Sedimentary Sequence And Palaeogeographical Evolution Of The Early-Middle Jurassic Coal Measures In Dongsheng Coalfield

Dongsheng Coalfield is located in the northeast of Ordos Basin,which is rich in coal resources in Early-Middle Jurassic strata.In this study,field measurement,petrological characteristics and sedimentological characteristics will be used to determine the lithofacies and lithofacies combination developed in the study area,and determine the sedimentary environment of each period in the study area.The sequence of the study area is divided,and isochronous sequence stratigraphic framework covering the study area is established through single well and connected well sections.The quantitative lithofacies paleogeographic map of the study area is drawn by the method of "single factor analysis and multi factor synthesis".Finally,from the main factors of coal control,the genesis of coal seam in the study area is studied.The main conclusions are as follows:(1)In the study area,10 lithofacies and 3 lithofacies combination types are identified.By analyzing the environment of single lithofacies development,the sedimentary environment represented by lithofacies combination is determined,and three sedimentary environments are identified:meandering river,braided river and delta.Coal seams are mainly developed in delta plain environment.(2)According to the method of sequence stratigraphy,the sequence boundary and system tract boundary are identified in the study area.The study area is divided into 8 third-order sequences and 20 system tracts in the early and Middle Jurassic,and the isochronous sequence stratigraphic framework in the study area is drawn according to the single well and connected well sections.(3)The quantitative lithofacies paleogeographic map of the study area is analyzed and drawn by iso line map of single factor such as stratum thickness,mudstone thickness,coal seam thickness and sandstone percentage.The first member of Yanyi formation is dominated by braided river sedimentation,and the second to fifth members of Yanyi formation are dominated by delta sedimentation.(4)Combined with the paleogeographic map,the coal accumulating rule of the study area is analyzed.The coal rich belt is mainly developed in the delta plain deposit,mainly located in the western area of the study area,consistent with the subsidence center of the stratum.