孤北地区断阶式坡折沉积层序特征与隐蔽圈闭预测

CHARACTERISTICS OF DEPOSITIONAL SEQUENCE WITH THREE FAULT-STEPPED BREAKS AND THE PREDICTION OF SUBTLE TRAPS IN THE GUBEI AREA

根据地质、钻井及地震等资料的综合分析,将下第三系划分为1个一级层序、4个二级层序、10个层序组、17个三级层序和54个体系域,建立了层序地层格架,并以体系域为单位恢复了古环境和沉积体系。研究表明,可容纳空间的变化可与裂陷作用、盆地底板沉降作用、基准面变化、海洋与大陆气候影响作用、局部褶皱、断裂、底辟、火山活动等、多物源沉积物充填及古环境变迁等相联系。在孤北地区通常具有3阶坡折,沉积物具有逐阶搬运、扩散的特征:首先在靠近盆地的三阶坡折附近形成低水位扇,在靠近孤岛凸起的一阶坡折之下形成特殊的低水位期冲积扇或辫状河道沉积或沉积物呈过路作用;接着在二阶坡折和三阶坡折之间形成低水位楔;最后在一阶坡折之下和二阶坡折附近发生下切河谷充填作用。低水位期形成的低水位扇、低水位楔、辫状水道及下切谷充填砂都可能构成有效储层,成为非构造圈闭赋存的重要位置。

Based on the geological data, well logs and seismic sections, the authors establish the Tertiary sequence framework including 1 firstorder sequence, 5 secondorder sequences, 17 thirdorder sequences and 54 systems tracts. The study indicates that the accommodation changes are controlled by the factors of rifting,subsidence of basin floor, base level changes,effects both of oceanic climate and continental climate, local folding, local faulting, diapiring, volcanics, filling of multimaterial resources, and the revolution of paleoenvironment. There exits 3 faultstepped breaks, and the sediments are transported and dispersed one by one across them. Lowstand fan is formed near the third faultstepped break,and the special lowstand sediments such as alluvial fan and braided channel or no deposition for the bypassing near the first faultstepped break below the Gudao Uplift.Then lowstand wedge is deposited between the first faultstepped break and second faultstepped break. The filling of incised valley is occurred between the second faultstepped break and third faultstepped break. It is very important for finding subtle traps in the sandstones of lowstand fan, lowstand wedge, braided channel and incised valley formed during the lowstand period.