川西坳陷中段上三叠统须家河组须五段层序格架与沉积相

SEQUENCE STRATIGRAPHY AND SEDIMENTARY FACIES OF THE FIFTH MEMBER OF UPPER TRIASSIC XUJIAHE FORMATION OF THE MIDDLE PART OF THE WEST SICHUAN DEPRESSION,SOUTH CHINA

随着四川盆地非常规天然气勘探开发的进展,在实践中长期被视作烃源岩与盖层的上三叠统须家河组五段的勘探潜力逐渐受到重视。综合研究野外露头、岩芯、测井、地震等地质资料,结合前人对四川盆地须家河组的认识成果,依据高分辨率层序地层学理论,建立了川西坳陷须五段层序格架:将须家河组确定为一个巨旋回,须四段—须六段确定为一个超长期旋回,须五段确定为一个长期旋回,并将其细分为三个极不对称的中期旋回。通过野外露头与岩芯观察认为,须五段沉积时研究区处于三角洲前缘远端—滨浅湖的低能环境,特征为水体浅、河流能量弱、供砂不足,使得三角洲砂体以河口坝、远砂坝为主,水下分流河道常在短暂推进后快速退缩,三角洲末端常被波浪作用改造为滩坝沉积。须五段沉积相带的展布受层序演化的控制,6个中期基准面半旋回中,MSC1上升半旋回、MSC2上升半旋回和MSC3下降半旋回的三角洲砂体较为发育。

The Fifth Member of Upper Triassic Xujiahe Formation in the West Sichuan Depression has be- come a target for unconventional gas exploration. The high-resolution sequence stratigraphic of this strati- graphic unit is investigated using outcrop, drill core, log, and seismic data. The Xujiahe Formation repre- sents a megacycle, and the Fourth to Sixth members a super long term cycle, and the Fifth Member as a long-term cycle which was further divided into three asymmetric mid-term cycles. Based on outcrop and core observations, the Fifth Member is interpreted as representing the distant end of a delta front and low- energy shallow lake environment characterized by shallow water depth, weak stream energy, and inade- quate sand supply. Thus, mouth bars and distal bars were the primary microfacies, whereas distributary channels usually advanced transiently and retreated quickly, so that the distant end of delta fronts were of- ten modified into beach-bar microfacies. Distribution of sedimentary facies of the Fifth Member was con trolled by sequence stratigraphic evolution; there were more delta front sandbodies in the rising half-cycle in MSC1 and MSC2, and in the falling half-cycle in MSC3.