Toward Understanding the Processes That Control the Hydrologic Properties of Thrust Faults

Permeabilities of thrust faults in sandstones can range from 100 mD to 1 nD, a range of 8 orders of magnitude. This variability reflects in larger part the permeability of the protoliths from which these fault rocks are derived. These faults typically have a permeability that is 1-6 orders of magnitude lower than the host rock from which they are derived. Understanding the ways in which fault rocks with these low permeabilities develop is crucial for understanding how thrust faults will influence subsurface fluid flow. This presentation describes the formation of thrust faults in a variety of clean sandstones from a shallow, young fluvial sand from California with porosities ranging from ~30% to Mesozoic, well-lithified sandstone from Wyoming with a porosity as low as a 1%.

There are a variety of mechanisms that lead to the formation of fault rocks. The mechanisms include compaction, cataclasis, authigenesis and mineralization. While there are a variety of mechanisms that operate to create fault rocks, it is apparent that the lowest fault permeabilities occur in fault rocks that are derived from low permeability host rocks. Thrust fault in the unconsolidated California sands have a reduction in permeability of almost 6 orders of magnitude. However, since the permeabilities of the host in this case are on the order of several Darcy, this reduction results in an absolute permeability only as low as 0.01 mD. The lowest fault permeabilities (~ 1 nD) occur in faults that are derived from rocks with permeabilities of ~0.01 mD. This indicates that regardless of deformation mechanism, in order for extremely low permeabilities to exist, fault rocks must be derived from low permeability host rocks.