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Sources of Clay in Fault Rock and Implications for Permeability Inferred from XRF and Field Mapping

N. A. Anyamele1, N. C. Davatzes1, and J. G. Solum2
1Earth and Environmental Science, Temple University, 1901 N.13th Street, Beury Hall, Philadelphia, PA, 19122
2SIEP-EPT-RXN Shell International Exploration and Production, Inc., Bellaire Technology Center, 3737 Bellaire Blvd., Houston, TX, 77025
Corresponding author: [email protected]

Clay in gouge of faulted sedimentary rocks is a critical control on the sealing capacity of faults. However previous work on the sources of clay in gouge have been largely limited to mechanical incorporation of detrital clays. This hypothesis is mainly based on field observation of folding, smear and attenuation of clay-rich shale beds into the fault zone. This work investigates neoformation of clay minerals as a significant source of clay enrichment in gouge.

To test the soundness of this proposal, careful mapping was conducted at five sites along normally offset sandstone and shale at the Moab Fault, Utah. Detailed mapping revealed distinct structural zones including: (1) smeared shale, layers of gouge rich in clay and separated by slip surfaces that rarely include sandstone breccia; (2) faulted sandstone hosting deformation bands, slip surfaces, joints, and veins. Elemental composition was mapped with a portable X-Ray Fluorescence scanner. Initial protolith rock type and elemental heterogeneity were characterized along measured sections. Preliminary results show: (1) increased concentrations of Ca and Sr along veins and slip surfaces, indicating that veins and slip surfaces are sinks for soluble elements, (2) variation of elements associated with illite such as Ti, K, Rb, and Zr, in the gouge, indicating the gouge is mainly illitic, consistent with results elsewhere. In the future, mineralogy will be quantified by X-Ray diffraction techniques. These observations suggest the clay minerals within the gouge were introduced by neoformation in addition to mechanical incorporation of detrital clays.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas