Influence of poroelastic behavior on the magnitude of minimum horizontal stress, Sh, in overpressured parts of sedimentary basins

In many sedimentary basins of the world the minimum horizontal stress, [S.sub.h], is greater in overpressured zones than in normally pressured zones at equivalent depths. A common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress, [S.sub.v], and [S.sub.h] within certain bounds, and the difference is smaller under lower effective stress (i.e., higher pore pressure, [P.sub.p]). However, in the overpressured parts of the central North Sea graben, United Kingdom, and the Sable subbasin of the Scotian Shelf, Canada, conventional friction envelopes underestimate the magnitude of [S.sub.h]. These data instead indicate that [S.sub.h] increases at a rate proportional to but less than the rate of increase of [P.sub.p], a condition consistent with a [P.sub.p]-induced deformation of the rock called poroelastic behavior. This paper argues that, whereas friction may govern [S.sub.h] in normally pressured basins, poroelastic behavior is responsible for the unusually high Sh in the overpressured parts of these same basins. Data on the [P.sub.p] and [S.sub.h] gradients from these basins suggest that [Delta][S.sub.h]/[Delta][P.sub.p] [approximately] 0.7.