Discussion on development of polygonal fault systems: a test of hypotheses

Abstract

D. M. D. James writes: the mechanics of formation of polygonal faults are widely acknowledged as not yet fully understood and it is welcome that Goulty & Swarbrick (2005) attempt a discriminatory test between rival hypotheses. It is however important that such a test bears some relation to physical reality. The ‘test’ is based on the assumption that the ratio of horizontal to vertical effective stress as deduced from leak-off tests in two wells in the North Sea at depths of about 2 km yields the co-efficient of earth pressure at rest therein and hence the residual friction governing the post-nucleation development of polygonal faults, apparently globally and at any depth. It is not clear if the authors consider that the polygonal fault tiers at the well sites are currently active. Neither is it clear if they assume that the coefficient of residual friction along the fracture is a reasonable approximation to the coefficient of initial friction which governs the dip of the propagating fracture tip (and the fault surface as a whole) relative to the orientation of the vertical effective stress. It is hardly surprising that the ratio of horizontal to vertical effective stress ( k ) is very high in the wells analysed; the leak-off tests are from highly overpressured horizons (ratios of pore fluid pressure to total vertical stress approximately 0.80 and 0.84) and the effect of pore fluid pressure coupling (Hillis 2003) will inevitably decrease the value of the horizontal effective stress to lesser extent than it does the vertical effective stress. It is certainly not yet demonstrated that polygonal faults require overpressures of this magnitude for their formation, by any mechanism. Leak-off tests tend to overestimate the horizontal effective stress if no correction is made for the excess mud pressure necessary to overcome the tensile strength of …