Reservoir-Scale Characteristics of Fine-Grained Sheet Sandstones, Tanqua Karoo Subbasin, South Africa
Philip R.C. Dudley, Donald E. Rehmer, Arnold H. Bouma, 2000. "Reservoir-Scale Characteristics of Fine-Grained Sheet Sandstones, Tanqua Karoo Subbasin, South Africa", Deep-Water Reservoirs of the World, Paul Weimer
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Outcrop exposures of the Permian Skoorsteenberg Formation, in the Tanqua Karoo subbasin, South Africa, show a succession of five laterally continuous submarine-fan systems. Thick basin shales (> 25 m) bound each fan. We concentrated on Fan 4, over 60 m thick, which exhibits seemingly laterally continuous sheet sandstones that can be subdivided into six distinct depositional packages. The reservoir-scale (cm-scale) attributes of the basal package, a thickening-upward succession, include excellent overall lateral and vertical connectivity and continuity.
Abundant sole marks, rip-up clasts, shallow bedding-plane scours, and parallel laminations/ ripple cross-laminations, in association with vertical stacking patterns and the absence of channels, suggest deposition from unconfined turbidity flows in a proximal to intermediate depositional lobe (sheet sandstone) setting. The reservoir-scale observations reveal that: (1) apparently tabular, plane-parallel, laterally continuous sandstones in fact vary in thickness and thin gradually to rapidly; (2) the system is dominated by amalgamated, very fine-grained, structureless (massive) sandstones; (3) lateral continuity of sandstone beds exceeds 2 km; (4) dip-oriented shale bed lengths average less than 600 m whereas strike-oriented shales are more laterally continuous (>1 km); (5) basin-floor topography strongly influenced the distribution of sediment; (6) lateral switching of relatively narrow sandstone bodies (one to tens of km) comprise the depositional lobe; (7) good reservoir connectivity exists for a theoretical well spacing of 2 km; and (8) correlation of gamma-ray logs on a kilometer scale is achievable, with thick-bedded sandstones (>20 cm) and/or amalgamated beds of equal thickness showing the best correlation.