Effective Characteristics and Main Controlling Factors of Tight Gas Reservoirs in Southern Shenfu area

Tight gas reservoir heterogeneity is strong and the effective reservoir prediction is difficult. In this paper, the main gas-producing layers within Carboniferous-Permian Formation in the southern Shenfu area were taken as the research object. The effective characteristics and the main controlling factors of effective reservoirs were studied by means of thin section observation, physical property test, mercury injection experiment and productivity analysis. The results show that the reservoirs are mainly composed of lithic sandstone and feldspar lithic sandstone with low porosity and permeability. Combined with productivity, the lower limit of reservoir physical properties can be manifest as 7% porosity and 0.1 mD permeability. Mercury intrusion-fractal characterization shows that the effective reservoir pores are mainly distributed in the range of 750-3 900 nm, accounting for more than 50%, corresponding to the dissolution pores. However, the tight reservoir pores are concentrated in the range of 0-80 nm, accounting for more than 80%, corresponding to the clay intercrystalline pores. The effectiveness of underwater distributary channel accounts for more than 70%, which is a high-energy facies belt for effective reservoir development, and can be proven by sorting and particle size analysis. The pore type is mainly the dissolution pore with secondary origin, accounting for more than 85%. Generally, the dissolution pore formed after the dissolution of feldspar by organic acids in the late diagenesis stage. The larger the amount of feldspar dissolution, the higher the porosity and permeability. Based on the above analysis, the coupling of high-energy microfacies and secondary dissolution is the main controlling factor of effective reservoir development.