Stratigraphic Development of a Submarine Slope to Shelf Edge, Karoo Basin, South Africa: Lessons for Reservoir Prediction

Flint, Stephen; Hodgson, David; Brunt, Rufus L.; Van Der Merwe, Willem C.; Jones, George; Morris, Emma

Deepwater submarine slope settings exhibit stratigraphy that, although locally complicated, follows a repeating pattern of erosion- and deposition-dominated processes that are preserved in a hierarchy of components that build reservoir and seal systems. The Laingsburg Karoo system exposes at outcrop a complete basin floor through 800 m thick slope succession to shelf deposits. The slope stratigraphy comprises five composite sequences, 80 m to 150 m thick, which have been mapped over 3500 km2. Each composite sequence typically contains three sequences that commonly show a progradational-aggradational-retrogradational stacking pattern. Composite sequences are arranged into three composite sequence sets that show an overall progradational trend. The position and approximate volume of distributive fan deposits down dip of each erosion/bypass dominated slope valley has been quantified. Positioning of successive lowstand fans is controlled by compensational stacking at composite sequence scale and by interaction with topography created by mass transport deposits derived from a lateral confining slope. The transition from slope to shelf is marked by a 80 m thick mudstone unit, which is overlain by a set of 5 wave-dominated, locally fluvially influenced parasequence clinothems. The lower four clinothems show a progradational stacking pattern interpreted as a highstand systems tract, followed by aggradational stacking of the fifth parasequence, followed by a backstepping 20 m thick mudstone. Only clinothem 4 shows upper slope gullies and a basinward thickening wedge of upper slope turbidites. The data are consistent with a type 2 sequence boundary above parasequence 4, with sand delivery over the shelf edge at a time of low (but still positive) shelf edge accommodation and no evidence for subaerial exposure or inclsed valley formation. The volume of sand supplied to the slope at this time was far less than in the underlying slope succession and underscores the point that deepwater delivery slope systems are commonly overlain by quite different shelf systems.

Key messages:

1. The key scale for mapping and making stratigraphic prediction in the slope to basin setting is the composite sequence but on the overlying shelf it is at parasequence set.

2. Minimum volumes of sediment bypassed through slope valleys can be constrained

3. Sand can be delivered across the shelf edge without subaerial exposure of the shelf but volumes are low

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013