Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Facies Variability in Shelf-Edge Strata of the Lower Kookfontein Formation, Tanqua Karoo Basin, South Africa: An Analog for Reservoir Heterogeneity in Shelf-Edge Settings
(1) The University of Texas at Austin, Austin, TX.
The earliest-Triassic fill of the Tanqua depocenter, Karoo Basin, South Africa contains stacked deltaic sand wedges deposited in an outer-shelf to upper-slope setting with excellent exposure. A dataset of photomosaics and >2000m of measured sections in 51 locations is used to present a detailed description of the shelf-edge successions.
The shelf-edge deposits studied reveal sedimentary architecture complexities associated with transition from outer shelf to upper slope. Heterogeneities of an analog shelf-edge reservoir will be controlled by: 1) small gradient changes; 2) the variability of the degree of bed deformation; 3) the transition from confined (erosional) to unconfined (non-erosional) flows from outer shelf to upper slope.
The study area offers three-dimensional (3-D) exposures over an area of 100km2. In outcrop, the shelf edge is identified by a subtle basinward gradient increase (<1°) of stratal surfaces parallel to regional paleo-flow directions. In outermost-shelf areas, mapped parasequences contain 2 to 8m thick fine-sand deltaic terminal distributary mouth sands that include units of syn-depositional, soft-sediment deformation features such as sub-vertical dewatering pipes and sediment loading structures that disturb the strata up to thicknesses of 2m. These features were formed by deformation on a low gradient with little lateral movement of sediments. Toward the shelf edge the strata become more heterogeneous, the distributary mouth sands are interbedded with 0.5 to 3m thick slumped and debris-flow units indicative of down-slope mass movement. Some of the down-slope movements occur in concave scours that cut undeformed distributary mouth sands suggesting that slumps are derived from collapsed mouth-bar deposits. The deformed units have a higher fine-grained component than the undeformed distributary mouth sands with which they are interbedded. The undeformed mouth-bar sands pinch out within 2km basinward of the shelf break and most slumps and debris flows pinch out basinward within 4km of the shelf break. On the upper slope the slumps and debris flows are not confined to erosional scars as they are at the shelf edge and they are instead deposited as laterally continuous sheets. Farther down the slope, parasequences are finer-grained, characterized by thin (1-3cm) inter-beds of sandy turbidites and laminated muds with few isolated, sand-rich turbidites that are confined to channels with depths of up to 15m and widths of over 200m.