A classification method for shale gas reservoir types with free gas as the core parameter: A case study of Lower Silurian Longmaxi Formation marine shale gas reservoirs in the southern Sichuan Basin

There is a set of organic rich shale (TOC> 2%) of 30-50 m in thickness in the Lower Silurian Longmaxi Formation of the southern Sichuan Basin, but during its development, the shale gas production rate varies greatly in different layers. There is a big contradiction between the classification result according to the original evaluation standard and the actual productivity. To this end, this paper discusses the reasons for the contradiction between the original evaluation standard and the evaluation result based on the comparison of shale gas reservoir evaluation indicators, by analyzing a large number of experimental data. Then, combined with the production performance data, the shale gas reservoir classification method was studied, and a new shale gas reservoir classification method and standard with free gas as the core parameter was established. Finally, the new method was verified based on the practical shale gas development in the Weiyuan Block of southern Sichuan Basin. And the following research results were obtained. First, the reason for the above mentioned contradiction is that the control effect of shale gas occurrence mode on the gas production rate is not fully considered. Second, the new standard is mainly composed of TOC, adsorbed gas volume, brittle mineral content, porosity and free gas saturation. The main difference between the new standard and the old standard is that adsorbed gas content and free gas content are distinguished and the free gas content is replaced with the easily obtained gas saturation in the new standard. Third, the shale gas reservoir in the study area can be classified into three types, namely free type, adsorption type and transition type (mixed type). The free-type siliceous shale gas reservoir is the main high-yield interval of Longmaxi Formation shale in this area, and the transition-type shale gas reservoir is the potential favorable target interval. Fourth, the practice has proved that the new classification standard is of good applicability. In conclusion, the new classification method can distinguish different types of shale gas reservoirs more effectively and define the high-yield shale gas intervals accurately. © 2021, Natural Gas Industry Journal Agency. All right reserved.