Local Process-Regime Variations in Shelf-Edge Deltas: Implications for Shelf-Margin Progradation, Stratigraphic Traps, and Deepwater Sediment Delivery

Abstract

Research on deepwater reservoirs has traditionally focused on changes in relative sea level (RSL), where a fall in RSL promotes bypass of sand to deepwater, and a rise promotes storage on the shelf. However, shelf-edge deltas (SEDs) can play a critical role in deepwater sediment delivery under all RSL conditions, as well as in the accretion of the shelf margin. In addition, SEDs can act as important hydrocarbon reservoirs. Though some recent studies have highlighted a link between dominant SED process regime (i.e., fluvial, wave, tidal) and sediment delivery, the impact of variable process regimes within the SED on deepwater sedimentation has been overlooked. Here, we document that process regime variations in a SED complex had significant impact on progradation of the SED and shelf margin, slope channelization, deepwater sedimentation, and reservoir connectivity. In the South African Tanqua Karoo Basin, the Permian Kookfontein Formation is exposed in 3D on seismic-to-bed-scales, and the stacked shelf-slope clinothems can be walked out for 21km along depositional strike and dip. A quantitative approach was used to assess the impact of process-regime variability along the shelf edge on downslope sedimentation. Facies proportions were quantified from sedimentary logs and digitized photopanels. These data were integrated with a grid of robustly mapped key surfaces to construct stratigraphic and quantitative cross sections. Differences in proportions of facies and architectural elements associated with fluvial and tidal processes highlight two main types of shelf-edge depositional environments within the same SED system. Where the shelf-edge deposition occurred from river-dominated deltas, the slope is sand-rich and channelized, with channels widening downslope, and rich in slump and collapse features. Where the SED deposits indicate considerable tidal reworking, the slope is sand poor, SED deposits are thinner and pinch out closer to the shelf edge, and only one small channel was observed. This analysis suggests that process regime variability within SEDs exercises significant control on the character of shelf-edge progradation, slope channelization, and deepwater sediment delivery. In particular, tidal processes reduced SED collapses and deflected sediment along the shelf edge to collapse-dominated fluvial zones linked to channelized slope regions. Moreover, our quantitative analysis aids in predicting the complex connectivity that occurs between these zones.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015