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Architectural Characteristics of Fine-Grained Submarine Fans: A Model Applicable to the Gulf of Mexico
Arnold H. Bouma (1,3), Henry DeV. Wickens (2), James M. Coleman (3)
Submarine fan deposits in the Gulf of Mexico and in many areas of the Atlantic Ocean fall into the category of fine-grained, low overall sand/shale ratio basin-floor fans. Models published over the years that have been applied to either exploration or development are primarily based on sand-rich fans, most of which were deposited in active margin settings and are not applicable to the areas mentioned above. Observations from the excellent outcrops of the Permian Tanqua Karoo in the southwestern part of the Karoo Basin, South Africa, the Jackfork Formation (Pennsylvanian) in Arkansas, and the Deep Sea Drilling Project Leg 96 on the Mississippi Fan make it possible to construct a macro-scale model that addresses the architecture of both the macro- and meso-scale depositional features of fine-grained submarine fans deposited in generally unconfined settings.
At the entrance to the basin floor in the base-of-slope area, the large conduit that was initially carved out across the basin slope, may start to widen. Most of the high- and low-density flows moving from the submarine canyon in the upslope area through the conduit (fan valley or upper fan channel), will be too small to occupy the entire width of the lower part of the conduit. The result is a complex build-up of channel fills incised into each other, forming amalgamated sandstone bodies interbedded with remnants of shale-rich levee-overbank deposits. Once in the basin proper the degree of erosion of flows will be reduced and channel-levee complexes result. The channel fills are primarily amalgamated sandstones whereas the levee-overbank deposits are often thin-bedded, low-contrast, low-resistivity sandstones with high porosity and permeability. The interbedded shales reduce vertical conductivity of fluids but proper production techniques make these deposits potentially very productive. The channels gradually become smaller and loose their influence to govern the direction of the head of density flows. A gradual widening of the deposits results, known as sheet sands or depositional lobes. They have high sand/shale ratios and the overall sheet sand deposition suggests lateral switching resulting from seafloor compensation rather than frequent channel progradation.
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