Dynamic evolution and differential enrichment of deep coalbed methane: A case study in Qinshui Basin

Abstract

Gas content is a critical factor in evaluating the resource potential and predicting the productivity of deep coalbed methane reservoirs. This study investigates the evolution of gas content in deep coal reservoirs over geological time and explores models of differential gas enrichment. Using Carboniferous-Permian deep coal reservoirs in the Yushe-Wuxiang block of the Qinshui Basin as a case study, fluid inclusion analysis, basin modeling, and theoretical gas content calculations were combined to quantitatively reconstruct the pressure and gas content evolution during the uplift of these coal reservoirs. Fluid inclusion analysis revealed homogenization of temperatures ranging from 85 °C to 155 °C in the coal-bearing strata, indicating trapping pressures between 33.16 MPa and 46.42 MPa. These inclusions were trapped between 228.05 and 216.09 million years ago, coinciding with the period of maximum burial depth, and exhibit a pressure coefficient of 1.05–1.19, confirming an overpressure state. During the uplift phase, decreasing temperature and pressure led to the conversion of free gas into adsorbed gas. This transition resulted in a gradual increase in adsorbed gas content while free gas content declined. Intense tectonic activity during the Himalayan orogeny induced fracturing and depressurization of the coal reservoirs, causing substantial dissipation of free gas. Consequently, the deep coal reservoirs in the Yushe-Wuxiang block currently exist at normal pressure and are devoid of free gas. Comparative analysis of gas content and reservoir pressure evolution across different blocks led to the proposition of four distinct coalbed methane enrichment models: (1) deep burial and weak tectonic reworking, (2) deep burial and intense tectonic reworking, (3) shallow burial and weak tectonic reworking, and (4) shallow burial and intense tectonic reworking. The deep burial and weak tectonic reworking model are identified as a promising target for coalbed methane exploration and development in North China. This study elucidates the mechanisms responsible for differential gas content enrichment in deep coal reservoirs, providing valuable insights for guiding the exploration and development of deep coalbed methane resources.