Quantitative evaluation of organic porosity and inorganic porosity in shale gas reservoirs using logging data

Reservoir pores space characterization is the core content of shale gas reservoir evaluation. The reservoir pore space of shale gas reservoir is complex, which can be divided into organic pore, clay pore, crack pore, and debris pore. A method for evaluating the four porosity of shale gas reservoir by using logging data is presented in this paper. First, according to the volume model, shale rock is equivalent to three parts: matrix, organic matter, and pore. On this basis, response equations of density logging and neutron logging are constructed. The total porosity of shale reservoirs can be solved by combining the two equations. Second, organic pore is nanopore that remains in organic matter during the conversion of kerogen to oil and gas, which is related to the content and maturity of organic matter. An equation for calculating organic porosity using total organic carbon content and density logging is presented. The scale factor in the equation is obtained from argon ion polishing scanning electron microscope experimental data. Third, based on the GRI porosity experimental data of 8 samples in the study area, the relationship among clay porosity, clay bound water saturation, and clay content was analyzed. It is considered that the clay bound water saturation has a certain correlation with the clay content, but the correlation coefficient is not high, has a positive correlation with the clay content. Based on this, a statistical model of clay porosity, clay content, and total porosity was established. Fourth, based on the dual laterolog response characteristics of microcracks, the fracture porosity of shale reservoirs is calculated by positive and negative simulation proposed by Li et al. In addition, the detrital porosity is equal to the difference between total porosity and organic porosity, clay porosity, crack porosity. The above techniques are applied to the calculation of four porosities of two wells in the Longmaxi Formation of Sichuan basin, China, and good effect has been achieved.