下刚果盆地碳酸盐岩盐筏体沉积规律

Sedimentary laws of carbonate salt raft in Lower Congo Basin

基于盐筏活动过程中对古地貌和局部可容空间的影响,产生了对上覆碳酸盐岩沉积相带和地层展布样式的控制作用这一认识,通过对下刚果盆地A区块内碳酸盐岩盐筏体钻井岩相、沉积相与地震相特征分析,对盐筏体内部碳酸盐岩沉积规律进行了总结。研究结果表明:1区块盐筏体内共发育A型、B型和C型3类地震相,从盐筏活动初期至晚期,盐筏体内部地震相垂向上呈现C型—B型—A型演化序列。2C型地震相表现出不规则状杂乱反射、弱振幅、连续性较差的特征,为盐筏活动初期产物,主要为泥灰岩非储层相;A型地震相具有席状、强振幅、中—高频率、连续反射特征,为盐筏活动晚期产物,沉积稳定分布,以颗粒滩与砂质碎屑滩为特征,二者均是盐筏活动程度较弱背景下的沉积产物;B型地震相在盐筏强烈活动中形成,由蠕虫状、中等振幅、中—高频率、断续状的B1型地震相、层状、中等振幅、低频、连续反射的B2型地震相以及席状、中—弱振幅、中—高频率、连续反射的B3型地震相三者组合而成。3横向上表现出B1型—B2型—B3型—C型演化过程,由盐隆高部位向低部位依次呈现高能颗粒滩相向滩间海相转变的过程。4周期性"滑脱-沉积"作用下,形成盐筏体内以B型地震相为代表的帚状地层展布样式;根据盐隆高部位位置变化,盐筏体内常形成"原地顶隆型"和"偏移旋转型"2种沉积形态,主要体现在高能颗粒滩相带的空间叠置与迁移方向差异。碳酸盐岩盐筏体作为被动大陆边缘含盐盆地构造-沉积演化阶段产物,其发育特征具有广泛的适用性。盐筏体的形成过程不仅是储层建造的过程,而且常形成良好的油气运聚系统,具有重要的石油地质意义。

Based on the influences of salt raft activities on paleo-geomorphology and local accommodation,the control effect on overlying carbonate sedimentary facies belt and stratigraphic distribution pattern was identified. Through analyzing the drilling lithology,sedimentary facies and seismic facies of carbonate salt raft in Block A,Lower Congo Basin,the carbonate sedimentary laws in salt raft were summarized. The research results indicate that:( 1) Three types of seismic facies,i. e.,Type A,B and C,were developed in carbonate salt raft. From the early stage to late stage of salt raft activities,the seismic facies within salt raft show C-B-A evolution sequence in vertical direction.( 2) Type C seismic facies is characterized by irregular chaotic reflection,low amplitude and poor continuity as the product in the early stage of salt raft activity,dominated by marlstone non-reservoir facies; Type A seismic facies is characterized by sheeted high amplitude,mid-high frequency and continuous reflection as the product in the large stage of salt raft activity,where sediments are stablely distributed and characterized by grain shoals and sandy grain shoals. Both are sedimentary products in the context of weak salt raft activity. Type B seismic facies is formed in intense salt raft activities,consisting of wormlike imtermittent Type B1 seismic facies with medium amplitude and mid-high frequency,layered Type B2 with medium amplitude,low frequency and continuous reflection and sheeted Type B3 with mid-low amplitude,mid-high frequency and continuous reflection.( 3) B1-B2-B3-C evolution sequence is presented in lateral direction,and the transition process is from high-energy grain shoal facies in the high position of salt uplift to intershoal marine facies in the low position.( 4) Under the periodical 'slipsedimentation',broom-shaped stratigraphic distribution pattern is formed,represented by Type B seismic facies in salt raft. According to the changes in the high position of salt uplift,two sedimentary types are often formed in salt raft,i. e.,autochthonous top uplift and shifting rotation with the main difference in spatial stacking and migration direction of high-energy grain shoal facies belt. As the product of tectonic-sedimentary evolution in salt-bearing basin on passive continental margin,carbonate salt raft shows the development characteristics with wide applicability. The formation process of carbonate salt raft is not only a process of reservoirbuilding,but also has a favorable hydrocarbon migration and accumulation system with a great petroleum geologic significance.