Abstract - The Moab Fault is a 45km long, salt-related, normal fault of c.950m maximum surface throw offsetting a Pennsylvanian to Cretaceous sedimentary sequence. The surface trace of the fault comprises a simple southern segment, and a more complex array of fault splays in the north. Maximum surface throw occurs on the southern segment where the fault is associated with both a footwall high and a hangingwall anticline. Throw decreases to zero northwards where the fault is continuous with a monocline of < ca 100m amplitude. The fault trace is associated with a swarm of minor structures extending up to ca 50m from the fault, accommodating a very small proportion of the fault throw (<<10%). Minor structures are more intensively developed at sites of structural complexity, e.g. around branch-points, fault bends, overlap zones and fault-related folds. The fault was active from the Triassic until at least the mid-Cretaceous.
At outcrop the sharply defined fault zone is 1-10m wide and, typically, separated from undeformed wallrock by a pair of bounding slip surfaces. Dominant rocks within the fault zone are foliated shale or shaley gouge, often with entrained blocks of sandstone, sheets of breccia and blocks and sheets of slightly deformed or undeformed country rock, all of which may be intersected by internal slip surfaces. Slip band zones, up to several metres wide, occur immediately adjacent to the fault zone where the wallrock lithology is suitable, i.e. well-sorted sandstone. Rapid lateral changes in structure, content and width of the fault zone are usual and comparable vertical changes are likely. Relay zones and breached relay zones, on a range of scales, contribute significantly to complexity of the fault zone structure.
The dual bounding slip surface structure of the fault zone is attributed to a combination of tip-line and asperity bifurcation processes. Fault tip-line bifurcation is due to irregular tip-line propagation resulting in leading edge segmentation, fault overlap and eventual duplex formation. Asperity bifurcation removes a fault surface irregularity by the generation of a new slip surface which by-passes the original slip surface. Repetitions of either or both bifurcation processes give rise to a fault zone with a complex and unpredictable internal geometry. Fault zone thickening by bifurcation is intermittent rather than progressive and localised fault zone thinning may also occur.
The complexity of fault zone structure seen at outcrop has clear, negative, implications for fault seal prediction. Across-fault juxtapositions predicted from seismic data may be very different from those which are actually present and the complex distribution of fault rocks cannot be characterised realistically from well data. The irregular distribution of fault rocks suggests that prediction of fault seal by calculation of shale smear potential may be suspect, but the Moab Fault Zone nevertheless shows good preservation of shaley gouge even where the fault zone is narrowest. Shale smear methods of seal prediction have been widely applied with considerable success, which suggests that although the hydraulic properties of fault zones may be affected by their structural complexity, this influence must usually be insignificant relative to that of the shale content or net:gross of the faulted sequence. Moreover, the complexity of fault zone structure raises the question of whether the pursuit of disaggregated methods for fault seal prediction can ever provide a more reliable tool than the empirical/statistical methods currently employed.
Abstract of talk given to:
Faulting, fault sealing and fluid flow in hydrocarbon reservoirs University of Leeds, 23-25 September 1996