J.J. Walsh¹, C. Childs¹, T. Manzocchi¹, J. Strand¹, A. Nicol², M. Tomasso³, and M. Schoepfer¹

1Fault Analysis Group, University College Dublin, School of Geological Sciences, University College Dublin, Belfield, Dublin, D4, Ireland, phone: +35317162606, fax: +35317162607, [email protected]; ²GNS Science, Wellington, New Zealand; ³Bureau of Economic Geology, Bureau of Economic Geology, John A. & Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, 78758

Post-depositional normal faults within the turbidite sequence of the Late Miocene Mt Messenger Fm of the Taranaki basin are characterised by granulation and cataclasis of sands and by the smearing of clay beds. Clay smears maintain continuity for high ratios of fault throw to clay source bed thickness (ca. 8), but are highly variable in thickness, and gaps occur at any point between the clay source bed cutoffs at higher ratios. Although cataclastic fault rock permeabilities may be appreciably lower (~2 orders of magnitude) than host rock sandstones, the occurrence of continuous clay smears, combined with low clay permeabilities (10's to 100's nD), means that the primary control on fault rock permeability is clay smear continuity. A new permeability predictor, the Probabilistic Shale Smear Factor, is developed which incorporates the main characteristics of clay smearing from the Taranaki Basin. The PSSF method calculates fault permeabilities from a simple model of multiple clay smears within fault zones, predicting a more heterogeneous and realistic fault rock structure than simple deterministic approaches (e.g., SGR, Shale Gouge Ratio). Nevertheless, its averaging effects at higher ratios of fault throw to bed thickness provide a rationale for the application of simpler fault rock mixing models at appropriate scales.

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California