Canyon-confined pockmarks on the western Niger Delta slope Uzochukwu Benjamin a,b,⇑ , Mads Huuse a , David Hodgetts a a Basin Studies and Petroleum Geoscience Research Group, SEAES, University of Manchester, Manchester M13 9PL, UK b Department of Geology, Obafemi Awolowo University, Ile Ife, Osun State, Nigeria article info Article history: Received 23 December 2014 Received in revised form 24 March 2015 Accepted 25 March 2015 Available online 1 April 2015 Keywords: Pockmarks Shallow fluid flow Geohazard Niger Delta abstract Fluid flow phenomena in the deepwater Niger Delta are important for the safe and efficient exploration, development and production of hydrocarbons in the area. Utilizing 3D seismic data from the western Niger Delta slope, we have identified pockmarks that are confined within a NE–SW oriented submarine canyon system that has been active since the early Quaternary. The pockmarks, subdivided into ‘canyon- margin’ pockmarks and ‘intra-canyon’ pockmarks, on the basis of their plan-form distribution patterns, are found to be spatially and stratigraphically related to stratigraphic discontinuities created by erosion cuts associated with the submarine canyon system. We infer that stratigraphic discontinuities provided pathways for fluid migration within the buried canyon system, allowing fluids from deeper parts of the basin to reach the seafloor as indicated by abun- dant pockmarks above the partly buried canyon. The transportation of fluids from deeper parts of the basin into the buried segment of the canyon system was facilitated by carrier beds expressed as high amplitude reflection packages and by extensional normal faults. The prevalence of the ‘canyon margin’ pockmarks over the ‘intra-canyon’ pockmarks is attributed to the direct connection of the buried canyon margins with truncated reservoir facies in hydraulic connection with deeper reservoir facies. The forma- tion of the ‘intra-canyon’ pockmarks is interpreted to have been limited by fluid flow disconnection often caused by stratigraphic alternation of sand-rich and shale-rich channel deposits that constitute the can- yon fill. Muddy canyon fill units act as baffles to fluid flow, while connected sandy infill units constitute pathways for fluid migration. Occurrence of pockmarks throughout the length of the submarine canyon system is an indication of shallow fluid flow within buried reservoir facies. Systematic alignment of sea- floor pockmarks are clues to buried reservoirs and provide insights into reservoir architecture which could be crucial in frontier exploration of buried deepwater canyons reservoirs and for risk assessment of development activities on top of submarine canyons. A single mega pockmark linked by a gas chimney to a deeper anticlinal structure was discovered to the north of the canyon system. This structure may be indicative of subsurface geo-pressures close to the fracture gradient, highlighting a significant drilling hazard in this part of the study area. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Pockmarks are seafloor expressions of focussed vertical fluid flow and expulsion in sedimentary basins and were first identified by King and MacLean (1970) on the Nova Scotia shelf (Newman et al., 2008; Cathles et al., 2010; Hovland et al., 2010; Andresen and Huuse, 2011). They occur predominantly in fine-grained siliciclastic depositional settings, although a few case studies have been reported in carbonate settings (Gemmer et al., 2002; Judd and Hovland, 2007; Betzler et al., 2011). Seafloor depressions of vari- able sizes are also known to form from the expulsion of fluidized sediments from depth and the subsequent collapse of the overbur- den (e.g., Davies, 2003). Pockmarks tend to form quickly over a geologically short time intervals and are associated with episodic and occasionally catastrophic venting of formation fluids, often including methane gas and occasionally higher hydrocarbons (Hovland and Judd, 1988; Judd and Hovland, 2007; Cathles et al., 2010). In plan-view, pockmarks display a circular to elliptical geome- try of less than 1 m to more than 300 m wide (e.g., Haskell et al., 1999; Hovland and Judd, 1988; Judd and Hovland, 2007; Pilcher and Argent, 2007; Cathles et al., 2010; Sun et al., 2013). In cross- sectional view, they display variable morphology from U-shaped and V-shaped depressions to truncated cones (Newman et al., 2008). Pockmarks of greater than 250 m diameter are referred to as ‘giant’ pockmarks (Foland et al., 1999), while those with http://dx.doi.org/10.1016/j.jafrearsci.2015.03.019 1464-343X/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Basin Studies and Petroleum Geoscience Research Group, SEAES, University of Manchester, Manchester M13 9PL, UK. E-mail address: uzochukwu.benjamin@postgrad.manchester.ac.uk (U. Benjamin). Journal of African Earth Sciences 107 (2015) 15–27 Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci