MINERALOGICAL CONTROLS ON FAULT-ZONE ARCHITECTURE AND PERMEABILITY STRUCTURE IN SANDSTONES

Mineralogy influences fault-zone architecture and permeability structure in sandstones in several ways. We focus on two: the affect of cement mineralogy on mechanical and hydrologic properties of the rock, and the influence of grain and cement mineralogy on fault-zone deformation.

The amount of cement required to produce a measurable impact on mechanical or hydrologic properties varies substantially with cement mineralogy. This variability is fundamentally related to mineralogical controls on the spatial distribution of cement, which influences pore connectivity and tortuosity as well as grain contact strength. In quartz arenites, for example, quartz cement is relatively uniformly distributed. Progressive quartz cementation produces systematic changes in both hydrologic and mechanical properties. High porosity quartz-cemented quartz arenites fail through the formation of deformation bands, whereas low porosity examples form fractures. At the other end of the spectrum, calcite cement initially forms in a concretionary manner in quartz arenites. Uncemented, high porosity pathways for fluid flow and formation of deformation bands remain until cement patches coalesce. Thus, significant calcite cement must precipitate before sandstone exhibits significant reduction in permeability or formation of fractures. In contrast, where hematite cement is localized at grain-grain contacts, fractures form in lieu of deformation bands in quartz arenite with porosities as high as 30-35%. Because cement volume is very low the permeability should not be strongly affected, even where the rock is strong enough to support through-going fractures.

Whereas cement mineralogy affects whether or not rocks fail through formation of deformation bands or fractures, grain mineralogy controls the nature of deformation bands. Specifically, it can determine the magnitude of cataclasis in bands formed at shallow depths. At low confining pressure, quartz grains exhibit microscopic evidence of spalling and flaking at grain margins whereas feldspar grains experience transgranular fracture and lithic clasts show evidence of either transgranular fracture or distributed microcracking. Grain mineralogy thereby controls the magnitude of cataclasis and associated reduction in permeability.