Lower limits of evaluation parameters for the lower Paleozoic Longmaxi shale gas in southern Sichuan Province

Here we present six key parameters that have been applied to evaluate the Silurian Longmaxi shale gas accumulations of southern Sichuan Province. These parameters include richness of organic matter, single layer shale thickness, burial depth of the shale, maturity, shale properties, and brittle mineral content. In combination with drilling data, the lower limits of these six parameters for marine shale gas have been evaluated and the following conclusions are drawn: (1) Organic carbon recovery, gas content inversion, and production tests have been applied for the first time to measure the lower limit of TOC, which is determined to be 1.0%. (2) Based on the hydrocarbon generation and expulsion theory for source rocks and the characteristics of Longmaxi shale, the lower limit of the single-layer thickness is estimated to be 30 m, of which a thickness of at least 15 m should contain high-quality shale. (3) Based on the correlation of TOC and porosity, the lower limit of gas-filled porosity is 1.2%. (4) Other parameters, such as maturity, burial depth, and brittle mineral content have also been studied with regard to the geologic conditions of southern Sichuan. Based on the study results, an evaluation index system has been established for highly evolved marine shale gas, and the system has been applied successfully to geologically screened shale gas occurrences in southern Sichuan Province.