In marine geological settings, pockmarks are evidence of highly focused fluid expulsion at the seabed. The modern seafloor of the Lower Congo Basin (LCB, offshore West Africa) is covered by densely packed arrays of thousands of pockmarks, whose distribution reflects in part the spatial organization of underlying seal bypass features. This study describes and analyses the variable distributions of seabed pockmarks using 3D seismic and spatial statistics, in order to infer subsurface processes that control the fluid migration routes and understand the overall shallow plumbing system of the area. The 3D seismic visualization of feeding conduits (pipes) allowed the identification of the source interval for the fluids expelled during pockmark formation. Pockmark formation may be linked to gas hydrate dissociation and/or expulsion of free gas beneath the GHSZ. Spatial statistics were used to show the relationship between underlying discontinuities and seabed pockmarks distributions, and revealed that pockmark occurrence is not considered to be random. Several different types of geo-mechanical controls were recognised and divided into 1) stratigraphic or depositional controls, 2) strati-structural controls, and 3) structural controls, corresponding to increasing stages of deformation affecting basin sediments. Furthermore, from the wide variability of pockmark sizes present in the area and the local geomorphology, it is possible to conclude that pockmark size is related 1) to the sub-surface depth at which the fluid source interval occurs and 2) to lateral variations of the degree of overpressure. The results of this study are relevant for the understanding of shallow fluid plumbing systems in offshore settings, with implications on our current knowledge of overall fluid flow systems in hydrocarbon-rich continental margins. This is relevant for the understanding of shallow fluid plumbing systems in offshore settings and overall fluid flow systems in hydrocarbon-rich continental margins.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:742832 |
| Date | January 2017 |
| Creators | Maia, Ana |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/111446/ |
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