Research progress of aquifer of Zhiluo Formation in northern Ordos Basin and suggestions on water hazard prevention

The raw coal output of Jurassic coal field in Ordos Basin continues to increase, which plays an important role in the national energy security. In recent years, with the increase of mining depth, the threat of roof water hazard with the Zhiluo Formation aquifer as the source of water inrush has become increasingly prominent. This paper systematically expounds the spatial occurrence characteristics of the Zhiluo Formation in the northern Ordos Basin, the spatial relationship with coal seams, the characteristics of mine water inflow from the Zhiluo Formation aquifer as the water source, and the occurrence characteristics and migration laws of the Zhiluo Formation groundwater. The problems and prospects of the prevention and control of water hazards in the Zhiluo Formation aquifer are also summarized. The results show that the Zhiluo Formation can be vertically divided into upper, middle and lower members, of which the lower member is a set of fluvial facies deposits under semiarid climate conditions. On the plane, the Zhiluo Formation is widely distributed in the northern Ordos Basin. During the depositional period of the Zhiluo Formation in the North Yushen District and the South Shenfu District, there was a Paleochannel in the northwestsoutheast direction, forming a largescale riverdelta filling sedimentary sand body. The palaeochannel sand bodies of the Zhiluo Formation strongly scoured the Yan’an Formation, so that the 1-2, 2-2 and 3-1 coals of the Yan’an Formation were missing in different areas. On the whole, the fifth stage of the Yan’an Formation has the strongest scouring effect, followed by the fourth stage. Only a few boreholes can see the scouring effect of paleochannel sand bodies in the lower member of the Zhiluo Formation on the third member of the Yan’an Formation. The mine water inflow from the aquifer of the Zhiluo Formation has the characteristics of large water inflow, long duration, small water level variation, strong ability of stable “water supply”, and strong concealment. The thickness of sandstone sand bodies in the lower member of the Zhiluo Formation, the ratio of sand to ground in the lower member of the Zhiluo Formation, and the thickness of waterblocking rock formations in the lower member of the Zhiluo Formation control the occurrence of groundwater in the Zhiluo Formation macroscopically, with good porosity and permeability. The occurrence of the groundwater provides a good microscopic storage space for groundwater. In the coverage area of the Anding Formation in the west of the study area, there is basically no hydraulic connection between the Zhiluo Formation and the groundwater of the upper Luohe Formation and the Quaternary Salawusu Formation. It mainly accepts slow lateral supply, and the supply conditions are poor, and it is mainly based on static reserves. In the bedrock weathering area in the east of the study area, the groundwater of the Zhiluo Formation and the upper Quaternary Salawusu Formation undergoes relatively strong hydraulic exchange, and the recharge conditions are good, mainly lateral recharge and vertical recharge of the upper aquifer. The runoff speed is accelerated, the alternation is frequent, and the dynamic reserves are the main. In view of the Zhiluo Formation aquifer in the north of Ordos Basin and its water hazard prevention and control, indepth research should be carried out from the aspects of the water control mechanism of the Zhiluo Formation sedimentary system, hydrogeological conditions, the relationship between the mine water inflow and the continuous “water supply” of the Zhiluo Formation ancient channel sand aquifer, the water prevention technology of the groundwater source, the water supply of the aquifer system and the ecological value.