Sequence Stratigraphic Analysis And Coal Accumulation Of Middle Jurassic Xishanyao Formation In The Middle Of Eastern Junggar Coal Field, Xinjiang

Junggar Basin is one of the most important Jurassic coal basin in northwest China. This large intracontinental overlapped basin developped from Late carboniferous until neogene. Easten Junggar coal field is located in the north margin of the uplift zone in the east of Junggar Basin, at the south foothill of Kelameili Mountain. A mass of coal resources have been proved up in this field.Based on the information from the drilling cores, well logs and the analysis of the coal facies, applying the theories and methods of sequence stratigraphy, sedimentology and coal geology, the sequence stratigraphic framework of Xishanyao Formation is established, the formation of sedimentary facies and the evolution of depositional systems are discussed in detailed, the significant stratigraphic boundaries in the single thick coal seam are identified based on the index of the coal facies, the dominative factors of the accumulation of the coal seam are discussed. Finally, the distribution of thick coal seams are analysed, all of those helps to the further exploration, the compare of the coal seams and the comprehension of the accumulation The main achievements of this paper are as follows:(1) The middle Jurassic Xishanyao Formation can be divided into two third-order se quences (Sql and Sq2) in the middle of East Junggar coal field, which Sql is the main coal-bearing sequence, containing lowstand system tract (LST), lake expansion system tr act (EST)and highstand system tract (HST). 6 parasequence sets and 11 parasequences ca n be identified. Only one single incomplete LST is preserved in Sq2. SB1 is a large-scal e erosion. SB2 is a large-scale erosion near the Kelameili mountain, and a integrated int erface away from the source area. SB3 is a parallel unconformity interface which means the absence of the strata.(2) The Xishanyao Formation mainly were deposited in three types of depositional systems: braided river, braided river delta and shallow lake. Which braided river and braided river delta were mainly formed in LST of Sql, braided river distributed in the central and the northern area, and braided river delta distributed in the southern fringe. Along with the retrogradation of the braided river to the northwest and the expansion of the shallow lake, each type of depositional systems can form in EST of Sql. Early HST contained the similar sedimentary system with the late EST, The whole area were silted during the middle HST.(3) Clastic sediments mainly came from the northwest, and a northeast source direction was developed during HST. Kelameili Mountain is the source area. Whether there was a south source area is to be confirmed until the drilling informations in the southen area are sufficient.(4) The coal facies analysis of the thick coal seam in the middle of the Easten Junggar coal field reveals high Inertinite content, low ash yield and gelification Index, comparatively high tissue preservation index, which shows that the coal forming environment attributes to a comparatively dry peat bog, which may related to that the rate of the peat growth is quickly than that of the basement subsidence in the peat swamp.(5) Five parasequences sets and eight parasequences are divided from the single thick coal seam based on the vertically changes of the coal facies. EST is divided into two parasequences sets from three small sequence compositions, which showed regression singly but transgression as a whole. HST is divided into three parasequences sets from five small sequence compositions, The lower three of which shows regression and the upper two shows transgression.(6) The basement subsidence rate, the water level changes and sedimentary systems control the coal accumulation. Firstly, the basement subsidence means the expanding accommodation, the distinction of which result in transferring of the center of the coal formation. Secondly, the expasion or shrinkage of the lake influence the progress of the coal formationt, which was terminated by fast transgression, and beneficial after the regression of lake. It is disadvantageous for the growth of the peat swamp by vast development of the river. With flourishing plants, warm and humid ancient climate conditions, no influence of the source debris, Those area keeping the durative balance of the basement subsidence and growth of the peat is advantaged for the formation of regional thick coal.(7) Coal thickness of Xishanyao Formation in this exploration area distributed regularly that unstable thin coal seam was mainly been found in LST, Regional thick seam always distributed in EST and HST, which thichness decreased to the direction of the source and the centre of the basin.