Pore structure evaluation of tight reservoirs in the mixed siliciclastic-carbonate sediments using fractal analysis of NMR experiments and logs

The study on mixed depositional reservoirs is widely attended in recent years. However, the mixed depositional reservoirs are generally characterized by complex pore structure, which is caused by strong heterogeneity, complex mineral compositions, and varying pore sizes, presenting a major challenge to describe the difference in pore structure between different samples. To address this challenge, the pore structure of mixed depositional reservoirs was evaluated by the fractal dimensions, and the influences of various mineral compositions on pore structure were analyzed. The fractal dimensions derived from NMR measurements or NMR logging data can be divided into two segments at T-2cutoff values, representing different fractal values and characteristics of pore structure. For the E-3(2) Formation in Yingxi field, the fractal dimension of movable fluid pores ranges from 2.628 to 2.917 (average 2.765), whereas the bound fluid pore systems are not following the fractal theory. Meanwhile, the fractal dimensions show a negative relationship with movable fluid porosity and permeability. The effects of various mineral compositions on fractal laws of the pore networks are different. Fractal dimension increases with the increases of clay mineral contents, and the decrease of dolomite content, while the effect of calcite content on fractal laws of the pore system is uncertain, and quartz from different genetic type represents different effect on fractal dimensions. This study is contributed for petroleum exploitation in mixed depositional reservoirs and shows that the fractal dimension can comprehensively indicate the complexity of the pore structure in mixed depositional reservoirs.