Natural gas accumulation characteristics and orderly distribution pattern of gas reservoirs in the whole petroleum system of the Carboniferous-Permian coal measures, Ordos Basin

Since 2021, significant breakthroughs have been achieved in the exploration of deep coalbed methane (CBM) or coal-rock gas in the Carboniferous-Permian coal measures of the Ordos Basin, revealing broad development prospects. Meanwhile, wide attention has been directed to the genetic correlations and commingling production potential between CBM gas and other natural gas, such as tight gas, across multiple Carboniferous-Permian sequences within the basin. Based on the theory and a quantitative evaluation method of the whole petroleum system, we analyze the genetic mechanisms and interrelationships of various natural gas reservoirs that have already been identified in the basin. Accordingly, we establish the accumulation model of natural gas and define the orderly distribution pattern of gas reservoirs in the whole petroleum system of the coal measures. The results indicate that the boundaries of the whole petroleum system in the Carboniferous-Permian coal measures extend across the entire basin. The whole petroleum system exhibits three dynamic boundaries from shallow to deep: the buoyancy-driven hydrocarbon accumulation depth (BHAD), the hydrocarbon accumulation depth limit (HADL), and the active source-rock depth limit (ASDL). Accordingly, three dynamic fields of hydrocarbon accumulation are developed between the Earth’s surface and the three dynamic boundaries, namely the free-hydrocarbon dynamic field, the confined-hydrocarbon dynamic field, and the bound-hydrocarbon dynamic field from top to bottom. During the evolution of the whole petroleum system, hydrocarbons expelled from source rocks migrate and accumulate predominantly under the action of buoyancy in the free-hydrocarbon dynamic field of the intermediate to shallow strata, and then enter into high-porosity, high-permeability reservoirs within traps to form conventional natural gas reservoirs. In contrast, within the confined-hydrocarbon dynamic field of the intermediate to deep strata, hydrocarbon migration and accumulation from source rocks are predominantly driven by differences in capillary pressure between the source rocks and reservoirs. Consequently, these hydrocarbons migrate into adjacent low-porosity, low-permeability reservoirs, resulting in the formation of tight gas reservoirs. In the bound-hydrocarbon dynamic field, primary hydrocarbons retained in source rocks migrate and accumulate predominantly through adsorption, contributing to the formation of CBM reservoirs characterized by integrated source rocks and reservoirs. The natural gas reservoirs in the basin generally display an orderly distribution pattern, with CBM reservoirs, tight gas reservoirs, and conventional gas reservoirs occurring sequentially from bottom to top. In the eastern Ordos Basin, the intermediate to shallow strata contain only limited conventional gas due to the overall uplift and the presence of tectonic fractures. In contrast, the intermediate to deep strata in this region demonstrate widely distributed tight gas, substantial thickness of target strata, varied gas-rich plays, and considerable resource potential, establishing them as an important target for sustainable exploration and development.