INTERACTION OF MECHANICAL AND DIAGENETIC PROCESSES IN FAULT ZONES AND THEIR IMPACT ON FLUID FLOW

Fault zones made up of fault cores and surrounding damage zones are the loci of both physical and chemical processes that affect their fluid flow behavior. In order to characterize the architecture of such faults and assess their impact on fluid flow, we present two examples of these processes: One from clastic rocks, and the other from carbonates. The fault zones in the Aztec Sandstone crop out in the Valley of Fire, Nevada, and are formed by lateral shearing of opening mode fractures (joints) and the ensuing cataclastic comminution responsible for fault rocks and associated slip surfaces. There is evidence that subsurface fluid flow focused in open fractures and slip surfaces by dissolving material from the fractured rocks of the damage zones and precipitating minerals along the fault cores. The next example is from the large normal faults flanking the Fucino Basin, Italy. These faults initiated at depth within platform carbonates, and during exhumation eventually juxtaposed the massive limestones against the basin filling clastic rocks. The fault cores include cemented and uncemented narrow fault rocks. The cemented fault rocks have a distinctive textural anisotropy induced by micro veins that parallel the main slip surfaces. The uncemented fault rocks are made of tiny survivor clasts embedded in a fine calcite matrix, and thus behave as granular media. The present day properties of the fault rocks in both cases are characterized by low cross-fault permeability and significant capillary pressure to seal and compartmentalize the field.