Radial Canyon System, Volta Fan Fold Belt, Ghana
Authors: Nibbelink K.A. , Huggard J.D. 
Published: October 2008

Abstract
A radial canyon system is observed with 2-D and 3-D seismic data in 1500 to 2500 meters of water on the Volta River Fan in Ghana, West Africa. Interpretation of seafloor topography in a deep-water hydrocarbon system is critical for migration and seep detection, potential shallow drilling hazards, seismic imaging problems and drill site or production facility location. This paper defines the seafloor canyon geometry, proposes a model of genesis and discusses the affect on deeper seismic imaging.
Eight straight canyons form a radial pattern on a 2 to 3 degree incline in the middle slope of the Volta Fan. They are 20 to 30 km long, 150 to 200 m deep and 2 to 4 km wide, and have flat floors with high seismic amplitude. The straight canyons are distinctly separated from shelf edge, V-shaped, meandering canyons on the 4 to 5 degree upper slope. Between the radial canyons, a weak seismic reflection interval is interpreted as gas hydrate ridges. Below these hydrate ridges a thick interval of high amplitude is interpreted as free gas in the shallow sediment. Both gas hydrate and free gas zones are interpreted to occur only on the crest of the regional Keta Arch. The interface between the gas hydrate and free gas zone is interpreted as a decollemont surface, which allows slumping and progressive headward erosion of the canyon system.
Gas seeps on seismic data and oil slicks on radar images demonstrate a working hydrocarbon system and potential thermogenic source for the gas hydrates as seen in the Niger and Congo Fans. The variation in amplitude associated with the free gas zone may cause masking of amplitudes in deep structures. From the early recognition of a working hydrocarbon system and correctly imaging deep seismic data to shallow drilling hazards identification and positioning production facilities, understanding the seafloor is critical to the success of any deep-water program.

Introduction
Seafloor mapping has become standard practice in deepwater exploration to understand bathymetry, slope stability for drilling and production facilities and reservoir depositional models. The Volta River Fan in Ghana, West Africa exhibits a radial canyon system in the Middle Slope between 1500 to 2500 meters of water, (Figure 1). These canyons present significant challenge for potential deep-water oil and gas development. The gas hydrate slump model provides for a method of generating these canyons and implies oil and gas in the deeper sediments. Structure, isochron, amplitude and variance maps from 2-D and 3-D define the geometry of the canyon system and the distribution of seabed lithology.

Methodology
Canyon topography on the seafloor was observed on early 2-D seismic, however a seismic program acquired in July 2000 defined the radial pattern for the canyons over the Keta Arch, (Figure 1). The seafloor, hydrate and free gas zones were mapped on pre-stack time migration sections and depth sections. Gridding, contouring and 3-D visualization of the 2- D seismic were used to develop the gas hydrate slump model for the mid-slope canyons.