James P. Evans¹, Zoe K. Shipton¹

(1) Utah State University, Logan, UT

ABSTRACT: Analyses of Permeabilities of Faulted Sandstone in a Subseismic Fault

We determine the structure and permeability variations of a 4-km-long normal fault by integrating surface mapping with data from five boreholes drilled through the fault (borehole- to tens of metre-scale). The Big Hole fault outcrops in the Jurassic Navajo Sandstone, central Utah. A total of 363 m of oriented core was recovered at two sites with displacements of 8 m and 3-5 m. The main fault core is 0.3 - 1.2 m thick and is composed of low-porosity amalgamated deformation bands with slip-surfaces on one or both sides. Fault thickness is not a function of displacement, and can vary over strike distances of several metres. Minipermeameter measurements show a permeability decline from >2000 md to 1.5 md near the fault. Whole-core tests show that fault core permeability is less than 1 md, and individual deformation band permeability is 0.9-1.3 md. Using these results we calculate bulk permeability across and along the fault zone. Calculated transverse permeability over 5-10 metres is 30-40 md: approximately 1-4% the value of the host rock. Fault parallel transmissibilities are 70-90% of the host rock. An inverse power mean calculationfor a complex array of orthorhombically oriented faults yields transmissibilities 20-60% of the host rock. The bulk fault zone permeability is most sensitive to fault core thickness, which exhibits the greatest variability of the fault components examined.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado