Study On The Coal Environment And Coal Accumulation Law Of The Permian Leping Xuanwei In The Qilian Area

Upper Permian of the South China and the Southwest Guizhou, which is the target Formation of coalbed methane exploration and exploitation, is rich in coal resources. The current research area is located in the south of Sichuan basin, the west part of Yangtze plate, bordering on 3 Provinces (Sichuan, Yunnan,and Guizhou). Due to the natural sedimentation of Sichuan and Guizhou in the early of late Permian, seawater flew into the east of sichuan basin from the southeast of Guizhou Province. There was a general slope from the southwest to the northeast in Junlian-Gongxian, an offshore piedmont plain region, so the transgressive scope limits in the east of Gusong. Then, terrestrial facies sand and mud-stone sediment, which is mainly composed of lacustrine and swamp faces, were formed in the west area of Gusong forms. In the late of Permian, with the increasing transgressiveness and west-forwarded coastline, Gusong was completely submerged by seawater, which resulted in alternant tidal flat and coast shoals in the west area. Therefore, a symmetric and complete tidal flat coal-bearing deposit develops well in this area. With the constant west-forwarded transgression, there is gradual migration to the west for the beneficial coal-forming environments and coal-accumulating centers.Xuanwei FM is divided into upper and lower sections for a detailed study of the characteristics of the coal distributive. The thickness of the lower is approximately 100 meters and the upper between 40 to 50 meters; the thickness of the planes on the two sections varies little. The upper section of Leping Fm is subdivided into three small layers (the first small layer contains C1?C2?C3 coal seams,the second small layer contains C4?C5?C6?coal seams, and the third small layer contains C1?C2?C3 coal seams). The thickness of each small layer varies between 10 to 18 meters, with little variation for the thickness of the planes. Through the analysis of coal-bearing strata sedimentary facies, three types of sedimentary facies (meandering river deposit, tidal flat and mixed platform) are identified and they are further divided into 6 kinds of subfacies and 13 kinds of microfacies, among which tidal flat swamp microfacies and meandering river swamp microfacies are likely to form coal seam. By the vertical and horizontal compaision of From 19 coal seams, C2?C3?C7?Cg beds are found to be the major coal seams; the third coal seam is the thickest while the second coal seams is thinnest.The coal forming environment of the area is divided into the meandering river facies and tidal flat facies. The former is thinner with unstable transverse distribution little commercial value, while the latter is thicker with stable transverse distribution and significant commercial value. The factors such as thickness of coal seams, metamorphic grade,ash content,burial depth, gas content and concentration of methane are studied with the maps of hydrogeological condition and the criteria of favorable zones selection. The present study discovers a "?" favorable zone and a ? favorable zone in the the third layer of upper Leping FM; and "?" favorable zone and ? favorable zone in the the third layer of up Leping FM.