Shale pore volume prediction based on statistical method

The reservoir space of shale gas is closely related to the characteristics of the mineral characteristics of reservoir.The paper, taking the shale in Longmaxi Formation in the eastern margin of Sichuan basin as example, a series of tests were conducted. Based on the results of mineral composition, microelement analysis, and geochemical parameters, as well as the low temperature nitrogen adsorption and high resolution images, a pore volume prediction equation was formulated through multivariate statistical method. Then the pore diameter distribution and its affecting factors were explored with this new model. It is showed that in Longmaxi shale formation the mineral composition has big difference in the middle and the bottom, and the abundant biogenic quartz is the main reason for the high quartz content in the bottom of Longmaxi Formation. The nano pores in shale mainly rank 2~5 nm, and contributes about 64.2%~70.1% of the total pore volume. The formulated equation indicates that pore volume has a strong correlation with shale mineral composition,and standardization of the regression coefficient shows that organic matter, clay minerals and brittle minerals have a diminishing effects on pore volumes. Brittle mineral pores, inter-granular pores in clay pieces and intra-granular pores in clay minerals are the main pore types in clay-rich shale, and silt pores is the most common, usually larger than 2nm in diameter. The pore volume of organic pores is mainly controlled by the value of TOC, and the surface porosity which is primarily contributed by 2~5 nm pores is 8.8%~12.5%.The TOC value and thermal maturity of shale are the primary controlling factors for the pores smaller than 2nm in diameter, while the development of pores larger than 50 nm is mainly controlled by clay minerals, quartz and feldspar.