FACIES ARCHITECTURE OF KOOKFONTEIN SHELF EDGE DELTA, TANQUA-KAROO BASIN (SOUTH AFRICA): IMPLICATIONS FOR FACIES ANALYSIS AND MODELLING

Abstract

In recent years, the construction of accurate geological models and subsequent upscaling for reservoir simulation are commonly built from an integration of subsurface and surface datasets. Surface data can supply additional information on facies variability and small scale heterogeneity needed for a robust reservoir model, yet impossible to attain from subsurface data. This study therefore presents an outcrop characterisation study of the Permian Kookfontein deltaic succession using a combination of detailed sedimentary log, gamma-ray log and photopanel analysis. 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. Following a hierarchical procedure, 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, represents a deposition of mid-slope to shelf-margin succession exhibiting an overall upward thickening and coarsening with progradational stacking pattern typical of a normal regressive prograding delta. However, in detail, cycles show some anomalies from a purely thickening and coarsening upward succession. Deposition of each cycle is believed to result from: 1. primary deposition by episodic and probably sporadic mouthbar events governed by stream flow dynamics; and 2. secondary remobilisation of sediments under gravity. 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. Widespread distribution of soft-sediment deformation structures, their growth-style and morphology within the studied succession are empirically related to progradation of Gilbert-type mouthbars over the shelf break as well as the slope gradients of the Kookfontein deltaic clinoformal geometry. Apparently, low-slope gradients may have favoured mostly loading and dewatering structures rather than large-scale slumps. Although shale content (shaliness) estimation from gamma-ray logs offers a quick and straightforward way for assessing reservoir potentiality of encountered facies in the field, this is rather crude and therefore its reliability requires detailed petrographic characterisation for confirmation. The described internal heterogeneity in this work is below the resolution (i.e. mm-scale) of most conventional well-logs, and therefore could supplement well-log data especially where there is no borehole image and core data. 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.