Experimental constraints on the diagenetic self-sealing capacity of faultsin high porosity rocks

A thorough understanding of fault seal processes is important in many pract ical and geological applications, which depend on subsurface flow of fluids . While the mechanisms involved in fault sealing are well known, the micros cale processes involved and their relative contribution to sealing remain d ebatable. In particular, the extent to which diagenetic processes overprint cataclastic fault sealing has not been resolved, mainly due to the long ti me scares required to measure these effects. Here, we report results from a novel suite of room temperature experiments that combined continuous analy sis of dissolved silica using on-line high performance liquid chromatograph y, with low strain rate creep loading on sandstone cores. This technique al lowed changes in silica concentration during different phases of deformatio n to be resolved, and revealed a 7-fold increase in overall silica concentr ation immediately after dynamic faulting by localised cataclasis. Calculati ons based on these results show that the mass of dissolved silica from the resultant fault gouge increased by up to two orders of magnitude relative t o that from the intact rock over the same time scale. This increase represe nts the first stage of the inherent diagenetic sealing capacity of the faul t, presumably through localised diffusive mass transfer. Post-test microstr uctural studies suggest that the magnitude of diagenetic self-sealing depen ds on lithological and mechanical attributes of the host rock, which contro l fault gouge microstructure. Our experiments suggest that diagenetic proce sses may account for permeability reduction of up to two orders of magnitud e, comparable to reductions due to cataclasis alone. Together, these two pr ocesses account for the 5-6 orders of magnitude reduction of permeability o bserved in natural faults and deformation bands. (C) 2000 Elsevier Science B.V. All rights reserved.