COTERILL, K., Elf Exploration; S. K. DHOLAKIA*, J. COLEMAN, A. CHAMPAGNE, D. MAROTTA, M. PASLEY, G.TARI, -BP-Amoco; L. BINGA, Sonangol; and H.VAN DIERENDOCK, Shell

Abstract: Sinuous Morphologies in Submarine Channels - Scale and Geometries in Seismic and Outcrop Indicating Possible Mechanisms for Deposition

Increasingly higher resolution 3D seismic data acquired from deepwater West Africa depict and resolve submarine channel systems at a scale equivalent to or greater than that of water bottom side scan sonar or conventional CDP seismic.

Workstation interpretation allows cross-section, plan, and 3D views of seismic geometries with geomorphologic significance. Interpreting depositional processes from seismic facies and attribute analyses is strongly influenced by the observer's scale of reference, data resolution, and physiography of depositional analogs. Our understanding of submarine depositional systems is further challenged by outcrop data that supports a variety of process interpretations at the seismic scale.

Seismic displays of submarine fans show highly sinuous meander forms, levee-overbanks, and lobate-sheets.These features do not appear to have the same process-facies relationships as subaerial morphologies of similar form. Downcurrent migration of fluvial meander belts is generally associated with continuous flow. Combinations of suspended and bed load transport with helical dynamics result in lateral point bar accretion with opposing cut-bank erosion.

In contrast, episodic flows of suspended sediment are dominantly considered responsible for submarine deposition. Differentiating suspension deposits and submarine turbidity currents from submarine "point bars" and bed load 2D-induced traction currents requires a geologic model that accounts for variable depositional mechanisms.

Channel forms identified in seismic are approximately 100 meters wide and 25-50 meters deep. Associated levee and proximal overbanks are approximately 1 kilometer wide. Lobate sheets may extend 10's of kilometers. Each 100-meter channel has a meander form. These forms amalgamate and/or stack into seismic facies units typically referred to as "a channel", "amalgamated and nested channels", or "incised valleys." Axial views of the channels reveal deposition is most often composed of longitudinal accretions. Seismic displays in a current parallel direction exhibit relatively long, continuous reflections that offlap and prograde down-current. Cross-sectional views show discontinuous, rather chaotic patterns. Few examples of later accretion are seen in these data (Figure 1).Although opposing cut-bank erosion may be associated with these accretions, updip and downdip meander bends show either avulsed migration or submarine "scroll" patterns (Figure 2). Consequently, fluvial type meander belt migration does not appear to be a consistent causal mechanism for these sinuous channel forms.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England