Fault Geometry Control from Fluid Migration and Localisation of Diagenesis Around the Moab Fault, Utah

Stuart D. Burley¹, Stuart M. Clarke², and Ross I. Garden^3
1BG Trinidad, c/o BG Group, Reading, United Kingdom
²Basin Dynamics Research Group, Department of Earth Sciences, University of Keele, United Kingdom
³BG Oman, c/o BG Group, Reading, United Kingdom

The Moab Fault, a NW-trending, normal fault with ca 960m maximum surface throw outcrops for ≥ 45km within the NE part of the Paradox Basin, Utah, U.S.A. The fault displaces a sedimentary sequence of Pennsylvanian to Cretaceous age. Outcrop mapping indicates that diagenetic cementation occurred in the ca 50m wide deformation zone around the fault but was concentrated in high strain areas, such as bends and branch points along the fault. Volumetrically, the most significant authigenic minerals are ankerite and calcite, which show increased abundance proximal to the fault and extend up to 200m from the fault at branchpoints. Reduction of iron oxide and cementation with pyrite is also concentrated at the same high strain locations but extends up to 7km from the fault and is best developed at structural highs and in high permeability Jurassic sandstones below capping mudstones.

A combination of structural and fluid flow modelling, with spatial analysis of diagenetic mineral distribution indicates that large scale fluid flow along the Moab Fault is concentrated at structurally complex sites. Away from these sites, there is only minor fluid migration along the Moab Fault. Diagenesis is attributed to ingress of hydrocarbon and aqueous fluids into high permeability sandstones. Fault-related fluid flow occurred during a relatively short lived episode of fault valving. Three-dimensional structural modelling indicates that up-dip hydrocarbon migration from hanging-wall kitchens was focused towards cross-fault migration pathways at branch points and subsequent migration within the footwall was directed towards the footwall high, sealed by a well-developed clay gouge rich fault zone. Hydrocarbon migration was driven by overpressure generation which resulted from gas generation within Paradox Formation traps.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas