四川盆地上三叠统须家河组二、四、六段以砂质沉积为主,砂体厚度大,分布面积广泛。弄清砂体大面积分布的成因,对预测优质储层分布具有重要的实践意义。通过系统的层序地层划分、物源分析、砂体展布研究和沉积相编图,并结合水槽实验和现代地质调研,认为晚三叠世四川盆地古地形平坦,发育多个物源。以冲积扇、河流、三角洲和湖泊4类沉积体系为主。河流—三角洲沉积体系分布广泛,相带稳定,厚度均一;湖泊沉积体系分布于川南,以浅湖亚相为主,向盆地西南存在出水口。四川盆地上三叠统可划分出4期三级沉积层序,须二、四、六段均位于各三级沉积层序的低位体系域,沉积物供给速率大于等于基准面上升速率,物源供给充分。须二、四、六段可进一步划分出多期4级沉积层序,湖平面进退频繁。四川盆地上三叠统砂体分布面积主要受地形坡降、物源供给和水体频繁进退控制。小的地形坡降提供了地质背景。它不仅造成大面积浅水区的发育,而且造成水流的侧向侵蚀能力、水流流态等发生改变。充足的物源供给提供了物质保障。多物源体系的存在,造成砂体从多方向进入盆地,在平面上叠置连片。而水体中砂质含量增高,水流的搬运机制、侧向侵蚀能力差异,有利于砂体向前推进。水体频繁进退是砂体大面积分布的重要途径。水体频繁进退过程中,三角洲前缘长期遭受的河流,波浪,以及沿岸流的强烈改造,发生强席状化,分布面积扩大。%the second,fourth and sixth members of the Upper Triassic（Xujiahe） Formation in Sichuan basin are controlled by sandy sediments,with sand body characterized by large thickness and widespread distribution.To elucidate genesis of large area distribution of sand body brings great practical significance for the prediction of high-quality reservoir distribution.Based on systematic work regarding sequence stratigraphic division,analysis of provenances,analysis of sand body distribution and workout of sedimentary facies maps,combined with study results of flume experiment and geological investigation,it is considered that Sichuan Basin was characterized by flat ancient landform,the development of multiple provenances and four dominant sedimentary systems（alluvial fan,fluvial system,deltaic system and lacustrine system） during Late Triassic.Of the four sedimentary facies,the fluvial-deltaic sedimentary system has widespread distribution,relatively stable facies belts and uniform thickness;and lacustrine sedimentary system develops mainly in South Sichuan,with shallow lacustrine microfacies dominant and water outlet toward Southwest Sichuan Basin.The Upper Triassic of Sichuan Basin can be divided into 4 third-order sedimentary sequences,in which the second,fourth and sixth members of Xujiahe Formation are all located in the low stand tract system of each third-order sedimentary sequence,with the rate of sediment supply more than and equal to the rate of base level rising,and sufficient source supply as well.In addition,the three members can be subdivided into multi-period fourth-order sedimentary sequences according to frequent transgression and regression of lake level.Distribution area of the Upper Triassic sand body in Sichuan Basin is controlled by three main factors： topographic slope,source supply and frequent transgression and regression of water body.By providing geological background,small topographic slopes not only cause widespread development of shallow water area,but also bring many changes of water flow（e.g.lateral erosion and flow pattern,etc.）.Material guarantee can be assured by sufficient source supply.The existence of multiple-provenance system allows sand body entering into from various directions,and finally superimposing and connecting one another horizontally possible,and the increase of sandy sediment content,associated by transportation mechanism and lateral erosion difference of water flows,is very favorable for extending forward of sand body.Frequent transgression and regression of water body plays an important role in widespread development of sand body.During frequent transgression and regression of water body,owing to long-term strong reformation process of river,wave and coastal current,delta front experienced strong blanket deposition and expansion of development area.
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