Coal Accumulation And Coal Character Of Chaoshuibasin In Alxa Right Banner, Inner Mongolia

Chaoshui basin, located in northern Hexi Corridor, is a faulted-depressed superposed basin of Mesozoic and Cenozoic Eras, China. Exploration datas show that there are abundant coal and oil resourses in the basin.Based on the sesmic, logging, drilling cores and other datas, and the theories of sequence stratigraphy, sedimentology and coal geology, combining geotectonics, sedimentology, geophysics and geochemistry, this paper researches the occurrence, stratigraphic framework, sedimentation unit, coal accumulation, coal petrography, coal quality and coal facies characteristics of the Early-Middle Jurassic coal-bearing strata in northwestern Chaoshui Coal Basin, and analyses the main governing factors of coal accumulation and coal quality., The main achievements of this paper are as follows:1.Four sequence boundaries(SB1,SB2,SB3 and SB4) were identified in Jurassic stratum unit. SB1 is a first-order sequence boundary, an angular unconformity interface between Jurassic strata and Cambrian basement. SB2 is a second-order sequence boundary, which shows an angular unconformity between the late Jurassic Shazaohe Formationand the middle Jurassic Longfengshan Formation. As third-order sequence boundaries,SB2 and SB3 divide the coal-bearing strata of Jijigou and Longfengshan Formation in Early-Middle Jurassic into three third-order sequences (sq1,sq2, and sq3, respectively).There are lowstand system tract (LST), lake expansion system tract (EST)and highstand system tract (HST) in Each third-order sequence.2.The mainly sedimentary facies were alluvial fan, meandering river,braided river, braided river delta, fan delta and shallow lake in the area. Clastic sediments mainly derived from the north and northeast. In sq1, LST mainly consisted of alluvial fan and braided river delta, EST mainly of shallow lake mudstone, and HST of alluvial fan and braided river delta. In sq2, LST mainly consisted of braided river, EST mainly of shallow lake mudstone, and HST of braided river and meandering river. In sq3, LST mainly consisted of alluvial fan and braided river delta, EST mainly of shallow lake mudstone, and HST of alluvial fan and braided river delta in northern and shallow lake mudstone in southern.3. The major coal macerals of theworkable coal-seam in the study area is vitrinite and secondary is inertinite. Medium gelification Index and high tissue preservation index show that the coal facies is mainly of wet forest swamp facies, and partly of shallow forest swamp facies. 4. Characterized by medium moisture, low ash and sulfur, and high volatile contents, the coal rank is between bituminous and subbituminous.The main minerals in coal are quartz and kaolinite, with some pyrite, calcite, siderite and trace of amazonitemicrocline.5. Except Sr,B and Cs,,the contents of other microelements are relatively low in coal. Cr, Ga, Pb,Li, Cu, Ge, V, Sc, Be, W, Th, As, Ti, and Nb show positive correlations with aluminosilicates; B, and Sr may mainly exist in the form of carbonate minerals; S and Mo may mainly exist in the form of sulfide minerals.In addition, the high correlations between Ti, Zr, Nb, and Ta show that they may exist in the form of heavy minerals.6.Peat accumulation were mainly controled by tectonic subsidence, lake level change and depositional system distribution in the study area. Rapid tectonic subsidence provided huge accommodation for peat accumulation, the settling velocity differentiation provided a favorable coal-forming environment, such as the slope break of the fault-depression and the gentle slope aboveit,. The rapid growth of coal-forming plants and peat accumulation may be controlled by the rapid lake transgression and regression, meanwhile, types of depositional systems also controled the distribution of peat bog.Controled by these governing factors, braided river delta and river depositional systems are well developed in the the slope break of the fault-depression and the gentle slope above it, while coal seams of larger thickness and better continuity were developed in the low-lying land of alluvial plain or in the silted shallow lake,far from the river channels.7.Coal seams in the study area are regularly distributed. In each third-order sequence, only some unstable thin coal seam formed in LST and EST, while the main coal seams were developed during HST periods, especially in highstand system tract (HST) of sq2. The coal-rich region was narrow, with coal seams becoming thin, splitting and die out gradually towards the direction of the source area and the centre of the basin.