Geodynamic Modeling of Sedimentation-induced Overpressure, Gravitational Spreading, and Deformation of Passive Margin Mobile Shale Basins
Markus Albertz, Christopher Beaumont, Steven J. Ings, 2011. "Geodynamic Modeling of Sedimentation-induced Overpressure, Gravitational Spreading, and Deformation of Passive Margin Mobile Shale Basins", Shale Tectonics, Lesli J. Wood
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We investigate the differential loading and pore-fluid pressure required for failure and subsequent prolonged gravitational spreading of passive margin shale basins using two-dimensional analytical limit analysis and plane-strain finite element modeling. The limit analysis, supported by the models, indicates that narrow margins (slope regions that are approximately 50 to 100 km [31 to 62 mi] wide) require pore-fluid pressures that are 80-94% of the overburden weight for failure to occur, whereas for wider margins (~400 km [~250 mi] wide) like the Niger Delta, the corresponding values are 95-99%; these ranges depend on the intrinsic strength of sediments.
In the large deformation models, gravitational spreading in response to sedimentation-induced overpressure caused by delta progradation is investigated. Shale is modeled as a visco-plastic Bingham fluid that is frictional-plastic below yield and has a yield criterion that depends on the effective pressure (mean stress minus pore-fluid pressure). The velocity of the postyield flow of the shale is limited by the viscosity of the Bingham fluid, chosen for this study to be 1018 Pas. Pore-fluid pressure is predicted parametrically to be proportional to the local sedimentation rate during progradation, where the proportionality constant, kc, depends inversely on the hydraulic conductivity. Varying the sediment progradation rate, the depth of onset of excess pore pressure, and kc produces model deformation patterns consistent with seaward-directed squeeze-type flow of overpressured shale (Poiseuille flow) or wholesale sea-TOC ward motion of the shale and overburden (Couette flow), depending on the overall mobility of the model.
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The phenomenon of rocks moving under their own means has always fascinated both scientists and nonscientists alike. The 2006 AAPG Hedberg Conference on Mobile Shale Basins was held in response to a need to gather industry and academic communities in a common forum to address the very existence of mobile shales. Stimulating and informative discussions at that Conference led to this special volume on shale tectonics. AAPG Memoir 93 documents shale tectonics from a variety of basins around the world, including the southern Beaufort Sea; the Krishna-Godavari Basin, India; eastern offshore Trinidad; offshore Brunei; and along the westernmost portion of the Mediterranean Sea. The book also provides information on the petrographic framework, behavior, geometries, and geodynamic models of shales. Publication of this Memoir coincides with a growing interest in shales as hydrocarbon reservoirs, and will add to the body of literature that significantly addresses both extrusive and intrusive shales.