Role of Sedimentation-Controlled Overpressure Development in Mobilizing Salt and Shale Substrates: Implications for the Tectonic Evolution of Passive Margins

Markus Albertz, Christopher Beaumont, Steven J. Ings, and Sofie Gradman
Dalhousie University, Halifax, NS

Mobile substrates, such as salt or shale allow passive margins to undergo large-scale gravity-driven deformation involving updip extension, downdip contraction and compensating translation of overburden, yielding first-order similarities in structural style. However, major rheological differences between salt and shale suggest that the timing and interplay between sedimentation and the structural evolution of passive margins may differ significantly, depending on whether deformation is accommodated by salt or shale substrates. For example, salt is viscous and begins to deform as soon as it is differentially loaded with sediments. In contrast, the rheology of shale is poorly constrained but critical levels of pore fluid pressures seem to be essential for shale mobilization. Based on known material properties, industry reports, and logical reasoning, a simple model for shale rheology can be developed in which shale is frictional-plastic in the undeformed state but behaves viscously during finite deformation when fluid pressures have become sufficiently large to cause failure.

We use 2D, vertical cross-section finite element models of passive margins to investigate the role of sedimentation on overpressure generation and mobilization of shale. The rate of deltaic sedimentation parametrically controls pore fluid pressures in a marine shale substrate. Thus, the zones of overpressures, mobile shale, and unstable overburden correlate with the region of greatest sedimentation rate at the delta front. The results are consistent with stepwise outbuilding of the delta and stratigraphically distinct depocenters. We compare the structural style and dynamic evolution of our shale tectonic models with equivalent salt models as well as case studies.