Conditions for hydrofracture and the fluid permeability of accretionary wedges

Abstract

The dewatering of fine grained sediment piles in accretionary wedges is controlled by zones of active deformation. Episodic hydrofracture in these zones enhances permeability, enabling fluid flow to be faster than the speed of underthrusting. Here I investigate the mechanics of simultaneous shear failure and hydrofracture in muddy rocks as a function of the faulting mode. The physical properties of model rock types are chosen to represent uncemented mud or cherty/marly mudstones that have undergone significant diagenesis. During thrust faulting, hydrofracture always requires pore fluid pressures in excess of the lithostatic load, whereas shallow level wrenching or normal faulting permits hydrofracture at hydrostatic pressures significantly lower than lithostatic load. All three faulting modes are expected to occur in an accretionary wedge attempting to dynamically maintain its critical taper. The model computations indicate that in the upper 80–120 m of a section of “soft” mudstone hydrofracture inevitably accompanies wrench or normal faulting. “Hard” mudstones inevitably hydrofracture to depths of up to 1400 m during normal faulting. These differences in mechanical behaviour have profound consequences for fluid venting capabilities of accretionary wedges. They influence hydrocarbon trapping, and play a role in the localisation of mud diapirs and mud volcanism.