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Facies Distribution in Mud-Prone Sandy Deep-Water Lobes: Outcrop Validation of Physical Models


Deep-marine fan systems represent significant challenges for reservoir prediction at both exploration and development scales. Today's ultra deepwater sub-salt targets, such as the Wilcox, exemplify these challenges, and are commonly characterized by relatively mud-rich reservoirs of overall marginal quality. Complex facies distributions there have been attributed to the interplay of flow behaviour (turbulent, transitional and laminar) and slurry rheology within a basin-floor channelized lobe setting. A series of physical experiments designed to investigate rheological controls on the three dimensional morphological development of subaqueous depositional systems are presented herein. A range of clay-rich sand-laden flows with systematically varying rheology were run over a sloping basin floor allowing the analysis of both flow evolution and deposit morphology. The experiments demonstrate the evolution of radial lobes formed under turbulent flow conditions, through strongly heterogeneous feather-edged lobes with finger-like channels formed under transitional flow, to elongate tongue-like bodies formed under laminar flow. Deposit grain size samples illustrate the probable facies development, inferred from the degree of sorting and vertical variability, and their lateral transitions in lobe sub-environments. These data are related to inferred analogs from Fan 3 of the Permian Skoorsteenberg Fm. Tanqua Karoo. It is demonstrated that predictable facies distributions arise through the rheological evolution of the flows as they evolve from initially turbulent in the proximal and axial fan areas, to progressively transitional to laminar in medial and distal areas. This progression relates to an increase in near bed flow concentration due to deceleration and collapse in response to flow expansion and mud entrainment. These data provide some constraints on facies distributions, and quantification of their lateral transitions, associated with morphometric parameters of architectural elements within subsurface targets.