Geomorphometric analysis of seabed pockmarks, offshore western Niger Delta: A case study of the Freeman Field

The morphometry and spatial distribution of seabed pockmarks have been used as proxies for subsurface conditions and local hydrodynamics. We have characterized and analyzed the distribution of seabed pockmarks in the Freeman Field, offshore western Niger Delta using a high-resolution 3D seismic data to understand the relationships between pockmarks and their controlling factors. We identified a total of 684 pockmarks in the Freeman Field at water depths between 1461 and 2395 m. The pockmarks are circular, elliptical, and elongated in plan view, having U-, V-, and W-shaped geometries in cross-sectional view. The average length, width, and depth of the pockmarks are 210, 111, and 15 m, respectively. More than 80% of the pockmarks were randomly distributed. From statistical analysis, most of the pockmarks occurred within a water depth range of 1600-2100 m. The randomly distributed pockmarks occurred at variable water depths whereas the pockmarks that were aligned along fault planes occurred at shallow water depths (approximately 1400-1700 m). However, those confined within the canyon occurred at deeper water depths (approximately 1700-2400 m). Our results show no correlation between the water depth and any of the pockmark dimensions; therefore, we hypothesize that changes in water depth had no effect on any of the pockmark dimensions because the Freeman Field is located at water depths greater than 1000 m where the current velocity range is lower (0.2 and 0.42 m/s). Hence, pockmark dimensions were comparatively uniform throughout the study area. We suggest that the variation in pockmark morphometry is linked to seafloor currents and the activity history of the pockmarks whereas the spatial distribution is linked to structural and stratigraphic discontinuities. Furthermore, our results give insights to the factors that should be considered during risk assessment before hydrocarbon exploration and production.