Unconventional Natural Gas Charging and Enrichment Characteristics in Coal-Measure Tight Reservoirs of Southeast Ordos Basin, North China.

Abstract

Coal-measure unconventional natural gas resources are widespread globally. However, due to the diverse types of reservoirs, thin interbedding, and complex pore-fracture system structures, their exploration and production face challenges. This study aims at the core scientific problem of unclear enrichment mechanism of unconventional natural gas in coal-measure tight reservoirs in southeastern Ordos Basin. The dynamic gas-water distribution characteristics and controlling factors of coal, mudstone, sandstone, and limestone reservoirs were systematically analyzed by gas charging simulation experiment based on nuclear magnetic resonance combined with single- and multifractal theory. The results show that the gas saturation of coal and mudstone is significantly limited (12.10-34.52%) due to small pores and strong heterogeneity. Due to the advantages of large pores and connectivity, sandstone and limestone have significantly higher gas saturation (sandstone reaches 42.68-51.19%). Quantitative analysis of mineral composition revealed a strong positive correlation between quartz content and gas saturation (<i>R</i> ² = 0.84), while clay minerals inhibited gas occurrence (<i>R</i> ² = 0.64). Organic geochemical analysis further confirms that the gas saturation of coal is controlled by organic matter maturity and total organic carbon, while the gas saturation of sandstone depends on the large pore volume of >0.1 μm and pore throat connectivity. Nearby coal or mudstone with high organic matter content, reservoirs with effective pore volume of >0.1 μm, and low clay mineral content may be natural gas enrichment desert areas. This study reveals the charging and enrichment law of unconventional natural gas in multitype tight reservoirs of coal measures and provides theoretical support for the prediction of gas-water distribution in complex heterogeneous reservoirs and the exploration of unconventional natural gas in coal-measure tight reservoirs.