Logging evaluation of free-gas saturation and volume content in Wufeng-Longmaxi organic-rich shales in the Upper Yangtze Platform, China

Abstract The Wufeng-Longmaxi shale is the most important target for the exploration and production of shale gas in China. There are two important parameters of great importance to shale gas evaluation: free gas saturation and content. However, current models that have been proposed based on the use of sandstone are not applicable to this shale gas reservoir due to differences in conductivity mechanisms between these two types of reservoirs. In this study, after the pore volume of gas shale had been classified into three phases, those being the adsorbed phase pore, free phase pore, and bound water pore, we proposed a method to calculate the free gas saturation and content from well logging data based on the pore volume model. Our method utilizes the following procedures: 1) determining the total porosity on the basis of the matrix value of dry clay; 2) establishing a quantitative correlation of the bound water content to the clay content and then determining the bound water porosity; 3) calculating the absorbed gas content by Langmuir formula; 4) calculating the adsorbed phase porosity according to the principle of mass conservation after determining the underground density of the adsorbed phase (in the range of 0.4–1.2 g/cm 3 ); and 5) determining the free gas saturation and free gas content. Field examples show that the predicted shale porosity, free gas, and adsorbed gas contents calculated by using our proposed model accurately coordinate with the core derived results, which verified that the proposed method was valuable to evaluating the Wufeng-Longmaxi shale gas reservoir.