Paleogeography and coal accumulation model of the Middle Jurassic Xishanyao Formation in western Dananhu Coalfield of Turpan-Hami Basin

The Dananhu Coalfield in the southeastern part of the Turpan-Hami Basin contains abundant coal resources, but the coals in this area have multiple layers and vary greatly in thickness, resulting in increased difficulty in coal exploration. In order to explore the coal accumulation regularity of the Xishanyao Formation in the Dananhu Coalfield, the coal-accumulating models, palaeogeography, and distribution of coal-accumulating centers of the middle member of Xishanyao Formation were analyzed based on borehole cores and geophysical loggings data. Three types of sedimentary facies were identified in this member, including fluvial, delta, and lacustrine facies. The coal-accumulating swamps were developed from the fluvial overbank, interdistributary bays of the delta plain, and shore-shallow lakes, among which the interdistributary bay swamp was the most favorable coal-accumulating environment. The paleogeography and coal thickness changes of the middle member of Xishanyao Formation have been analyzed, which shows that the major paleogeographic units included alluvial plain, delta plain, delta front, and shore-shallow lake. The two coal accumulation centers with coal thickness greater than 95m were found to be located in the interdistributary bay area. The coal accumulation was influenced by a combination of paleo-tectonics, paleoclimates, paleo-vegetations, and paleoenvironment conditions. The relationship among coal seam thickness, total stratal thickness, and the sandstone to mudstone thickness ratios is further analyzed using the vector cloud diagram. The result shows that the area with a moderate stratal thickness and a low sandstone to mudstone thickness ratio is associated with the thicker coal seams, indicating that the preferred coal-forming environment was the interdistributary bays where the moderate subsidence rates and relatively less supply of coarse-grained sediments were maintained. Based on these paleogeographic analyses, we proposed that the coal accumulation centers were mainly distributed in the central area of the No.1 mine field and the central north of the F1 mine field. A coal accumulation model has been established in which the swamps of interdistributary bays of the delta plain had better coal accumulation than the swamps developed from the fluvial overbank and shore-shallow lakes.