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Lobe Types and Facies Associations from Fan 3 of the Permian Tanqua Karoo Turbidite System

Abstract

Superb exposures of a sandy, fine-grained, Permian Tanqua Karoo turbidite system provide an excellent example of the spatial distribution of sedimentary facies that comprise lobe and lobe complexes. The distribution of sedimentary facies within these lobes were mapped spatially by high-resolution dGPS system inorder to build 3D reservoir models and training images of facies architecture of lobe elements. Two types of facies associations comprise lobe complexes in Fan 3. The most common type, – type I, consists of proximal axis of amalgamated Ta and lesser Tae beds with mm mudstone drapes. Scouring and loading is common and disrupts/erodes mudstone beds. Vertical stacking of amalagamated beds common. Laterally, lobe deposition is characterized by waning and depletive flow. Grain size decreases overall away from the axis, and subcritical climbing ripples cap the Ta beds. Laterally the climbing ripple beds lack the basal Ta, and grade to the normal Tce or Tcde at the lobe fringe. Proximal to distal facies changes in the axial part of the systems are characterized by gradually waning flow. Facies changes along the axis occur over a distance between 2-4 times lateral facies changes. The typical trend is similar to the lateral changes except that the climbing ripple facies is uncommon. The type II lobesdiffers from type 1 in that it contains both turbulent and laminar flow. The proximal part is similar to type I, except for slightly higher organic and clay content, especially at the top of the flow. Lateral facies changes show similar patterns to type I lobes, except an increased organic content in the low density turbidites (LDT) and a very thin muddy sandstone (slurry flow) deposits at the lobe fringe. The proximal to distal trends show greater variation than the type 1 lobe. Distally, the high density turbidites (HDT) are capped by a very thin organic-rich cap which thickens into a muddy sandstone bed capping the HDT. These HDT packages can remain thick to the edge of the lobe where they show a rapid decrease in thickness. The muddy sandstone “slurry beds”, often are capped by Tde beds that extend basinward, as lobe fringe facies. These lobe deposits form discrete end members with different reservoir properties, bed connectivity and heterogeneities. Recognition and incorporation of these variations into reservoir models should allow better deterministic range of reservoir properties and performance.