Research On Distribution Pattern And Mobility Of Coal Measures Gas And Water In Shixi Block

The Permian Lower Shihezi Formation and Shanxi Formation in the Shixi Block on the eastern margin of the Ordos Basin belong to low-porosity and low-permeability reservoirs,with strong heterogeneity and unclear distribution of coal-measure gas and formation water,which affect the effective production of coal-measure gas resources.Guided by the relevant theories of sedimentary petrology,coalbed methane geology,reservoir geology,logging geology and hydrogeology,this thesis uses core,logging and gas testing data to analyze the Shixi area.The reservoir distribution characteristics,gas-formation water layer identification criteria,gas-formation water distribution laws and control factors have been studied in depth and in detail in the research interval of the block.Mainly obtained the following knowledge:The sandstone types in the study area are mainly lithic sandstone and lithic quartz sandstone,and the interstitial material is a muddy matrix;The macroscopic coal composition of the coal seam is mainly bright coal and mirror coal,followed by dark coal,and the microscopic composition is mainly vitrinite.The average value of R_o,maxof the coal seam is 2.42%.The sand body of Shan 2³ Member is 16.5 m thick on average,thick in the west and thin in the south,with 5 layers;the sand body of He 8 Member is24 m thick on average,thick in the east and thin in the west,with an average number of 7 layers.The total thickness of the coal seams in the Shanxi Formation is 5.68m on average,of which the average thickness of the Shan 2³ Member coal seam is 4.15m,which is thinner in the north and thicker in the south,with three layers,and the burial depth is shallow in the northeast and deep in the south and west.The average thickness of mudstone in Shan 2³ Member is 16.02m,thin in the east and thick in the west,with7 layers,and the burial depth is shallow in the northeast and deep in the southwest;The average thickness of the mudstone in the He 8 Member is 17.29m,which is thin in the east and thick in the west.The number of layers is 6,and the burial depth is shallow in the northeast and deep in the southwest.The Shan 2 and He 8 member lithologic assemblages can be divided into 7 types,among which the Shan 2 member reservoir assemblage is mainly sandy mudstone-mudstone-coal seam-sandy mudstone type;The He 8 Member is dominated by sandy mudstone-fine sandstone-medium sandstone-sandy mudstone type.The methane and nitrogen concentrations of Shan 2 and He 8 members decrease with the increase of burial depth,and increase with the increase of lateral migration distance.The concentrations of ethane and carbon dioxide increased with the increase of burial depth and decreased with the increase of lateral migration distance.The gas layers of Shan 2³ and He 8 members are mainly distributed in the west of the study area,and the water layers are mainly distributed in the north and east.Coal-measure gas is a thermogenic coal-formed gas generated from the source rock of type III kerogen,and the formation water mainly comes from atmospheric precipitation.There are three types of fluid distribution patterns in the study area:water retention in low parts,water above gas,and isolated lens water.It is pointed out that the distribution of gas and formation water in the study area is mainly controlled by the reservoir,structure and hydrocarbon generation.It is shown in detail as follows:lithologic combination mode,coal seam thickness,thermal evolution degree,and the gas-rich part in the high structural part,while the sand body distribution and the low part of the structural part control the water..A prediction model of movable water saturation is established,and it is pointed out that reservoirs with gas saturation greater than 60%and movable water saturation greater than 20%are located in the western part of the area;reservoirs with gas saturation between 40%and 60%and movable water saturation greater than 20%are mainly distributed in the northwest-southeast;reservoirs with gas saturation lower than40%and movable water saturation lower than 35%are located in the northeast.