Characteristics of natural fractures in the upper Paleozoic coal bearing strata in the southern Qinshui Basin, China: Implications for coalbed methane (CBM) development

2020 
Abstract Natural fractures, as the effective storage space and fluid flow pathway of hydrocarbons, play a critical role in the exploration and development of unconventional gas resources in the coal bearing strata. In the present study, to reveal the effects of natural fractures on coalbed methane (CBM) preservation and development, the types, origins and development characteristics of natural fractures in the upper Paleozoic Permian Shanxi Formation in the southern Qinshui Basin were systematically described and analyzed based on outcrops, cores, borehole image logs, thin sections, and SEM. Four types of natural fractures have been identified in the Shanxi Formation according to their geologic origins, including the endogenic fractures (cleats) and exogenous fractures in the No.3 coal seam, and tectonic fractures and diagenetic fractures in its roof and floor. Tectonic fractures can be further divided into small faults, intraformational shear fractures, intraformational open fractures and slip fractures, while diagenetic fractures are mainly bed-parallel fractures. The statistical results show that the development and distribution characteristics of those types of fractures are significantly different, which are mainly influenced by lithology, layer thickness and geological structure. Further, natural fractures with different types and distributions show different influences on CBM preservation and well productivity. Exogenous fractures can effectively improve the seepage capability of coal reservoirs and are conducive to CBM wells productivity, whereas endogenic fractures have less contributions to coal permeability due to their poor effectiveness. The intraformational open fractures in the fold axis zones, and intraformational shear fractures and small faults in the fault zones are generally excessive-developed. Those fractures may result in gas dissipation and external water supplement, which have negative effects on CBM preservation and production. However, moderate-developed intraformational shear fractures in the sandstone roof can favorably affect the pressure reduction of coal reservoir and improve the recovery potential of unconventional gas resources. These investigations may serve as a geological basis for unconventional gas exploration and development of the upper Paleozoic coal bearing strata in the southern Qinshui Basin.
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