Multichannel seismic, acoustic and bathymetric data from the Lower Congo Basin

Effective seal breaching is a major contributor to methane seepage from deep sea sediments as it ensures the migration of gas and liquid hydrocarbons from buried reservoirs to the seafloor. This study shows two giant pockmarks on the lower slope of the Lower Congo Basin associated with salt-tectonic faulting and the buried Pliocene Congo deep sea fan. The progressive burial of Pliocene fan deposits results in mobilization of methane from gas hydrates at the Base of the Gas Hydrate Stability Zone which migrates through the hemipelagic seal towards the seafloor along salt-induced faults. Seal-breaching in this part of the Lower Congo Basin relies solely on salt-tectonic faulting contrasting with upslope seafloor seepage settings where polygonal faulting within the hemipelagic seal occurs. Dedicated 2D and 3D seismic and acoustic surveying allows the detailed reconstruction of the evolution of pockmarks which appear to have been active for the last 640 kyr. We also show indications that the modern seafloor depression formed due to reduced sedimentation in the vicinity of active seepage. The presented seafloor seepage features illustrate the mode of gas release from the Pliocene fan in the Lower Congo Basin, which contrasts with previously investigated seepage environments further upslope.

有效盖层破裂是深海沉积物甲烷渗漏的主要驱动因素之一，其可促使气态与液态烃类从埋藏储层运移至海底。本研究在下刚果盆地下斜坡区域识别出两处巨型麻坑，该麻坑与盐构造断裂及埋藏的上新世刚果深海扇密切相关。上新世扇体沉积的持续埋藏，使得天然气水合物稳定带（Gas Hydrate Stability Zone）底界处的天然气水合物中的甲烷得以活化，并沿盐诱导断裂穿过半远洋盖层向海底运移。下刚果盆地该区域的盖层破裂仅依赖盐构造断裂，这与斜坡上部的海底渗漏环境形成显著差异：后者的半远洋盖层内发育多边形断裂。通过专项二维、三维地震及声学勘测，我们可对麻坑的演化过程进行精细重建——这类麻坑在过去640 kyr间似乎一直处于活动状态。本研究还发现，活动渗漏区域附近因沉积速率降低形成了现代海底凹陷。本次呈现的海底渗漏特征，阐明了下刚果盆地内上新世扇体的天然气释放模式，这与此前在斜坡更上部区域研究的渗漏环境存在显著差异。