Sedimentary modelling and petrophysical characterisation of a Permian Deltaic Sequence (Kookfontein Formation), Tanqua Depocentre, SW Karoo Basin, South Africa

[摘要] ENGLISH ABSTRACT: This study presents an outcrop characterisation and modelling of the excellently exposedPermian Kookfontein Formation of the Ecca Group in the Tanqua-Karoo sub-Basin. Thesedimentary modelling (i.e. facies architecture and geometry) and petrophysicalcharacterisation followed a hierarchical and deterministic approach. Quantitative outcrop datawere based on the thirteen sedimentary cycles that characterise this stratigraphic succession atthe Pienaarsfontein se Berg locality; and these data were analysed using a combination ofdetailed sedimentary log, gamma ray log and photopanel analysis, as well as petrographicthin-section and grain size-based petrophysical analysis.Based on texture and sedimentary structures, twelve depofacies are recognised which arebroadly grouped into four lithofacies associations i.e. sandstone facies, heterolithic facies,mudstone facies and soft-sediment deformation facies; these depofacies and lithofacies formthe basic building blocks for the flooding surface-bounded facies succession (i.e. cycle). Also,based on sediment stacking and cycle thickness patterns as well as relative position to theshelf break, the succession is sub-divided into: (1) the lower Kookfontein member (i.e. cycles1 to 5) exhibiting overall upward thickening and coarsening succession with progradationalstacking pattern; representing deposition of mid-slope to top-slope/shelf-margin succession,and (2) the upper Kookfontein member (i.e. cycles 6 to 13) exhibiting overall upwardthickening and coarsening succession with aggradational stacking pattern; representingdeposition of top-slope/shelf-margin to outer shelf succession. Lateral juxtaposition ofobserved vertical facies variations across each cycle in an inferably basinwards directionexhibits upward change in features, i.e. decrease in gravity effects, increase in waves anddecrease in slope gradient of subsequent cycles. This systematic upward transition in features,grading vertically from distal to proximal, with an overall upward thickening and coarseningprogradational to aggradational stacking pattern indicates a normal regressive progradingdelta. However, in detail, cycles 1-3 show some anomalies from a purely thickening andcoarsening upward succession.Deposition of each cycle is believed to result from: (1) primary deposition by periodicand probably sporadic mouthbar events governed by stream flow dynamics, and (2)secondary remobilisation of sediments under gravity. The facies distribution, architecture andgeometry which governs the sedimentary heterogeneity within the deltaic succession istherefore mainly a consequence of the series of mouthbar flooding events governed bysediment supply and base-level changes. These series of flooding events resulted in the delineation of the studied stratigraphic interval into two main parasequence sets, i.e.transgressive sequence set and the overlying regressive sequence set. This delineation wasaided through the identification of a maximum flooding surface (i.e. maximum landwardsshift in facies) above Cycle 3 in the field. The architecture and geometry of the ensuing deposystemis interpreted to have been a river-dominated, gravitationally reworked and waveinfluencedshelf edge Gilbert-type delta. Widespread distribution of soft-sedimentdeformation structures, their growth-style and morphology within the studied succession areempirically related to progradation of Gilbert-type mouthbars over the shelf break as well asthe slope gradients of the Kookfontein deltaic clinoformal geometry. Analysis of hypotheticalfacies stacking and geometrical models suggests that the Kookfontein sedimentary cyclicitymight not be accommodation-driven but rather sediment supply-driven.The workflow employed for petrophysical evaluation reveals that the distribution ofreservoir properties within the Kookfontein deltaic sandbody geometries is stronglyinfluenced both by depositional processes and by diagenetic factors, the latter being moreimportant with increased burial depth. The reservoir quality of the studied sandstonesdecreases from proximal mouthbar sands, intermediate delta front to distal delta front facies.The major diagenetic factors influencing the reservoir quality of the studied sandstones aremechanical compaction, chemical compaction (pressure solution) and authigenic pore-fillingcements (quartz cement, feldspar alteration and replacement, calcite cement, chlorite andillite). Mechanical compaction was a significant porosity reducing agent while cementationby authigenic quartz and clay minerals (i.e. illite and chlorite) might play a major role inpermeability distribution. The porosity-permeability relationship trends obtained for thestudied sandstones show that there is a linear relationship between porosity and permeability.The relative timing of diagenetic events as well as the percentages of porosity reduction bycompaction and cementation indicates that compaction is much more responsible for porosityreduction than cementation.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 especiallywhere there is no borehole image and core data. The combination of ‗descriptive' faciesmodel and schematic geological model for this specific delta, and petrophysicalcharacterisation make the results of this study applicable to any other similar ancient deposystemand particularly subsurface reservoir analogue.

[发布机构] Stellenbosch University