Reservoir-Scale Characteristics of Fine-Grained Sheet Sands, Tanqua Karoo, South Africa

Dudley, P.R.C., D.E. Rehmer, and A.H. Bouma, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70809

Abstract

Outcrop exposures of the Permian Skoorsteenberg Formation, Tanqua Karoo, South Africa, show five successive laterally continuous submarine fan systems. Each fan is bound by thick basin shales (> 25 m). Fan 4, measuring over 60 m, exhibits laterally contin uous sheet sands that can be subdivided into six distinct depositional packages. The reservoir-scale attributes of the basal package, a thickening-upward sequence, are documented to show excellent overall connectivity and continuity.

Abundant sole marks, rip-up clasts, shallow bedding plane scours, and parallel laminated/ ripple cross-laminations in association with vertical stacking patterns and the absence of channels, suggest deposition from unconfined turbidite flows in a proximal to intermediate lobe setting. The reservoir-scale observations reveal that:

1. apparent tabular plane-parallel laterally continuous sandstones in actuality show variations in thickness and gradual to rapid thinning trends;
2. the system is dominated by amalgamated, very fine-grained massive sandstones;
3. continuity of sandstone beds exceeds 2 km;
4. dip oriented shale bed lengths average less than 500 m while strike oriented shales become laterally continuous (>1 km);
5. grain size distribution is either uniform or varies slightly in both vertical and lateral (>2 km) directions;
6. basin floor topography strongly influences the distribution of sediment;
7. lateral switching of relatively narrow sand bodies comprise the depositional lobe;
8. theoretical well spacing of 2 km shows overall good reservoir connectivity; and
9. correlation of gamma-ray logs on a kilometer scale is achievable with thick-bedded sandstones (>20 cm) or amalgamated beds of equal thickness correlating the best.