Fluid Expulsion Above a Gravity-Linked System: 3-D Seismic Reflection Case Study from the Orange Basin, Offshore Namibia

Abstract

Subsurface fluid flow and leakage is a widespread process manifested in petroliferous basins. However, the underlying subsurface configuration and geological processes controlling fluid plumbing and sustaining long-term fluid escape along continental margins remain poorly understood especially in data-starved deep-water settings. In this study, we aim to constrain subsurface fluid plumbing systems in the post-rift megasequence along the inner slope of the southern Namibian margin. Here, a largescale gravity-linked system developed in response to the margin collapse during the Late Cretaceous. We analysed a remarkably high-quality post-stack depth migrated 3-D seismic reflection dataset where over 50 pockmarks (up to 300 m in diameter and 100 m deep) have been characterized along the seafloor. These pockmarks are restricted to the region devoid of recent mass wasting events along the seafloor. Also, a unique kilometre-scale pipe structure emanating directly from the Cretaceous megaslide complex and terminating on the seafloor is well-imaged in the north-western part of the study area. Our findings reveal the spatio-temporal evolution of these focused fluid flow features is related to potential fluid sources within the post-rift basin fill which includes: (1) near seafloor contourites deposits and (2) deeper Cretaceous mega slide complex. The thrust fault systems related to the mega slide complex coupled with other younger deformation may have served as efficient pathways for channelling fluids towards the shallower stratigraphic levels where the contourites sediments may have acted as transient reservoirs. Under elevated pore-pressure catastrophic venting of fluids and fluidized sediments resulted in the formation of the pockmarks. However, the fluids employed in the pockmarks may be essentially pore-water locally sourced from the contourite with no contribution from the deep-seated migrating fluids possibly sourced from the Cretaceous organic-rich shales. The results from this study have implication for the analysis of fluid flow on continental margin characterized by large-scale gravitational collapse structures. Likewise, the Cretaceous megaslide complex may serve as attractive elements for deep-water exploration along the Namibian margin. Importantly, the discovery of these new fluid-escape features may be associated with expulsion of climate-forcing gases with implication for the carbon budget from this segment of the South Atlantic Ocean.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018