Understanding the Stratigraphic Evolution of the Upper Slope and Shelf Edge: The Key to Deepwater Sand-Shale Distribution

Richard Wild¹, David M. Hodgson², and Stephen S. Flint^2
1Deepwater Reservoir Characterization, Chevron ETC, Houston, TX
²Stratigraphy Group, The University of Liverpool, Liverpool, United Kingdom

The Permian Ecca Group of the Karoo Basin, South Africa preserves an extensive well-exposed siliciclastic basin floor, slope, and shelf-edge delta succession. The Kookfontein Formation includes multiple sedimentary cycles that display clinoform geometries and are interpreted to represent the deposits of a lower slope to shelf succession that reflects decreasing depositional gradient, and volume of sediment bypassed to the deeper basin, through time. The lower slope to shelf succession exhibits a progradational followed by aggradational stacking pattern and the corresponding rising shoreline trajectory. Variations in depositional facies and architecture are used to develop models to predict the presence and nature of turbidite sands in the clinoform strata and the partitioning and distribution of sand and shale in deepwater depositional systems within a structurally simple, low-relief setting. Low-angle or flat shoreline trajectories suggest a high efficiency slope dominated by bypass processes and the contemporaneous development of deepwater or basin floor sands. Conversely, a rising trajectory implies a low efficiency slope with the majority of the sand budget being accreted to the upper slope and / or stored on the shelf with coeval muddy slopes and basin floors. This detailed knowledge of spatial and temporal variations in facies and architecture is key to our understanding of submarine slope depositional systems and illustrates how the detailed analysis of depositional facies and stacking patterns can be used to predict the relative volumes of sediment accreted to the slope or bypassed to the deeper basin, thereby improving reservoir prediction and characterization. This technique is especially useful on datasets that lack high quality seismic data or where large-scale depositional geometries are not readily resolved.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas