Sequence strata and coal accumulation of Xishanyao Formation in Tiaohu sag of Santanghu Basin,Sinkiang

Taking Tiaohu sag of Santanghu Basin as the research subject,this paper focuses on key problems of se- quence stratigraphy subdivision and coal accumulation. Meanwhile it makes full use of the theoretical methods such as sequence stratigraphy of coal-bearing strata to discuss the coal accumulation of the Xishanyao formation. Detailed stra- tigraphic analyses of data from boreholes for the Xishanyao formation have been conducted in Tiaohu sag of Santanghu Basin and it is divided into three third-order sequences. Lowstand systems tract (LST),lake transgressive systems tract (EST),highstand systems tract (HST) are subdivided by the stacking patterns of stra-tigraphy,changes in lithology and lithofacies. A series of sequence-specific paleogeographic maps have been con-structed based on the contours of li- thological parameters. The paleogeographic units include braided river,braided river delta and lacustrine sedimentary system facies. The preferred sites of coal accumulation is the transition region between braided river delta and lacus- trine from the margin to the center. Furthermore,there are a range of thick coal seams at the contact interface between the delta front and the delta plains. The coal-accumulation features in sequence stratigraphic framework are clarified. Through analyzing coal seam change features,coal-rich zone,the accommodation rate in relation to peat production would have controlled on coal formation and preservation. During initial and late lake transgressive systems tract depo- sition,coal seams are thick and continuous while only thin,discontinuous and scattered coal seams are formed in the highstand system tract. The comprehensive coal-accumulation model is established based on the researches mentioned and the major coal seams are accumu-lated in the transition of the lake to the delta of the early and late EST. The re- sults will not only provide a se-quence stratigraphical model and paleoenvironmental framework for understand of the whole Santanghu Basin for this interval,but also are of practical significance for after coal resources exploration in San- tanghu Basin.