Structure and content of the Moab Fault Zone, Utah, USA, and its implications for fault seal prediction

This paper is included in the Special Publication entitled 'Faulting, fault sealing and fluid flow in hydrocarbon reservoirs', edited by G.Jones, Q.J. Fisher and R.J. Knipe. The structure and content of the Moab Fault zone are described for 37 transects across the fault zone where throws range from less than 100 m to c.960 m. The 45 km long fault trace intersects a sedimentary sequence containing a high proportion of sandstones with good reservoir properties, interspersed with numerous mudstone layers. Typically, the fault zone is bounded by two external slip zones with the fault zone components separated by up to nine internal slip zones. Fault zone components are tabular lenses of variably deformed sandstones and sandstone cataclasites and breccia, with a wide size range, usually enclosed in a matrix of shaley fault gouge containing mm to m scale entrained sandstone fragments. Neither fault zone structure nor content can be predicted by extrapolation over distances as little as 10 m. Although variable in thickness, shaley gouge is always present except where the mudstone is <c.20% of the faulted sequence. The distribution of shaley gouge conforms with existing algorithms for predicting the presence or absence of shaley gouge in subsurface fault zones. The fault zone heterogeneity is attributed to tip-line and asperity bifurcation processes.