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Donald E. Rehmer1, Philip R.C. Dudley1, Arnold H. Bouma1
(1) Louisiana State University, Baton Rouge, LA

Abstract: Architectural and Reservoir Characteristics of Fine-Grained Depositional Lobes, Tanqua Karoo, South Africa

Outcrop exposures of the Permian Skoorsteenberg Formation, Tanqua Karoo (South Africa) reveal five successive basin floor fans, separated by thick basin shale deposits (25 m). Fan 4 is well exposed and characterized by stacked, laterally offset depositional lobes that may exhibit a thickening-upward cyclicity. Seismic-scale features are apparent in measured sections, photo-mosaics, and synthetic gamma ray logs. Fan 4 is composed of six sandstone packages, separated by significant (50 cm) shale breaks. Sand/shale ratios are high (90%) within the packages and throughout the entire 60 m thick succession. Amalgamation of sandstone beds, shallow bedding plane scours, rip-up clasts, sole marks, parallel laminated/ripple cross-laminations in association with vertical stacking patterns, and the absence of channels are indicative of a proximal to intermediate lobe environment. Distal lobe to lobe fringe facies patterns occur in a downdip direction; fan termination results in a digitate pattern with a 'pinch and swell' geometry.

Reservoir-scale observations from the basal package of Fan 4, a thickening-upward sequence, shows variations in sandstone thickness and gradual to rapid thinning trends. Connectivity is high due to amalgamated contacts while sandstone bed continuity exceeds 2 km. Shale bed continuity changes with respect to dip and strike orientations. Basin floor topography strongly influences the distribution of sediment but vertical and lateral changes in grain size vary slightly. Internal sand-body geometry of the basal package suggests that the lobe is a composite stack of laterally offset, relatively narrow, coalescing sand bodies.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana