Sedimentary System And Depositional Evolution Of Ordos Basin In Early Period Of Late Triassic

Ordos Basin is significantly abundant in coal, oil and gas. Yanchang formation, the stratum of Late Triassic is one of the most favorable intervals for hydrocarbon accumulating, which has rather more important research and economical importance. However, the discussing focus of Yanchang formation is still sand body distribution, and lake basin area and delta extent of Late Triassic also aren't confirmed. Especially, in the early period of Late Triassic, as the important lake centre migration period and one of the major source intervals, the sand body distribution, lake basin area and delta extent are still ambiguous and need to study deeply.Based on utilizing sedimentology theory and regarding large scale sedimentary system study as onset, the author had finished this paper by regarding sedimentary facies types, deposition characters, plane distribution rules and deposition evolution of Chang-9 and Chang-10 Member as an important whole part. Then the sedimentary modes had been established, so the achievement of which indicated more guidance meaning for further seeking favorable reservoirs.Ordos Basin was a multicycle congruence basin that developed in the North China Craton. During Late Triassic epoch, the weather was wet and vegetation was flourish, and had lots of greyblack sandstone, mudstone and oil shale layers with much organic matter and fossil organism.The early Late Triassic could be divided into Chang-9 and Chang-10 Member, the deposition thickness of which ranged from 160 m to 470 m and changed thicker southward and thinner northward. Based on the stratum correlation analysis, the upper interval, Chang 9 Member could be subdivided into Chang 9₁ and Chang 9₂ sublayer by useing fllowing methods, like controlling markers, adjacent wells tracking and cycle comparing, which made full use of basical key layers, assistant key layers and thickness change of adjacent wells.Based on comprehensive analysis and utilizing detail studying of sedimentary system marks which developed in outcrops, logging data, core and seismic data. the author found the main sedimentary systems are composed of fluvial, lacustrine and delta system, which could be subdivided into 10 subfacies and 20 microfacies. Then the author described characters of 10 subfacies and established the sedimentary models.In Early period of Yanchang formation, the sediments supplyed by basin margin and mainly derived from northeast, southwest and northwest were transported far into the centre of lack basin. In them, northeastern, northwestern and southwestern source were derived from Archaean metamorphic rock of Daqingshan, Archaean gneiss of Alxa Ancient Land and Early Palaeozoic gneiss and granide of the Qinling and Qilian fold mountain belt, respectively.Sedimentary belts and plane distribution characters of Chang-9 and Chang-10 Member were studied detailedly, and found the plane distribution characters of sandstone were controlled by sedimentary facies. On shore of lake basin, delta distribution was belt-like. Meandering river delta, braided channel delta and braided rive are developed in northeastern and southwestern portion of the lake basin and southwest Lingtai-Binxian belt, respectively. The lake strandline could be confirmed by utilizing borderline between delta plain and delta front.During early period of Late Triassic, the depositional evolution of lake basin could be divided into beginning and development period. During Chang-10 period, the basin formed NW-SE trending embryo lake, so the sedimentary facies mainly was shallow lacustrine subfacies in central area, and only developed delta front subfacies on eastern and western shore around lake, but fast turned into delta plain subfacies landward. During Chang-9 period, because of affecting by accelerated subsidence, lake basin was extended further, and water body become deeper, but still shallow. Deep lake was only developed along Qiaozhen-Ganquan-Zhiluo belt.