Sedimentary Control Factors Of Mud Shale Development In Shanxi Formation Of Qinshui Basin

By consulting a large number of relevant documents,the paper has understood the regional location,structural characteristics and stratigraphic characteristics of Qinshui Basin,and then the sand thickness and mud thickness data of several drilling and logging wells are counted.The data are analyzed and processed by CorelDRAW,Geomap,Resform and other related software,and then the sedimentary environment and shale development characteristics of Shanxi formation are studied in depth.Finally,the controlling effect of sedimentation on shale is discussed,which provides a strong basis for shale gas exploration in Qinshui Basin in the future.According to the research results,the Shanxi formation of Qinshui Basin is generally a continental deposit: Delta plain facies is developed in the north,delta front and lake are developed in the middle,and lakes are mainly developed in the south.The lithology is mainly sandstone,mudstone and coal.Shanxi formation can be regarded as an independent third-order sequence,and it can be divided into two system tracts with shale widely developed on No.3 coal as boundary.The thickness and distribution of sandstone in highstand system tract are larger,and the sand land ratio corresponding to sedimentary microfacies is also higher than that in transgressive system tract.The distribution of sedimentary facies in the two system tracts is almost the same,but some areas are different,in which the distributary channel is gradually widened,and the extension range of flank sheet sand is also far away.There are many shale layers in Shanxi formation with small thickness,which are developed in the north,middle and south.The shale in the highstand system tract is thick and widely distributed,but the thickness ratio of carbonaceous mudstone to mudstone is lower than that in transgression system tract in most areas of the basin.The overall buried depth of the two system tracts is about 700 m,but it is more than 1000 m in the north and middle of the basin,the burial depth at the top of the area with stable shale thickness is about 300-600 m.The trend of change is gradually increasing from the edge to the center of the basin.The average value of Ro of shale in the highstand system tract of Shanxi formation is smaller than that of transgressive system tract,both of them are in the mature over mature stage,favorable for pyrolysis to generate natural gas,showing the distribution of Ro value from west to East increasing first and then decreasing,and the burial depth has a direct impact on the maturity of shale organic matter.The sedimentary environment controls the distribution and thickness of shale and the proportion of the thickness of carbonaceous mudstone in the study area: The transverse continuity of mudstone in lake environment is better than that in delta environment,The organic matter content in the transgressive system tract of Shanxi formation is the highest,so the sedimentary environment under the transgressive system tract is more conducive to the development of organic matter.The sedimentary facies belt of the transgressive system tract of Shanxi Formation controls the thickness ratio of the carbonaceous mudstone to that of the mudstone,the thickness of carbonaceous mudstone accounts for a large proportion when sandstone is thick;However,the channel widely developed in the highstand system tract failed to wash through the thick mudstone in the upper part,which made the sedimentary facies belt in the highstand system tract have little influence on the thickness ratio of carbonaceous mudstone.