Seismic Facies Analysis and Reservoir Architecture of Canyon-Channel Complexes - Deepwater Offshore Mauritania
Caroline Julia McKnoe
Honours Degree 2003
University of Adelaide
Understanding geological processes involved in deepwater systems has become increasingly important for hydrocarbon exploration. Developing a greater knowledge of how these systems function can aid in reducing exploration risk and assist in the development of strategies to identify reservoirs in these regions.
Deepwater Offshore Mauritania is currently an active area of exploration but until recently, a limited number of wells existed. Of the wells that have been drilled, little emphasis has been placed on the shallow interval. The impact of seismic facies architecture on sand deposition and reservoir connectivity in a shallow subsurface environment is a key issue in assessing reservoir quality. This study develops a seismic facies classification for recent canyon-channel architecture using 3D and 2D seismic data. Depositional elements associated with canyon-channel complexes are identified using root mean squared amplitude extractions in association with the seismic data set
The study area covered exploration Blocks 2, 3, 4, 5 and 6 with the main emphasis being placed on the Block 2, containing the Koumbi Saleh 3D survey. The Koumbi Saleh 3D survey was divided into Northern, Central and Southern Canyon-Channel Complexes based on dominant sea floor expressions. Sediment transport to the distal reaches occurs through sliding, slumping and debris flow processes causing turbidity currents. Initial canyon formation results from failure of the shelf edge due to sediment loading and high slope gradients. Slope gradients of > 2º are seen along the middle slope with gradients < 2º along the outer shelf, upper and lower slope regions. Re incision of canyons occurs when the slope profile is not at grade, and if below grade, vertical aggradation of channels occurs due to the increased accommodation space. Lateral migration occurs due to limited accommodation space resulting from an at grade slope profile.
The canyons have narrow width:depth (W:D) ratios ranging from 5:1 to 39:1 increasing down-dip from V to U-shape. Internal channels show similar narrow W:D ratios and range from 5:1 to 25:1. Downstream and lateral migration of channels with time is minimal and is confined to the channel belts. Channel belt widths range from 815 m to 1650 m with channel widths ranging from 400 m to 700 m, both increasing down-dip. Vertical aggradation is always present within the channels, with lateral migration commonly seen in the upper and middle slope regions. Vertical degradation due to re-incision of younger channels is important in achieving fluid connectivity.
Despite the apparent similarity in geometry between meandering fluvial channels and deepwater channels, the difference in how they migrate across a region is important, as this will control where sand-prone sediments are deposited. Submarine canyons show minimal downward and lateral movement, which confines the sand-prone sediments to within a channel belt approximately twice the channel width.
Depositional elements have differing potential to host reservoir-prone lithologies. Sites for preferential sand deposition, inferred from high amplitude reflections, are outer channel bends and proximal regions of frontal and channel splays. Sand deposition along outer bends occurs due to flow stripping, which may produce offset stacked sand bodies. Frontal splays, when developed, tend to show high amplitude reflectors that are associated with sand-prone sediments in the region proximal to their initial development. Levees and overbank deposits are associated with fine-grained soft sediments, which are more likely to impede fluid flow, and are considered low priority exploration targets.