Abstract - The Moab Anticline is the most prominent hangingwall structure associated with the Moab Fault, a salt-related normal fault in the Paradox Basin, SE Utah. The fold is accommodated by a combination of normal faults, with maximum throws up to ca 100m, and bed-parallel flexural slip. Fold growth was synchronous with Triassic-Lower Tertiary fault movement on the Moab Fault and related salt diapirism. Geological and petrophysical analysis of the well exposed array of faults associated with the anticline allows detailed definition of the geometry and kinematics of the array and consideration of the likely effects of faulting on fluid flow at production time scales.
The anticline is an asymmetric fold developed parallel to the Moab Fault with a wavelength of 1km, amplitude of 350m and length in excess of 10km. The short limb of the monoclinal structure dips at up to 40¼ towards the Moab Fault, whilst the other limb dips 2¼-15¼ away from the fault. Numerous swarms of faults occur along the crest of the structure where they accommodate keystone-like collapse of the crest of the anticline. On the short limb of the anticline the larger of these faults are synthetic to the Moab Fault but are antithetic on the opposing limb. The array of normal faults is highly segmented and strongly clustered, with the range of maximum fault throws varying along strike proportional with changes in the fold amplitude. Where crestal collapse faults cut mudstone horizons the faults are often either offset by, or flatten out into, bedding plane slip horizons indicating that bedding plane slip and crestal collapse were coeval. The faults are particularly well developed within high porosity aeolian sandstones and represented by flow-banded shear zones, slip band zones, and slip surfaces. Consistent cross-cutting relationships indicate that flow-banded shear zones are the oldest structures and that slip band zones are of intermediate age. Slip surfaces are often the youngest structures and generally have displacements greater than ca 1m. This sequence of faulting reflects a change in shear mechanism from grain flow through cataclasis to discrete faulting, mainly due to increasing depth of burial and confining pressure.
Core permeametry results show reduced permeability, of 1-3 orders of magnitude, associated with individual slip bands. These data, combined with detailed mapping and measurement of slip band densities over a 2km2 area spanning the anticlinal hinge, permit assessment of the likely effect of slip bands on flow on a production time scale. Flow modelling results indicate that, on a reservoir scale, slip bands are likely to reduce fault-normal effective permeability by less than an order of magnitude. The heterogeneous densities and the clustering of faults, as observed in the Moab Anticline, will however introduce local fault heterogeneity and anisotropy of permeability which may have a significant negative impact on the performances of individual wells.
Abstract of talk given to:
Faulting, fault sealing and fluid flow in hydrocarbon reservoirs University of Leeds, 23-25 September 1996