Reverse Fault

Key Takeaways

• Thrust faults in fold-and-thrust belts propagate along detachment horizons (mechanically weak layers such as evaporites, overpressured shales, or coal seams that have low friction coefficients and allow the overlying rock package to slide as a coherent sheet), and the geometry of the thrust system is controlled by the mechanical stratigraphy of the sequence — which layers are competent enough to form thrust sheets and which are weak enough to serve as detachments; the Alberta Foothills thrust belt, for example, uses multiple detachment levels within the Paleozoic and Mesozoic section, creating a stack of east-verging thrust sheets that have each traveled tens to hundreds of kilometers eastward from their point of origin; the hydrocarbon reservoirs in the Canadian Foothills (principally Cretaceous sandstones and Devonian carbonates) are deformed by these thrust sheets into fault-propagation anticlines and fault-bend folds whose precise geometry determines the size and location of the structural traps that have been productive throughout the basin; seismic imaging of these faulted reservoirs requires pre-stack depth migration (PSDM) with carefully built velocity models that account for the complex lateral velocity variations created by stacked thrust sheets of different rock types.
• Fault-propagation folds and fault-bend folds are the two principal structural trap styles associated with reverse faulting in fold-and-thrust belts, and their geometry follows predictable patterns related to the fault geometry that can be used to predict trap size and closure from limited seismic data: fault-bend folds form where a thrust fault changes dip angle (bends), causing the hanging wall rock to fold as it rides over the bend, with the fold crest forming directly above the bend point in the fault; fault-propagation folds form at the tip of a thrust fault that has not yet broken all the way to the surface, with the folding concentrated in the volume of rock above the fault tip where strain is distributed through ductile deformation rather than brittle faulting; both fold styles produce anticlinal closures that can trap hydrocarbons, and the kinematic models of thrust systems (originally developed by Suppe and Dahlstrom from the Alberta Foothills and later applied globally) allow restoration of the original pre-deformation stratigraphy from the deformed geometry, providing a framework for predicting the geometry of unexplored portions of a fold-and-thrust belt by extrapolating from the known geometry of exposed or drilled structures.
• The seismic expression of reverse faults differs from normal faults in ways that affect their detectability and accurate imaging: normal faults dip in the same direction as the hanging wall displacement (away from the upthrown block), creating a geometry where the fault plane dips into the downthrown block and is illuminated by seismic waves coming from the surface; reverse faults dip in the opposite direction from the hanging wall displacement (toward the upthrown block), and the overturned or steeply dipping strata in the hanging wall above a blind thrust create seismic shadow zones where reflections from beneath the overturned section are difficult or impossible to image with standard processing; the imbricate thrust systems common in fold-and-thrust belts create multiple overlapping fault surfaces and highly variable velocity structures (from slow shales in footwall synclines to fast carbonates in thrust horses) that cause complex raypath geometries and focusing/defocusing effects on reflected energy that standard time-migration cannot adequately handle, requiring full waveform inversion velocity model building and reverse time migration for reliable subsurface imaging.
• Reverse fault reactivation in normally faulted basins occurs when regional stress orientation changes from extension to compression, converting normal faults (which formed under extensional stress with the maximum principal stress vertical) into reverse or strike-slip faults (under compressional stress with the maximum principal stress horizontal) if the paleostress and current stress orientations are appropriately aligned; this inversion tectonics is particularly relevant in the UK and Norwegian North Sea, where Mesozoic rift basins were partially inverted in the Cenozoic as the Alpine collision changed the regional stress field; inverted basin margins show former normal faults that have been reactivated as reverse faults, uplift of the former hanging wall (now the footwall), and erosion of the structural high with consequent breaching of previously sealed hydrocarbon traps that had accumulated oil during the extensional phase; recognizing inversion structures on seismic data (characterized by the hanging wall syncline above the inverted fault being shallower than the equivalent footwall syncline, and by the presence of truncated reflectors and unconformities above inversion anticlines) is important for correctly assessing the trapping geometry and breach risk of hydrocarbon structures in formerly extensional basins.
• Injection-induced seismicity on reverse faults is less commonly discussed than normal fault reactivation (which is the dominant mechanism of induced seismicity from produced water disposal in the U.S. midcontinent) but has been documented in regions where the regional stress regime is compressional: when fluid injection raises pore pressure on a critically stressed reverse fault (a fault that is already close to failure under the compressional stress field), the reduced effective normal stress can trigger slip, generating earthquakes on a fault whose orientation is more consistent with reverse motion than with normal faulting; the recognition that induced seismicity can occur on reverse as well as normal faults is important for induced seismicity hazard assessment in areas where horizontal hydraulic fracturing in tight reservoirs occurs in compressional tectonic settings, such as the Duvernay play in Alberta where some large-magnitude induced seismic events (M4+) have been attributed to fault reactivation on reverse faults in the pre-existing compressional fault network.