Study On Sequence Stratigraphy And Sedimentary Facies Of The Depositional Period Of The SSQ3 In Yanchang Formation In Ordors Basin

Ordos basin is located in the midwest of China.In the geological history,it is a large,multi-cycle and complex cratonic basin.The Yanchang formation of middle-upper Triassic is not only the strata which obtain oil and gas,but also the oil and gas exploration target for decades.Based on the high rosolution sequence stratigraphy and modern sedimentology,the sequence stratigraphy of Yanchang formation is researched by using lots of outcrops,core and logging data,and the author select the third two-order sequence to research high-resolution stratigraphy,sedimentary faces and systematic.This paper is to provide theoretical basis to effectively solve the finding of the lithological reservoir.Yanchang formation is divided into four second-order sequence,and the third two-order sequence is divided into three third-order sequence and nine fourth-order sequence by identifing sequence boundary,flood surface and base level cycle.On the basis of the sequence stratigraphic division of typical well and outcrop,the isochronous stratigraphic framework of the whole basin and the internal basin of the SSQ3 sedimentary period are established,and this papaer systematically research the stratigraphic characteristics in every third-order sequence.Based on sedimentary facies analysis of the typical wells and outcrops,connected wells are compared within the isochronous sequence stratigraphy framework,and the basic geological maps are plotted,such as the contour map of strata thickness and the contour map of sandstonestratum ratio.Based on the comprehensive use of the typical wells of sedimentary faces,the typical outcrops of sedimentary faces,the connected wells of sedimentary faces and the contour maps,the sedimentary facies map and the sedimentary facies pattern are plotted,and then the sedimentary framework and sandbody distribution pattern of this period are analyzed.Delta,river,alluvial fan,lake,gravity flow are five major types of sedimentary facies in the research area.The delta and lake have the maximum deposition range.The multi-source from surrounding mountains or farther distance are to the centre of the basin,forming the main sedimentary system of delta-river-alluvial fan-gravity flow-lake.The northeastern basin is mainly composed of meandering river and meandering river delta.The facies belt is wide,and the sediments are characterized by fine grain size.The southwest basinis mainly composed of bian river and bian river delta.The facies belt is narrow,and the sediments are characterized by coarse grain size.The alluvial fan is developed in the edge of the basin.The lake is developed in the center of the basin.Gravity flow is developed in the delta front to the center of the lake basin.