Gas-bearing characteristics of coal measure strata and logging evaluation of fluid pressure systems

Within a single fluid pressure system, fluid migration occurs between different coal measure reservoirs, and the in situ accumulation of coal measure gas (CMG) represents the ultimate outcome of methane undergoing a series of dynamic migration processes. Clarifying the vertical distribution patterns of in situ fluids within coal measure strata is of critical importance for guiding the co-exploration and co-production of CMG. However, direct testing of such distribution patterns remains challenging. Geophysical logging provides an indirect characterization method by evaluating reservoir permeability and gas content. In this study, based on the physical property testing results of coal measure rock samples and their corresponding logging curves from the southeastern margin of the Ordos Basin, a progressive computational model of logging parameters, porosity, and breakthrough pressure was developed for quantitatively evaluating the sealing ability of coal measure strata. Additionally, a comprehensive predictive model for gas-bearing layer was constructed based on the P- and S-wave velocity of rock samples with varying gas-water saturation levels, the logging parameters of density and neutron from coal measure strata, and the logging results of hydrocarbon content. By combining the logging evaluation results of lithology, breakthrough pressure, and gas content, the coal measure strata in the study area were classified into three fluid pressure systems. The reliability of this classification was validated through the analysis of gas characteristic and reservoir pressure differences among the identified fluid pressure systems.