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Analysis of extensive exposures of the Permian Laingsburg Formation, Karoo basin, South Africa, have enabled a detailed reconstruction of the base of slope stratigraphy and palaeoenvironments in a deep‐water system characterized by a very narrow grain‐size range (fine sandstone). The deposits include an ≈ 4 km wide and 80 m thick channel complex, fringed by sandy sheet deposits that extend laterally for at least 6 km across depositional strike. Within the channel complex, individual channel fills are marked by shallow basal erosion surfaces draped by thin, parallel‐stratified beds of very fine sandstone and siltstone, interpreted as flow tails to largely bypassing flows. These thin beds are overlain by 0·4 to 5 m thick beds of structureless, fine‐grained sandstone that represent the majority of the channel fills. The basal packages may be partially to completely removed by localized scour in the axial zone of the channel complex but can be mapped laterally into overbank areas where they thicken and are dominated by rippled fine sandstones with intercalated siltstones. Axial confinement resulted from subtle topography on the basin floor, whereby the lower, dense parts of the initially erosive and bypassing flows were partially confined in the lows and the more dilute, slower moving upper parts of the flows deposited sheet‐like successions across slightly elevated overbank areas. The narrow grain‐size distribution prohibited the formation ofcoarse‐grained residual bypass deposits during the initial phases of channel formation. With decreasing magnitude, later flows became more depositional, filling remaining axial depressions with thick‐bedded structureless sandstone. The smaller volumes of late‐stage sediment were more axially focused, producing local scour‐and‐fill features and starvation of the overbank areas. Resulting grain‐size vertical profiles are complex. The basal flow tail packages and overlying massive deposits form a thickening and slightly coarsening‐upward trend in the channel fills. The overbank deposits show a thinning‐ and fining‐upward profile as a result of less bypass plus late‐stage starvation of sand. Application of traditional deep‐water facies models could therefore potentially lead to erroneous interpretations of the channel complex as a prograding lobe and the overbank sheets as channel‐fills.

The Ormen Lange Field is a gas reservoir offshore mid-Norway, developed in a combined structural–stratigraphic–hydrodynamic trap. The lobe-dominated turbidite deposits are mostly of excellent quality, but show a significant deterioration trend towards the fan fringe at its northern margin. Axial parts of the fan contain amalgamated sand-rich deposits, which pass laterally into layered sequences characterized by intercalation of low-permeability heterolithic drapes. Along its 40 km length, the field contains in excess of 400 linked polygonal faults attributed to de-watering of underlying shales. Despite pervasive faulting, reservoir connectivity on a geological timescale is proved by a common pressure gradient in pre-production wells and depletion seen in all later development wells. Recent appraisal drilling of the fan fringe, occupying the crest of the field, encountered only residual gas saturations, despite being located in an area delineated by a seismic direct hydrocarbon indicator. A hydrodynamic aquifer concept is the most plausible explanation for the fluid distribution, in which the gas from the crest of the structure is displaced, leaving behind a northward-thickening prism of residual gas. Dynamic simulation of the fluid-fill evolution over geological time showed the hydrodynamically tilted contact depends on rate of water flow across the aquifer, stratigraphic baffling and faulting, and reservoir quality, i.e. clean sand fraction and effective permeability. Optimal development of this deep-water reservoir depends on understanding the relationship between reservoir quality, connectivity, and the position of the free water level (FWL) in the field. A range of FWL in the north of the field, only weakly constrained by the wells, was empirically established from the hydrodynamically initialized models. This allowed a robust test of the production wells planned to drain the margin of the field. Modelled predictions of reservoir quality and pressures were confirmed by subsequent drilling.

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