Reservoir Architecture of Deepwater Sandstones: Examples from the Skoorsteenberg Formation, Tanqua Karoo Sub-basin, South Africa

Morris, W.R., ARCO Alaska Inc., 700 G Street, Anchorage, AK, M.H. Scheihing, ARCO Technology and Operations Services, 2300 West Plano Parkway, Plano, TX 75075, De Ville Wickens, Dept of Geology, University of Stellenbosch, Stellenbosch, 7600, South Africa, and A.H. Bouma, Dept of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803

Abstract

Outcrops of the Permian-age Skoorsteenberg Formation expose six major turbidite systems. These systems vary from 20 to 60 m in thickness, extend to 40 km in length and are vertically compartmentalized by basinal shales of varying thicknesses. Channel-fill deposits associated with levee-overbank systems occur as laterally stacked channel thalwegs (100 m wide, 10 m thick) or as aggradational channels (over 500 m wide, 25m thick). They are composed of amalgamated Ta beds having few internal shale 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 thick, extend over 1500 m away from channel axes. Such packages may exceed 90% net sand but have low kv/kh ratios. Associated crevasse channels are about 500 m wide, 14 m thick, and have a complex fill architecture. Vertically stacked fan lobe/sheets attain 60 m in thickness, are commonly off-stacked and show an upward increase in degree of amalgamation. Distal fan lobe/sheets thin to 13 m and are less well amalgamated. Dominant shale types in lobe/sheets include laterally continuous one to two meter thick Tcde intervals between major tabular bodies and thin, discontinuous shale drapes and heterolithic beds within amalgamated tabular bodies.

Shales would be the most important heterogeneity controlling fluid flow in this type of reservoir. Deterministic mapping of shale-bounded flow units would be most readily applied to lobe/sheet deposits. Small-scale shales must be treated as stochastic elements in a numerical description.