Lithofacies palaeogeography of lower coal group accumulation period of Carboniferous Permian in North China

Based on the systematic collection of borehole data,logging data and geological profiles as well as previous stratigraphic and palaeontological research results,the authors made a horizontal comparison of the lower coal group in North China,researched its distribution characteristics,lithofacies palaeogeography and spatial distance relationship between the upper surfaces of Ordovician system. According to the survey of field outcrop profiles and borehole data of Taiyuan Formation,the lower stratum of the lower coal group was divided into five sedimentary systems. A single factor analysis and multifactor comprehensive mapping method was applied to recover the palaeogeographic pattern within the lower stratum of lower coal group as an analyzing unit. The results show that in the north of North China,some fluvial and delta deposits are the main sediments,and alluvial fan deposit is developed in Chengde-Xinglong area. Transition to the south is the barrier-lagoon-tidal flat deposits. The carbonate platform deposit is dominant in the west of Ningxia,Liaodong Peninsula,east-central of North China,east of Henan and south of Anhui. With a boundary of 38°N,the most favorable coal accumulation environment of the lower coal group in the North China Basin has significant differences.The thick coal seams in north of 38°N are mainly formed in delta plain,followed by the lagoon-tidal flat transition zones. The coal seam in south of 38°N is mainly formed in the lagoon-tidal flat,with a relatively small thickness. By comparing different sedimentary environments and the formation mechanism of thick coal seams,it is concluded that the coal accumulation strength of lower coal group is in the order of interdistributary bay of delta plain,tidal flat,lagoon,and carbonate platform,and the thickness of coal seams gradually decreases and the number of coal seams increases. The research on the spatial distance between lower coal group and upper surface of Ordovician System shows that the distance in the mineable area is generally about 10-80 m,and the distance in the mineable area of south margin is generally within 30 m. The distance in the northern part of Ordos Basin,the northern part of Hebei,Beijing-Tianjin region,Hedong and Xishan area of Shanxi are about 50-80 m. The distance in eastern Henan-Huaibei-Southwest Shandong are about 30-60 m. And the distance in Jinan-Zibo-Laiwu of Shandong are about 30-50 m.