Overpressure Development on Passive Margins: Effects of Loading by Dynamic Deposition Fronts

Matthew Wolinsky, Duke University

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An outstanding question in continental margin research is the cause and timing of slope instability. The low slopes in passive margin settings suggest that low effective stress is an important factor in promoting instability. A primary cause of low effective stress is pore-fluid overpressure, which can be generated by rapid sedimentation over low-permeability units. These overpressures can also complicate offshore hydrocarbon exploration and recovery. Complex stratigraphy and depositional histories, as well as the long residence time of overpressure in low-permeability units, necessitate the consideration of a significant portion of a margin’s history when trying to understand relict overpressure. Here the development of overpressure in a siliciclastic passive margin is explored using a coupled stratigraphic and hydrogeologic cross-sectional process model. The stratigraphic model incorporates subaerial, nearshore, shelf, slope, and rise components, and simulates the development of depositional sequences. Transport of sand occurs via bedload transport in coastal plain rivers, and via mass wasting in nearshore deltas, both modelled with diffusion. This results in a Stefan-type moving boundary problem as the shoreline progrades seaward, which is solved using a fixed-grid finite volume method. Clay is transported in suspension and delivered to the river mouth. In the submarine environment, clays are suspended by Airy waves, travelling in a bottom boundary layer, and settle out below wave-base, building up the continental shelf and slope. The prograding nearshore and continental slope are created by two moving depocenters, and these patterns of depositon are used to force a pore-fluid model, which generates and diffuses overpressures within the growing deposit. A structured FEM grid is used to solve the pore-pressure diffusion, and remeshing is used to model the growing deposit. Model results will be presented which show the time-evolution of overpressure, and explore relationships between stratigraphic and hydrogeologic parameters and the development of overpressure.

Abstract Author(s): Matthew Wolinsky