Reservoir Architecture of Deepwater Sandstones: Examples from the Skoorsteenberg Formation, Tanqua Karoo Sub-Basin, South Africa
Published:December 01, 2000
William R. Morris, Mark H. Scheihing, De Ville Wickens, Arnold H. Bouma, 2000. "Reservoir Architecture of Deepwater Sandstones: Examples from the Skoorsteenberg Formation, Tanqua Karoo Sub-Basin, South Africa", Deep-Water Reservoirs of the World, Paul Weimer
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Outcrops of the Permian-age Skoorsteenberg Formation contain six major deep-marine systems. These systems vary from 20 to 60 m (66 to 197 ft) in thickness, are exposed up to 40 km (25 mi) in length and are vertically compartmentalized by basinal mudstones of varying thicknesses. Channel-fill deposits associated with levee-overbank systems occur as laterally stacked channel thalweg complexes (<200 m or 656 ft wide, 5 m or 16 ft thick), aggradational channels (over 1000 m or 3280 ft wide, 25 m or 82 ft thick), or erosional channels (500 m or 1640 ft wide and 18 m or 59 ft thick) that later have been back-filled by channel aggradation. They are composed of amalgamated Ta beds with few internal mudstone breaks and relatively high kv/kh ratios. Channel fills are in limited hydraulic communication with sandy levee-overbank deposits due to scour and interfingering. Packages of sandy levee-overbank deposits, about 10 m (33 ft) thick, extend well over 1500 m (4920 ft) away from channel axes. Such packages may exceed 90% net sand but have low kv/kh ratios. Associated crevasse channels, are about 430 m (1410 ft) wide and 26 m (85 ft) thick with a complex fill architecture. Vertically stacked fan lobes (sheet) attain 60 m (197 ft) in thickness, are commonly off-stacked and show an upward increase in degree of amalgamation. Distal lobe (sheet) thins too less than 13 m (43 ft) and is less well amalgamated. Dominant mudstone types within lobe (sheets) include laterally continuous 1 to 2 meter (3.3 to 6.6 foot) thick Tcde intervals between major tabular bodies and thin, discontinuous mudstone drapes within amalgamated tabular bodies.
Mudstones are the most important heterogeneity controlling fluid flow in this type of reservoir. Deterministic mapping of mudstone-bounded flow units would be most readily applied to lobe (sheet) deposits. Small-scale mudstones must be treated as stochastic elements in a numerical description.