Paleochannel Sand Bodies Of Zhiluo Formation In Northern Shaanxi Jurassic Coalfield And Their Water Control Mechanism

The coal mine water hazard is the main disaster in Jurassic coal field in Northern Shaanxi.Zhiluo formation is one of the main water bursting sources of coal mine flood.Taking the north of Jurassic Coalfield in Northern Shaanxi as the research area,the paper is guided by the relevant theories of sequence stratigraphy,sedimentology,hydrogeology and mine water damage prevention.It explores the spatial position and shape of paleochannel sand bodies about Zhiluo formation,analyses the control mechanism of the groundwater storage and migration and puts forward the water inflow model of productive mine under the paleochannel sand bodies by the combining outcrop investigation and borehole analysis,macro regional investigation and micro laboratory research,theoretical analysis and numerical simulation.The main conclusions are as follows:The Zhiluo Formation can vertically be divided into three sections in the study area.The sedimentary face of the lower member of the Zhiluo Formation is mainly braided river deltas.The subface is mainly delta plains and the sedimentary microfacies are mainly distributary channels,crevasse splays,interdistributary bays and interdeltaic bays.The pore development degree and connectivity of the lower member of Zhiluo formation are relatively good,mainly with medium porosity and medium permeability,and it belongs to the confined medium permeable pore water bearing formation as a whole.The pore development degree and connectivity of the middle and upper member of Zhiluo formation are relatively poor,mainly with low porosity and very low to super-low permeability.It belongs to the water bearing formation with low permeable pores under pressure as a whole.The micro-fractures are obviously developed,the porosity and permeability are more developed than the normal bedrock and the storage space is better when the paleochannel sand bodies is weathered.The whole section belongs to the confined medium-strong permeable fracture water-bearing rock group.The paleochannel complex sand bodies of the Zhiluo Formation studied in this paper refer to the fact that during the deposition of the Zhiluo Formation,due to the long duration of channel deposition.Large-scale superposed multi-layered sand bodies assemblages with good hydraulic properties are formed in the same area or in adjacent areas.Through the analysis of sequence stratigraphic correlation,hydraulic properties and scour contact relationship at the bottom of Zhiluo Formation,it is identified that the lower member of Zhiluo Formation is the development layer of paleochannel sand bodies.Hongjiannao field,Erlintu field and Jinjie field are mainly developed by the paleochannel sand bodies of the Zhiluo Formation.The width is about 25km.According to the analysis of the sand dispersion system and the development characteristics of the aquiclude rock formation,the paleochannel sand bodies of the Zhiluo Formation is divided into three grades.The thickness of the first-grade sand bodies is greater than or equal to 60m,the sand ground ratio is 0.8-1,the thickness of the aquiclude rock group is generally less than 10m,and the number of layers of the aquiclude rock group is generally less than 2,which is the main channel sand bodies development area.The thickness of the secondary sand bodies is generally 30-60m,the sand ratio is 0.4-0.8,the thickness of the aquiclude rock group is 10-30m,and the number of layers of the aquiclude rock group is generally 2-4,which is the branch channel development area.The thickness of the tertiary sand bodies is less than 30m,the sand ratio is less than 0.4,the thickness of the aquiclude rock group is generally more than 30m,and the number of layers of the aquiclude rock group is generally more than 4,which is the concentrated on the interdistributary bay and the interdeltaic bay.For the normal bedrock of the Zhiluo Formation,the thickness of the paleochannel sand bodies,the sand ratio,and the thickness of the aquiclude rock formation control the water-abundance of the Zhiluo Formation from the macroscopic scale.The physical properties and pore structure control the water-abundance of the Zhiluo Formation from the microscopic scale.The primary sand bodies(main channel)and the secondary sand bodies(branch channel)are mainly medium-grained and coarse-grained sand bodies.The physical properties and the pore structures are relatively developed and the water-abundance is strong.Then the third-grade sand bodies(interdistributary bay and interdeltaic bay)are mainly siltstone and fine sand bodies.The physical properties and the pore structures are poor and the water-abundance is relatively weak.The sedimentary grain size of the channel sand body is coarser and the overall cement content is less.After weathering,the upper network fractures develop and the lower dissolution fractures develop.The groundwater storage capacity is significantly enhanced and the water-abundance is strong.The macroscopic geometric connectivity and microscopic permeability of the paleochannel sand bodies of the Zhiluo Formation control the migration of groundwater.The weathering greatly improves the permeability of the paleochannel sand bodies.For the unweathered area of the Zhiluo Formation in the west,the Zhiluo Formation has weak hydraulic connections with other aquifers,and is dominated by the lateral supplement.The runoff has slowed to a crawl and the river has poor drainage.It is mainly dominated by the static reserves.For the weathering bedrock area of the Zhiluo Formation in the east,the groundwater is closely hydraulically connected between the Zhiluo Formation and the Salawusu Formation.The runoff conditions are good and it has smooth drainage conditions.With the relief of groundwater from the west to the east and under the action of infiltration and recharge of groundwater,the huge dynamic reserves are formed.Under the natural state,there are two modes of groundwater discharge in Zhiluo Formation:direct discharge and indirect discharge.Among them,the indirect drainage includes the recharge from the top to the Salawusu Formation,the leakage recharge to the Yan’an Formation from,and by the burnt rocks to excrete.In the mining state,the Zhiluo Formation fills the mine with water.The lower aquifer of the Zhiluo Formation directly fills the mine with water,the Quaternary groundwater fills the mine with water through the fissure network of the Zhiluo Formation,and the groundwater from the weathered layers of the Luohe Formation and Anding Formation passes through the Zhiluo Formation.There are 3 water filling modes for filling the mine with fissure network.The water-richness of the paleochannel sand bodies was evaluated by the Analytic Hierarchy Process in the Zhiluo Formation.The water-richness of the weathering bedrock area is generally stronger than that of the unweathered area.Meanwhile,the water-richness of the first-grade sand bodies(main channel)area is stronger than that of the second-grade sand bodies(branch channel)and the third-grade sand bodies(interdeltaic bay and interdistributary bay)area.The calculation of the height of the water flowing fracture zone in the first coal seam shows that the water flowing fracture zone is not connected to the bedrock of the Zhiluo Formation except for the northern of the Xiaohaotu No.1 mine field and the Dabaodang general prospecting local area.The other areas are connected to the paleochannel sand bodies of the Zhiluo Formation.Based on the analysis of the relationship between sand body classification,weathered sand body thickness,and mine water inflow,three basic water inflow modes for production mines under paleochannel sand bodies are proposed.The strong water inflow mode is mainly located in the distribution area of the first-grade sand body,and the thickness of the weathered sand body is generally greater than 40m.Moderate water inflow mode,mainly located at the edge of the primary sand body or in the area of the secondary sand body,and the thickness of the weathered sand body is generally 10-30m.The weak water inflow mode is mainly located in the tertiary sand body area,and the thickness of the weathered sand body is generally less than 10m.Through numerical simulation,the mine water inflow volume of the three modes is estimated,Jinjie coal mine working face 1 belongs to the strong water inflow mode,the maximum mine water inflow volume is 2309.2 m3/h,Hongliulin coal mine working face 2 belongs to the medium water inflow mode,the maximum mine water inflow volume is 1037.29 m3/h;Ningtiao Coal Mine Working face 3 belongs to the weak water inflow mode,and the maximum mine water inflow is 367.97 m3/h.