Responding mechanism of macro-micro characteristics and water abundance on Jurassic sandstone in Northern Shaanxi Province

The water abundance of sandstone aquifer is the basis of mine water prevention and control. With the gradual development of the Jurassic coal resources in northern Shaanxi areas, the study of water abundance of Jurassic sandstone aquifer is of great significance to ensure safe and efficient mining. However, how its heterogeneity affects water abundance, that is the influence mechanism is still unclear. Taking Dahaize Coal Mine as the research area, through the division of sedimentary facies and 38 typical sandstone samples casting thin section image analysis, mercury injection, nuclear magnetic resonance and so on laboratory tests, combined with the results of borehole pumping test, the sedimentary facies, micro pore structure and its relationship with water abundance of Jurassic Zhiluo Formation and Yan’an Formation were studied. The response mechanism of sandstone microscopic pore structure on water abundance is revealed. The results show that the lower Zhiluo Formation in Dahaize Coal Mine mainly develops river sand dam and flood plain sedimentary microfacies of river sedimentary system; The third section of Yan’an Formation mainly develops distributary channel and interdistributary bay sedimentary microfacies of delta sedimentary system. The river sedimentary system is generally more water abundance than the aquifer of delta sedimentary system. The water abundance of channel sand dam dominated by coarse sandstone and medium sandstone is stronger than that of flood plain dominated by medium sandstone and fine sandstone. The water abundance of distributary channel with large sandstone thickness and sand ratio is larger than that of interdistributary bay with small sandstone thickness and sand ratio. The microscopic pore structure could be divided into four types by physical properties, lithology, pore distribution, connectivity, which are type Ⅰ for large pore and coarse throat, type Ⅱ for mesopore and middle-fine throat, type Ⅲ for keyhole and fine throat, type Ⅳ for small- microporous and micro-fine throat, and the pore structure becomes worse and the water abundance becomes weaker from I to IV. The macroscopic sedimentary facies and microscopic pore structure of the sandstone jointly characterize the water abundance of aquifer. It provides a new idea for the characteristics and scientific prediction of water abundance of sandstone aquifer, and provides theoretical basis for mine water control.