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Drinkwater, Nick1, David Hodgson2, David Hodgetts2, Stephen Flint2
(1) Schlumberger Cambridge Research, Cambridge, United Kingdom
(2) University of Liverpool, Liverpool, United Kingdom

ABSTRACT: High-Resolution Geological Modelling of a Basin-Floor Fan – Simply a “Box” of Sand?

Fan 4 of the Skoorsteenberg Formation, Tanqua Karoo, South Africa, is the focus of a high-resolution geological modeling effort, investigating the geometry and stratigraphy of a high net: gross, basin-floor fan system. The model comprises a 16 x 20 km “postage stamp” selected from the better-preserved northern part of the outcrop belt, for stratigraphic Unit 4.2, the middle and largest of three recently identified, offset-compensating sub-units of Fan 4. Initially, the architecture appears simple and tabular; indeed, one critical architectural element of the system is laterally-extensive sandstone - siltstone/mudstone couplets that form the basis of the 23-strong model zonation. However, over the entire Fan 4 outcrop area, progradational, aggradational and retrogradational phases of fan evolution are identifiable resulting in a more complicated stratigraphic architecture than initially predicted. One key challenge of model construction is successful representation of basal, wedge-shaped zones in the progradational phase that would form important stratigraphic traps. Modelling also shows that depositional processes, such as variable amalgamation and local channel incision, are significant controls on lateral continuity within the predominantly thick, tabular-bedded zones of the aggradational phase. Other important trends needing retention into the model include distal bed thinning and grain-size fining throughout fan evolution. All zones have been successfully populated with facies association “objects” identified at outcrop (including sporadic channels) and conditioned to palaeocurrent measurements, probability maps and 40 logged outcrop sections. The resulting model demonstrates the importance of successfully capturing the subtle architectural variations in apparently simple, sheet-like, high net: gross submarine fan systems.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.