Characterizing the damage zone of the San Andreas Fault using core samples from the San Andreas Fault Observatory at Depth (SAFOD)

Alph Wright
Texas A&M University, Department of Geology & Geophysics College Station, TX, USA
[email protected]

Understanding the faulting process is fundamental to infer deformation history and fluid-flow properties of faulted reservoirs. Under diagenetic conditions, shear-induced grain-size reduction decreases permeability and increases capillary pressure. Faulted siliciclastic rocks from the western damage zone of the San Andreas Fault were retrieved at ~3km depth to characterize mechanical and diagenetic grain-size reduction in high strain faults. These samples capture a representative suite of faults with different magnitudes of shear strain and particle sizes. Quantifying particle size and shape as a function of strain and mineralogy allows us to inform predictive models for fluid flow properties, and test the hypothesis that the dominant comminution process changes when particles reach 1 micron in diameter. Our samples have shown that grain size distributions follow a power-law, N(S) is proportional to S-D, where N(S) is the number of grains larger than size S and D is the fractal dimension, correlating with the dominant deformation mechanism. Our previous analysis of particle size as a function of mineralogy, focused on particles larger than several microns, and showed that D ranges from ~1.3-1.7 for microbreccia, indicating that constrained comminution is the dominant deformation mechanism, and increases with increasing strain. Our findings are consistent with, but do not confirm, a change in scaling associated with a change in comminution processes at 1 micron. Therefore, further study will target particles between 50 nm and 2 microns, where we expect to find D~1, inferring a change from brittle to plastic deformation.

AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects