Pore-throat structure and fractal characteristics of dolomite reservoir in the Buqu Formation in the southern depression, Qiangtang Basin

Pore-throat structure of dolomite reservoir with strong heterogeneity, and with low porosity and permeability is very complex, so it is difficult to evaluate the pore-throat structure of the Mid-Jurassic Buqu Formation in Qiangtang Basin. Based on thin section observation, SEM, porosity and permeability test and mercury injection capillary pressure test of twenty dolomite core samples, to study the pore throat distribution and fractal characteristics, and to exploring the relationship between fractal dimension and physical parameters and pore throat structure parameters. The results show that:(1) The types of dolomite reservoir space include fabric-selective porosity, non-fabric-selective porosity and fracture, and according to the characteristics of capillary pressure curves, the pore throat structures were divided into 5 types, and the fractal curve has obvious turning point, a turning point for the distribution of pore throat peak, on behalf of the connectivity of good large-scale pore connectivity to the small scale of the pore throat of poor change. (2) The heterogeneity of small-scale pore throat is weak in layer and strong in interlayer. The fractal dimension of large-scale pore throat is smaller than that of small-scale pore throat, but the distribution range is wider and the difference between samples is obvious, which indicates that large-scale Pore throat is complex and diverse, interlayer heterogeneity is stronger than small-scale pore throat. (3) The fractal dimension of large-scale pore-throat has a good correlation with reservoir parameters and pore-throat structure parameters, which can be used to quantitatively characterize the pore-throat structure of Buqu Formation dolomite reservoir. The smaller the fractal dimension of large-scale pore-throat, the better the reservoir physical property, and the larger the proportion of large-scale pore-throat, the greater the contribution to the overall permeability of reservoir. (4) The reservoir space and pore-throat structure of Buqu Formation dolomite reservoir were based on the carbonate sediment pores formed by deposition. Dolomitization and dissolution jointly control the difference of pore-throat structure.