Outcrop-Based Facies Characterisation and Modelling: An Example of A Permian Tanqua-Karoo Shelf-Edge Delta

This study presents facies characterisation and modelling of the excellently exposed Permian Kookfontein Formation of the Ecca Group in the Tanqua-Karoo sub-basin. The sedimentary modelling (i.e. facies architecture and geometry) followed a hierarchical and deterministic approach. Quantitative outcrop data were analysed using a combination of detailed sedimentary log, gamma ray log, photopanel analysis, and petrographic thin-section and SEM analyses. Based on texture and sedimentary structures, twelve depofacies are recognised which are broadly grouped into four lithofacies associations i.e. sandstone facies, heterolithic facies, mudstone facies and soft-sediment deformation facies; these depofacies and lithofacies form the basic building blocks for the flooding surface-bounded facies succession (i.e. cycle). Lateral juxtaposition of observed vertical facies variations across each cycle in an inferably basinwards direction exhibits upward change in features, i.e. decrease in gravity effects, increase in waves and decrease in slope gradient of subsequent cycles. This systematic upward transition in features, grading vertically from distal to proximal, with an overall upward thickening and coarsening progradational to aggradational geometry indicates a normal regressive prograding delta, representing deposition of top-slope/shelf-margin to outer shelf succession. However, in detail, cycles 1-3 show some anomalies from a purely thickening and coarsening upward succession. Deposition of each cycle is believed to result from: (1) primary deposition by periodic and probably sporadic mouthbar events governed by stream flow dynamics, and (2) secondary remobilisation of sediments under gravity. The sedimentary heterogeneity within the depo-system is therefore mainly a consequence of the series of mouthbar flooding events governed by sediment supply and base-level changes. The architecture and geometry of the ensuing depo-system is interpreted to have been a river-dominated, gravitationally reworked and wave-influenced shelf edge Gilbert-type delta. Analysis of hypothetical facies stacking and geometrical models suggests that the Kookfontein sedimentary cyclicity might not be accommodation-driven but rather sediment supply-driven. The combination of ‘descriptive’ facies model and schematic geological model for our specific delta, make the results of this study applicable to any other similar ancient depo-system and particularly subsurface reservoir analogue.  

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California