Occurrence characteristics and water abundance of Zhiluo Formation in northern Ordos Basin

There are four coal bases in the Ordos Basin,namely Huanglong,Northern Shaanxi,Shendong and Ningdong,in which the Jurassic Yan’an Formation coal seams are mainly mined. Roof water damage is the main disaster of coal mining in the area. The Zhiluo Formation is one of the main water inrush sources. In order to study the waterrich characteristics of the aquifers in the Zhiluo Formation,the spatial occurrence characteristics of the aquifers in the Zhiluo Formation,the characteristics of the microscopic pore structure and their relationship with water richness were analyzed by means of geoscience information system mapping,microscopic pore structure testing and statistical analysis. The results show that controlled by the depositional environment,the Zhiluo Formation can be divided into upper,middle and lower members. The main waterbearing rock groups are weathered bedrock sections,coarsegrained sandstone,mediumgrained sandstone and finegrained sandstone sections. The main waterblocking rock groups are siltstone,sandy mudstone and mudstone sections. The weathered bedrock aquifer section is mainly located at 3.04-83.46 m at the top of the bedrock,and distributed in the middle of the study area in a northsouth strip. The thickness and weathering degree of the weathered bedrock gradually thins and weakens from the middle to the north and south sides. The sandstone waterbearing formation thickness and sandtoformation ratio of the lower member of the Zhiluo Formation are obviously larger than those of the middle and upper members,and the sand body distribution shape of the lower member shows a general connection type,and the continuity is obviously better than that of the middle and upper members. The cumulative thickness and number of layers of the waterblocking rock group in the lower member of the Zhiluo Formation are obviously smaller than those in the middle and upper member,and the heterogeneity of the waterblocking rock group in the middle and upper member is obviously stronger than that in the lower member. For the normal bedrock of the Zhiluo Formation,the sandstones in the middle and upper members of the Zhiluo Formation are dominated by low porosity and ultralow permeability to ultralow permeability. The sandstone in the lower member of the Zhiluo Formation is dominated by mesoporosity,and the permeability is dominated by medium permeability. Compared with the middle and upper members,the lower member has higher porosity and permeability. After the Zhiluo Formation was weathered,the rock microfractures developed obviously. Compared with the normal bedrock,the weathered bedrock has higher porosity and permeability. The porosity type is mainly mediumhigh porosity,and the permeability type is mediumhigh permeability. Mainly,the water storage space is more developed than the normal bedrock. The pore structure types of the Zhiluo Formation sandstone are divided into four types by fractal theory,the lower member of the Zhiluo Formation is mainly composed of type I pores,and the middle and upper members are mainly type II,III and IV pores. According to the correlation analysis results of the Pearson coefficient method,the sand body thickness of the lower member of the Zhiluo Formation,the sandtoground ratio of the lower member of the Zhiluo Formation,and the thickness of the waterblocking rock formation in the lower member of the Zhiluo Formation have obvious control effects on the waterrichness of the Zhiluo Formation on the macro scale. It is the main “contributing” interval of the waterrich Zhiluo Formation. According to three indicators,the sand bodies of the lower member of the Zhiluo Formation are divided into three grades. Through the analysis of the relationship between the pore structure and water richness of the lower Zhiluo Formation in typical boreholes,the pore structure has a certain control on the water richness of the lower Zhiluo Formation from the microscopic scale. The more developed the microscopic pores,the better the water richness. Through the correlation analysis and comparison of water pumping test data,it is found that weathering makes the water richness of the sand bodies in the lower part of the Zhiluo Formation significantly enhanced,and the thickness and degree of weathering of the weathered sand bodies in the lower part of the Zhiluo Formation have obvious control effects on the water richness of weathered bedrocks. The water richness of the lower member of the Zhiluo Formation is evaluated by the method of AHP. The water richness of the weathered bedrock area is generally stronger than that of the unweathered area,and the water richness of the firstgrade sand body area is stronger than that of the secondgrade sand body and the thirdgrade sand body area.