Despite the potential of sandstone-filled normal faults to significantly influence fluid transmissivity within reservoirs and the shallow crust, they have to date been largely overlooked. Fluidized sand, forcefully intruded along normal fault zones, markedly enhances the transmissivity of faults and, in general, the connectivity between otherwise unconnected reservoirs. Here, we provide a detailed outcrop description and interpretation of sandstone-filled normal faults from different stratigraphic units in central California. Such faults commonly show limited fault throw, cm to dm wide apertures, poorly-developed fault zones and full or partial sand infill. Based on these features and inferences regarding their origin, we propose a general classification that defines two main types of sandstone-filled normal faults. Type 1 form as a consequence of the hydraulic failure of the host strata above a poorly-consolidated sandstone following a significant, rapid increase of pore fluid over-pressure. Type 2 sandstone-filled normal faults form as a result of regional tectonic deformation. These structures may play a significant role in the connectivity of siliciclastic reservoirs, and may therefore be crucial not just for investigation of basin evolution but also in hydrocarbon exploration.
Bibliographical noteWe acknowledge the support of sponsoring companies of Phase 3 of the Sand Injection Research Group (SIRG). We would like to thank Chris Morley and another anonymous reviewer for constructive comments, and the editor for efficient handling of this paper. We also wish to thank and acknowledge the continuing help and access provided by the Bureau of Land Management and in particular Greg Middleton without whose enthusiasm and support our research would have been much more challenging.
- sandstone-filled normal fault
- sandstone intrusions
- extensional tectonics
- Great valley sequence
- San Joaquin valley
- Central California