Study on microscopic pore structure of extra/ultra-low permeability reservoirs.

The microscopic study of reservoirs in the Moliqing area is still not complete, and the understanding on microscopic pore structure characteristics, reasons for pore structure difference, and reservoir property controlling factors are unclear. By utilizing the thin casting slice, conventional and constant-speed mercury injection techniques combining multivariate analysis methods, the microscopic pore structure of the 2nd Member of Shuangyang formation reservoir in Moliqing fault depression is finely characterized. Based on the sedimentation and diagenesis, the reason for the pore structure complexity of extra/ultra-low permeability reservoirs is analyzed; and the multi-regression analysis is used to establish the prediction equation of reservoir permeability and the main controlling factors of reservoir physical properties are selected. The results show that reservoirs in the Moliqing area have the characteristics of extra/ultra-low permeability, complex pore structure and strong heterogeneity. The pore structure is mainly with the features of low discharge pressure, unobvious curve platform, poor sorting, and fineness, which can be divided into three types. Class I medium pore-low permeability-fine throat; Class II low port-extremely low permeability-micro fine throat; Class IIIa low pore-extremely low permeability-micro throat, and Class IIIb low pore-ultra low permeability-micro throat. The difference of original sedimentary conditions results in initial difference of microscopic pore characteristics, and the compaction during diagenesis and late carbonate cementation are another important factor leading to complex pore structure. The three parameters such as mean radius of the pore throat, pore structure coefficient, and expulsion pressure have significant effects on physical properties of reservoirs, where the effect of mean radius of the pore throat is the most significant. The research method and results can provide guidance to similar extra/ultra-low permeability sandstone reservoirs and subsequent reservoir classification evaluation. [ABSTRACT FROM AUTHOR]