Development geology of unconventional natural gas: Enrichment mechanisms, sweet spot evaluation, and development response

Unconventional natural gas has become an important contributor to the growth of global natural gas reserves and production expansion. However, pronounced heterogeneity among different reservoir types results in substantial variations in enrichment mechanisms, sweet-spot formation, and development responses, thereby hindering the establishment of a unified framework for unconventional gas development geology. This study systematically compares shale gas, tight gas, shallow coalbed methane (CBM), and deep CBM within an integrated framework that links enrichment mechanisms, sweet-spot evaluation, and development response. The results reveal distinct geological controls among these unconventional systems. The shale gas is characterized by source–reservoir integration, nanoscale storage, and favorable preservation conditions; the tight gas is controlled by near-source charging and permeability-limited flow; the shallow CBM is dominated by adsorption-controlled enrichment; and the deep CBM exhibits a coupled accumulation mode involving both free and adsorbed gas. Sweet spots are defined as reservoir intervals where favorable geological enrichment conditions can be effectively converted into engineering recoverability. Their identification requires the integrated evaluation of geological conditions, engineering parameters, and production performance rather than relying on individual indicator anomalies. Development responses reflect the evolution of enrichment characteristics during stimulation and production. Shale gas productivity is primarily governed by fracture-network conductivity and matrix gas supply, whereas tight gas development depends on reservoir connectivity and geological heterogeneity. Shallow CBM production is controlled by drainage-induced desorption and subsequent gas transport processes, whereas deep CBM production is characterized by an initial stage dominated by free-gas recovery followed by increasing adsorbed gas contributions. This study demonstrates that effective unconventional gas development requires an integrated understanding of enrichment mechanisms, sweet-spot evaluation, and production responses. Future research should focus on the quantitative characterization of enrichment processes, dynamic prediction of sweet-spot distribution, and cross-scale production modeling to improve the efficiency of unconventional gas development under complex geological conditions.