Critical tectonic events and their geological controls on gas generation, migration, and accumulation in the Weibei coalbed methane field, southeast Ordos basin

Abstract

The Weibei coalbed methane (CBM) field has become one of the most productive CBM areas in China in recent decades. Gas content and saturation are highly variable in CBM reservoirs in this area. To better understand the effects of critical tectonic events on the deposition–burial, geothermal, and organic maturation evolutions of the Weibei CBM field were investigated. Based on the evolution history analysis, a dynamic equilibrium model was used to evaluate the generation, retention, migration, accumulation, and dissipation process of gas in the No. 5 coal seam in the Weibei CBM field. The CBM reservoir formation history can be divided into six stages: the shallow-buried and immature bacterial gas stage; the deep buried and primary CBM generation stage; the uplifting to fluctuant buried and slowly mature stage; the accelerated mature secondary CBM generation stage; the large-scale uplifting and CBM dissipation stage; and the structural inversion and further CBM dissipation stage. The critical tectonic events of the Indosinian, Yanshanian and Himalayan orogenies affect different aspects of the evolution of CBM reservoir formation. The Indosinian orogeny mainly controls the primary CBM generation. The Yanshanian orogeny dominates the secondary gas generation and dissipation. The Himalayan orogeny is the main cause of variations in the gas distribution in this area.