissolved pore combination mode and independent fracture or pore mode. The first two combination modes have small resistance to fluid flow and they shall be focused on when sweet spots are selected.