The surface water bodies in this area are not well developed. Groundwater is controlled by lithology, structure and topography, and mainly occurs in the bottom of Quaternary system, bedrock fractures and karst fractures. It can be divided into pore water of Quaternary unconsolidated layer; Carboniferous-Permian sandstone fissure water; Karst fissure water in Taiyuan Formation limestone and Ordovician limestone. Shaanxi Coalfield Geology Bureau 13 1 team thinks that the Quaternary loose layer is weak in water abundance; Carboniferous-Permian water-rich is weak-medium, but water permeability is poor; Ordovician limestone is rich in water and permeable, but it is extremely uneven. The water content of coal measures strata is not large.

The water-rich characteristics are 1 1 # in the roof aquifer 1.4. 1 main coal seam.

(1) 3 # coal seam roof sandstone aquifer

3 # coal seam roof lithology is mainly medium-grained sandstone, fine-grained sandstone, mudstone and sandy mudstone. Medium-grained sandstone and fine-grained sandstone are confined aquifers with cracks in the roof of coal seam, and their water abundance is weak. Due to the interbedding of sandstone and sandy mudstone, the water permeability of rock stratum is poor, and the water level elevation is between 402.05 ~ 481.41m. The unit water inflow of borehole is 0.00301~ 0.18l/(s m), with an average of 0.0452l/(s m). The permeability coefficient is 0.0042 ~ 0.487m/day, with an average of 0.119m/day ... The fractures in mudstone and sandy mudstone are not developed, forming a water-resisting layer. Therefore, the roof aquifer is basically a composite aquifer without hydraulic connection.

(2) 5 # coal seam roof sandstone aquifer

The lithology of coal seam roof is mainly composed of thick medium-grained sandstone, fine-grained sandstone, siltstone and medium-thin mudstone. Medium-fine sandstone has developed fractures and good water permeability, forming a fractured aquifer, and mudstone is a relative aquifuge. The aquifer is 5 ~ 20m thick. The unit water inflow of borehole is 0.0028 ~ 0.000038L/(s m), with an average of 0.0014L/(s m), and the permeability coefficient is 0.0023 ~ 0.000078 m/d, with an average of 0.0015m.

(3) 1 1 # coal seam roof limestone aquifer

The lithology of coal seam roof is mainly composed of limestone and timely sandstone with a small amount of argillaceous rocks. The main aquifer is limestone fissure confined aquifer with a thickness of 8 ~ 10m, rich in water and permeability, and the static water level of the borehole is 2.53m higher than the surface. Water inrush may occur in fault structures or karst development areas.

1.5.2 Water-rich characteristics of Ordovician carbonate karst fissure aquifer in coal measure basement

The Ordovician stratum is a composite water-bearing body mainly composed of limestone and dolomite mixed with marl, and the water level in the area is relatively stable and unified (+380 m). The total thickness of the stratum is 465,438+00 ~ 520 m, and groundwater mainly exists and migrates in fractured karst, with uneven water content, complicated hydraulic connection and obvious structural control.

Division and distribution characteristics of (1) interval

Ordovician carbonate rocks in this area can be divided into three groups from bottom to top: Yeli liangjiashan Formation, Majiagou Formation and Fengfeng Formation, and can be further divided into eight lithologic sections according to lithologic combination (Table 1.2).

Table 1.2 Ordovician stratigraphic division and main characteristics table

The Liangjiashan Formation of Yeli is mainly exposed from Yumenkou to Panlong Estuary. Majiagou Formation is widely exposed in the whole region, but Majiagou Formation under Panlong River is the main one in the north and Majiagou Formation on Panlong River in the south. Fengfeng Formation is also developed in the whole region, but its occurrence degree varies greatly in the whole region due to the difference of ancient erosion degree and structural fracture strength. The second member of Fengfeng Formation is directly located at the bottom of coal measures north of Xiyuangou, and the second member of Fengfeng Formation from Xizhougou to Xiangshan basically disappears, and the first member of Fengfeng Formation also remains intermittently in some areas. In the south of Xiangshan (especially Yingshan), according to drilling data, the first and second members of Fengfeng Formation are generally developed. In a word, the distribution law of Ordovician carbonate rocks in the whole region is that the highest outcrop in the south and north areas south of Jushui River is the second member of Fengfeng Formation, in which the thickness of Sangshuping minefield is > 20m, and the thickness of this section south of Jushui River gradually increases, reaching nearly 100m south of Yingshan. The Fengwei Formation along the line from Jushuihe to Xiyuangou is distributed in fragments, and its thickness is generally less than 20m. Due to fault cutting, coal measures strata are connected with Shangmajiagou Formation in many places.

(2) the division of aquifer and relative aquifuge

According to the analysis of existing hydrogeological data, the general form of Ordovician limestone water in this area is a reticular water body based on structural zones (fault zones and fracture-intensive zones) (underground water inrush points are all related to structural fracture zones), so layered runoff is not the main form. However, as far as the water filling conditions of rocks are concerned, they still have the characteristics of layered water (still limited by regional stratigraphic units), and their aquifers and relative aquifers are divided as follows:

1) Relatively strong aquifer: ① Member 2 of Fengfeng Formation. Karst is developed, and the fracture rate is as high as 6.5%. According to the water inrush data recorded by Hancheng Mining Bureau, 1 May 9, 976, the original1belt inclined shaft in Sangshuping Mine gushed water, and the water quantity was 1530m3/h, which caused a well kick accident. The water inrush point was in the second thick limestone of Fengfeng Formation. Most other water inrush points in this mine are located near the top surface of Ordovician limestone, which is roughly equivalent to the second member of Fengfeng Formation. ② The second member of Shangmajiagou Formation. Karst is developed, and the average crack rate is 4. 19%. The strata in this section are mostly massive and thick layered dolomite, which is easy to be broken when the structure changes, so the cutting depth and opening of the solution gap are large. They may be favorable places for infiltration and accumulation of groundwater. According to the statistics of Hancheng Mining Bureau, more than 90% water inrush points in Magougou Mine are located in the second member of Shangmajiagou Formation. The most serious water seepage accident occurred on August 6th 1976+240m in the east of Shimen, and the maximum instantaneous water inflow reached 12000m3/h, and the average water inflow reached 5956m3/h. In the interbedded dolomite and limestone in Xiangshan Mine, the maximum water inflow was 414m3/h.

2) Strong aquifer: Member 2 of Xiamajiagou Formation. The fracture rate is 4%, and karst is also developed, but it is mainly concentrated at the bottom. Although there are many fractures in other parts, they are small in scale and strong in filling, so from their own characteristics, they are slightly worse than the water filling conditions of the above two stratigraphic units. However, during the excavation of the inclined shaft for discharging gangue outside the ditch of Xiangshan Mine in Hancheng (1975, 10, 18), there was also a water inrush record of 233m3/h in the local stratum, so this stratum can be regarded as a strong aquifer.

3) Weaker aquifer: Lower Ordovician Yeli liangjiashan Formation. High silica content and tight cementation, flint lumps or strips are distributed in layers, which is not conducive to the communication between layers. The fracture rate of this group is 0. 15%, which is enough to form a water-resisting rock section. However, it can be seen that there are also caves with the long axis of 1 ~ 2 m in the dolomites sandwiched at the mouth of Panlong River. The stratum in this section is fragile and easy to crack, and a large fracture dense zone can be developed, so the possibility of water filling cannot be completely ruled out. However, there is no reliable record of water inrush in this section, and it can only be classified as a relatively weak aquifer through analysis and inference.

4) Relative water-resisting layer: The first member of Fengfeng Formation and the first member of Shangxiajiagou Formation are stratigraphic units with high shale content, and their lithologic analysis shows that they all have relative water-resisting effect, especially the water-resisting effect of the first member of Fengfeng Formation is more practical. According to statistics, the fracture rate in this section is small, only 1.2%, which is also described in the data summarized by Hancheng Mining Bureau: 1.672m was sent to the first section of the roadway of Fengfeng Formation before the water inrush from the +240 m crosscut in Magougou Mine, and only water pouring and dripping occurred, and the total water inflow of the roadway was less than 96m3/h, which had a strong waterproof effect.

(3) Water conductivity of the fault structure

The fault structures or structural belts in the area are mainly concentrated in the shallow part of the mining area, and the medium and deep large and medium-sized faults are not well developed. Although these faults are mainly tensional, and the fault zones seen in the field are also broken and open, the 13 1 exploration team, which has been in charge of geological exploration in this area for a long time, believes that the water conductivity of large and medium-sized faults is poor. The development of joints and fractures is obviously controlled by strata. Although the local distribution is zonal, the vertical conductivity is not very strong. Generally, faults in coal seams are small in scale and have small vertical connectivity. According to the data revealed underground, although many faults have dripping phenomenon, the water quantity is very limited, which is not enough to cause water inrush or water inrush, and the effect on methane dissolution and escape in coal seam is not obvious. However, the fault structure connected with Ordovician strata in coal measures basement must be paid attention to. When a coal mine is driven below +380 m level and encounters a fault structure connected with an aquifer, it is common to conduct water or even flood the well (see relevant parts in this section). The occurrence elevation of middle and deep coal seams in this area is mostly below +380 m, and that of 1 1 # coal seams is mostly below+300 ~+200 m. After the pressure relief of each coal seam, especially 1 1 # coal seam, if the floor pressure is not enough to resist the water head pressure or the floor is seriously damaged by the fracture structure, it is likely to cause. Fracturing technology in coal seam methane mining will also aggravate the destruction of floor and the opening and conduction of inherent cracks. All these will bring difficulties to methane mining. These problems need further study.

To sum up, the hydraulic connection between the roof of each coal seam and the aquifer of overlying strata in coal measures is not close, which has little influence on coal seam methane. However, the influence of Ordovician limestone water on coal seam methane may be obvious only after the pressure caused by coal mining or gas production is released. According to the structural research results, the clear water tensile fracture zone in the south of the mining area and the tensile fracture zone in Dong Ze Village in the middle cut through the coal seam to varying degrees, which may have some influence on the water conduction of Ordovician limestone. When the fractures in the east-west, northwest and northeast directions are densely developed, Ordovician limestone water will also be carried out after the pressure is released. Attention should be paid to these structures or structural zones, and methane mining projects should be avoided as much as possible.