Equilibrium Profile and Baselevel in Submarine Channels: Examples from Late Pleistocene systems and Implications for the Architecture of Deepwater Reservoirs

Pirmez, Carlos, R.T. Beaubouef and S.J. Friedmann, ExxonMobil Production Research Company, P.O. Box 2189, Houston, TX 77252-2189

Abstract

Modern submarine channels in a variety of tectonic settings display relatively smooth longitudinal thalweg profiles despite the topographic irregularity of the adjacent seafloor. The erosional and depositional action of turbidity currents over periods of thousands of years leads to the development of a profile tending to an equilibrium condition, i.e., with a local slope such that the prevailing sediment discharge is carried to a local baselevel with minimum aggradation or degradation. Local baselevel is the deepest point within reach of sediment gravity flows travelling through the channel. In the absence of rapid tectonism, sedimentary processes dominate and the equilibrium channel tends to assume a concave-up thalweg profile, as illustrated by the modern Amazon Channel. Where the rates of tectonic deformation are comparable to the sediment flux, the thalweg profile smoothes irregularities but reflects, in part, the motion of active structures, as illustrated by intra-slope channels in the Gulf of Mexico and offshore Nigeria.

Disruption of the equilibrium profile occurs when the rate of tectonic deformation exceeds the sediment flux or when there is a rapid change in the sediment flux compared with the prevailing flow conditions. Alternatively, a new baselevel can be established as a result of channel avulsion or as local catchment areas are filled and subsequently by-passed. Profile disruption is expressed by local steepening or shallowing of the thalweg gradient. Sediment gravity flow response to these disruptions lead to aggradation/degradation of the thalweg and eventual establishment of a new equilibrium profile. 3D seismic and sidescan sonar images illustrate the various processes of disruption and stages associated with equilibrium re-establishment. These include thalweg downcutting and meander cut-offs updip of knickpoints, development of distributary channels and sheets as aggradation progresses downdip, and channel damming and redirection associated with down-to-the-shelf normal faults and folds. The mechanics of equilibrium profiles in submarine channels plays a key role in determining the type and distribution of reservoir elements in deepwater systems.