下刚果盆地A区块下白垩统Albian阶沉积层序与古地理演化

通过对西非被动大陆边缘下刚果盆地中的A区块沉积相和层序地层分析,确定Albian阶主要为一套具"间断-交叉"混积机理的碳酸盐岩混杂陆源砂、泥沉积,可划分出5个三级层序,经历了从混积缓坡→周缘混积台地→破裂混积台地→深水混积陆棚→深海盆地(深水扇)的沉积演化全过程,对应于威尔逊构造旋回中后裂谷拗陷阶段向大陆漂移的过渡阶段,其中发育主力储、产层的SQ2-SQ3层序为Ⅱ型周缘混积-破裂台地沉积层序,可进一步划分为TST和HST两个体系域。解析了混积碳酸盐岩层序地层与沉积充填响应特征,建立了研究区混积碳酸盐岩"滑脱-盐拱-转换"多因素联控的沉积演化模式,提出SQ3层序时期的盐拱构造顶部呈帚状分布和垂向偏转加积发育的破裂混积台缘浅滩为最有利的储集微相类型;以TST和HST体系域作为编图单元,编制了SQ2和SQ3层序各体系域的岩相古地理图并探讨了有利储集相带展布规律,认为A区块Albian阶SQ2-HST、SQ3-TST和SQ3-HST三个体系域中预测出的叠置台缘浅滩,应成为今后的勘探重点。

Sedimentary sequence and paleogeographic evolution of Albian Stage,Lower Cretaceous in Block A of Lower Congo Basin

Lower Congo Basin is located on the passive continental margin of West Africa.This paper studies the sedimentary facies and sequence stratigraphy of Block A,ascertains that Albian is a set of carbonate rocks mixed with terrigenous sand and mud forming in a mixed mechanism of "intermittence-cross" diamictic process.Albian can be divided into five third-order sequences in the area.The whole sedimentary and evolutionary process of the strata has undergone the sedimentary ramp→peripheral diamictic platform→fractured diamictic platform→deep-water diamictic continental shelf→deep sea basin(deep-water fan),corresponding to the transformation phase of the Wilson structural cycle from back-arc rift depression to continental drift.SQ2 and SQ3 sequences,the main reservoirs,are Type II sedimentary sequences of peripheral diamictic-fractured platform and each sequence can be further divided into TST and HST systems tracts.The response characteristics of sedimentary sequences and sedimentary filling of mixed siliciclastic-carbonate rock are analyzed.The sedimentary evolution model of the gang control of multiple factors including "slippage,salt arch and conversion" is established.Besides,at the SQ3 sequence stage,the fractured and diamictic platform edge shoal with a broom-like distribution and the development of vertical accretion on the top of the salt arch structure will be the most favorable microfacies in the area.By taking TST and HST systems tracts as the two mapping units,the authors draw a lithofacies paleographic map of each system tracts,discuss the distribution of the favorable reservoir facies,and then make a prediction about Albian in Block A.Finally the authors think that the superimposed platform edge shoal facies in the SQ2-HST,SQ3-TST and SQ3-HST system tracts should be the key areas in the future exploration.