断陷湖盆层序地层下降体系域的划分

湖泊沉积层序中的体系域四分性明显 ,其下降体系域发育 ,这是由断陷湖盆频繁的构造运动、丰富的物源因素决定的。研究认为 ,湖盆沉积地层中的不整合面或沉积间断面也是形成于湖平面快速下降期 ,但该时期在湖盆边缘地区仍接受沉积 ,因此 ,作为层序边界的不整合面或沉积间断面应位于下降体系域与低水位体系域和湖侵体系域之间。在一个完整的沉积层序中 ,下降体系域是每一个层序的最后一个体系域。同时 ,下降体系域沉积期在湖盆边缘地区砂体发育 ,易形成地层圈闭 ,具有良好的含油气前景

DISTINGUISHMENT OF FALLING STAGE SYSTEMS TRACT OF SEQUENCE STRATA IN A CONTINENTAL RIFT-SUBSIDENCE BASIN

The Tertiary Bohai Bay basin is a rift-subsidence lacustrine basin in east China. The sequence stratigraphy indicates that it is more suitable to divide a sequence into four systems tracts. The falling stage systems tract (FSST) is caused by the frequent tectonic movement of the rift\|subsidence lacustrine basin. The unconformity formed in the period of rapid falling in lake level which was controlled by tectonic movement. There might be deposits at basin margin. That is to say, the unconformity surface or the sedimentary discontinuity surface was located between FSST and lake systems tract. According to the defination of sequence, the sequence boundary is situated at the top of FSST, and the FSST is the last systems tract in a sequence. The sand body of falling stage systems tract at basin margin is developed, and the stratigraphic traps are easy to form. There is a good prospect of oil and gas bearing in the falling stage systems tract.