Characterizing the Formation of Clay-Bearing Fault Rocks: Techniques and Applications for Understanding Fault Seal Behavior

John G. Solum, Nicholas C. Davatzes, and David Lockner
U.S. Geological Survey, Menlo Park, CA

The occurrence of clays in fault rocks influences the mechanical and hydrologic behavior of fault zones. For example, enrichment of fault rocks in clays enhances sealing potential. This enrichment can result from incorporation and preservation of clays from faulted host rocks or from neomineralization. In either case, variation in the types of clays influences fault permeability and strength.

We focus on characterizing dioctahedral phyllosilicates, common in fault rocks. The ability to quantify discrete illite/mixed-layer I-S and 2M1/1Md polytypism allows for quantification of neoformed clays in fault rocks, usually characterized by mixed-layer I-S and/or the 1Md illite polytype.

We characterized clays by modeling X-ray diffraction patterns. The programs Newmod and Wildfire were used to generate libraries of hundreds to thousands of patterns of discrete illite/mixed-layer I-S and 2M1/1Md illite polytypes. Matlab routines are used to compare mixtures of the library patterns to natural samples. The goodness of fit of each of the test matches is tracked using the bootstrap method. This provides not only recognition of the best match, but also an estimate of the error of the match.

When this approach is applied to the Moab fault we find that clay gouge is enriched >30% in clays relative to protolith, and that >40% of those clays are neoformed. This neoformed gouge is weak (mu ~ 0.37), and has very low permeability (~3E-21 m2 at 40 MPa confining pressure). Previously published work shows that this mineralization coincides with a period of hydrocarbon migration, and therefore may have influenced reservoir formation.