GARDEN, ROSS, BG Research & Technology; ANDREW FOXFORD, Castle Farm; SIMON GUSCOTT, Amerada Hess Ltd.; STUART BURLEY, BG Research & Technology; JOHN WALSH, Fault Analysis Group, Liverpool & JUAN WATTERSON, Fault Analysis Group, Liverpool
Fault-related diagenesis along the Moab Fault, Utah takes the form of calcite veining and copper mineralization within the fault zone. and calcite cementation and into reduction of wallrocks. Calcite cementation is concentrated within 250m of the fault zone, whereas reduction is preferentially developed adjacent to the fault and extends >7km from the fault in high permeability, well connected sandstones. The diagenesis is volumetrically most significant at structurally complex sites along the fault trace, such as branch-points and fault bends and occurred over a relatively short time period of <10 Ma during the early Tertiary subsequent to major fault displacement. Calcite cementation and iron reduction are attributed to migration of aqueous and hydrocarbon fluids, respectively. These fluids utilized the fault and associated fracture system as a vertical flow conduit from deeper sources into high permeability Jurassic sandstones juxtaposed against the fault. The presence of hydrofracture-hosted vein networks that cross-cut the Moab Fault indicate that the fluids were overpressured and that fluid migration resulted from a short-lived episode of fault valving.
The concentrations of enhanced diagenesis at specific sites demonstrates the focusing of fluid flow along the fault. The location of such sites is due not only to the fracture permeability associated with the structural complexity but also to the fact that these sites are characterised by fault-bounded structural highs through which fluid migration was concentrated.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah