Multichannel reflection seismic data from the Lower Congo Basin, profile GeoB08-288, GeoB08-298, and GeoB08302

Active high intensity gas seepage is documented for the first time at the seaward edge of the salt occurrence in the southern Lower Congo Basin. Microbial methane release from the seafloor occurs on the crests of two 800 m high ridges formed by fault-propagation folding. Intense uplift is documented since the end of the Miocene by distinct onlapping reflections on the landward flank of these ridges. A paleo-pockmark structure suggests an onset of seepage coincident with this deformation period. High-resolution seismic imaging reveals methane migration along strata from Oligocene/Miocene fan deposits towards the ridge crests where large gas accumulations form beneath a discontinuous Bottom Simulating Reflection (BSR). Detailed mapping revealed that free gas and gas hydrate occurrences below and above the base of the gas hydrate stability zone are closely linked to sedimentary strata in the flanks of topographic ridges. Gas transport through the gas hydrate stability zone originates from the shallowest area of the BSR directly beneath the seafloor seep sites, suggesting pressure controlled venting. These sites represent the most seaward salt-related gas seepage features documented in the area and illustrate the initiation of long-lasting seepage at the front of an area of compressional tectonics at a passive continental margin.

本研究首次在下刚果盆地南部盐体分布带的向海边缘，记录到活跃的高强度气体渗漏活动。海底微生物甲烷释放过程，发生在两处由断裂传播褶皱形成的800米高海脊的顶部。通过该两处海脊向陆侧翼清晰可见的超覆反射特征，可证实自中新世末期以来区域发生了强烈隆升。一处古麻坑（pockmark）构造表明，气体渗漏的起始时间与该构造变形期相吻合。高分辨率地震成像结果显示，甲烷沿地层从渐新世/中新世扇状沉积体向海脊顶部运移，并在不连续的海底模拟反射（Bottom Simulating Reflection, BSR）下方形成大规模气体聚集。详细填图结果表明，赋存于气体水合物稳定带（gas hydrate stability zone）基底上下的游离气与天然气水合物，与地形海脊侧翼的沉积地层密切相关。穿过气体水合物稳定带的气体运移，源自海底渗漏点位正下方的BSR最浅区域，这说明渗漏活动受压力机制调控。该类渗漏点是本区域迄今记录到的最靠海的盐体相关气体渗漏构造，同时阐明了被动大陆边缘挤压构造带前沿长期持续渗漏活动的起始过程。