A geomechanical study of the Mesozoic and Tertiary sediments beneath the Scotian Shelf shows that two major weak layers exist at depth: the overpressured unit at 4000 m and the basal Argo salt, which is ductile and diapiric. A consistent relationship is observed between the overpressures and the stress state in the basin. First, the smaller horizontal principal stress increases dramatically just above the onset of overpressuring, and secondly, borehole breakouts--indicators of stress anisotropy--decrease in abundance within the overpressured zone. Neither tectonic shear nor rapid burial can explain these observations; however, overpressuring by fluid generation will result in horizontal stress increase and eventually lead to an isotropic stress state. Compelling evidence is found for hydrocarbon generation being a key cause of overpressuring in the Scotian Shelf. Present-day horizontal stress orientations in the sediments overlying the overpressured zone are aligned with contours of equal overpressure, but not with the traces of Tertiary listric normal faults or with the shelf edge. This implies that contemporary stresses in these sediments are causally related to overpressuring. The upward transmission of lithospheric plate movements and associated stress is unlikely because the ductile Argo salt effectively detaches the sediments from basement. Instead, the rocks of the Scotian Shelf are attempting to slide down the geomechanically weak overpressured zone in the directions indicated by the breakouts, but they are prevented from doing so by the bowl-shaped basement structure in the region.

J A Wade

Papers

Lithostratigraphy 11: overpressure 1