Morphological Signature of Fluid Flow Seepage in the Eastern Niger Submarine Delta (ENSD)
Authors: Riboulot Vincent , Cattaneo Antonio , Lanfumey Valentine , Voisset Michel , Cauquil Eric 
Published: May 2011

Pockmarks are crater-like depressions resulting from fluid flows at the seabed. Fluids may be of different types (water, gas, oil) and origins (dewatering, biogenic, thermogenic). In the Eastern Niger Submarine Delta (ENSD), pockmarks are generally circular depressions of variable size and shape which are well expressed on bathymetry obtained from AUV / deep-tow seabed surveys or from 3D seismic data. On bathymetric dip maps, circular features interpreted as pockmarks can be separated into 2 types with different shapes. The first group "Type-1 Pockmarks" is composed by pockmarks whose regular shape with a depression forms a circular crater and it is similar to many occurrencies worldwide well described in the literature. The second group "Type-2 Pockmarks" is composed of "irregular" pockmarks with a distorted shape and boundaries less pronounced than "regular" pockmarks. Both types are well documented in the ENSD between 200 and 800 m water depth. An integrated study including AUV bathymetry, 2D VHR and 3D HR seismic profiles, in situ samples and geotechnical data revealed that different pockmark morphologies may be associated to specific sedimentary processes. This paper proposes a geomorphological characterisation of distinct pockmark types and infers their originating processes only based on 3D seismic and VHR bathymetric data interpretation.

Introduction

The Eastern Submarine Niger Delta (ENSD) is a region of intense oil exploration and exploitation. Many targets for exploration and seafloor installations (e.g. pipelines) are located in areas with irregular seafloor morphology whose potential implications in terms of geohazard must be assessed. Among other seafloor features whose presence constitute a potential geohazard are fluid-flow related features such as pockmarks, requiring often dedicated surveys because of their relatively small size (Kvalstad, 2007). Fluid flow seepage has different expressions when studied in detail, and its impact on the seafloor has to be evaluated before any offshore field development. In general, the identification of the presence and areal distribution of pockmarks influences the choice of anchor/structure locations and the routing of pipelines in order to avoid the pockmarks as much as possible. In addition, it is important to estimate the possible evolution of pockmarks, their state of activity, and their association with other potentially hazardous geological features/processes.

Pockmarks are commonly defined as crater-like depressions resulting from fluid flows at the seabed (Hovland, 1984). We named here these pockmarks as Type-1. Recent studies showed that the morphologies of pockmarks may be the result of fluid seepage contained in buried structures (Pilcher et al., 2007). The analysis of buried structures (fault systems, channels, mass transport deposits, mud volcanoes) is important to detect pockmarks and to understand their origin.

It exists another type of pockmarks related to the presence of gas hydrates (Sultan et al., in press), named here Type-2. Type-2 pockmarks have a distorted shape and boundaries less pronounced than "regular" pockmarks. They might present a ring depression or circular moat at the perimeter of the pockmark, but other expressions at the seafloor reflect different stages of evolution linked to the dissolution/dissociation of gas hydrates (Sultan et al., in press). It is crucial to identify Type-2 pockmarks, because they are associated with the presence of potentially hazardous features such as gas hydrates, methane-derived authigenic carbonates and gassy soils, and their seafloor expression is more subtle than Type-1 pockmarks.