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 viscoplastic 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, k_c, depends inversely on the hydraulic conductivity. Varying the sediment progradation rate, the depth of onset of excess pore pressure, and k_c produces model deformation patterns consistent with seaward-directed squeeze-type flow of overpressured shale (Poiseuille flow) or wholesale seaward motion of the shale and overburden (Couette flow), depending on the overall mobility of the model.