Fault

A fault is a planar fracture or zone of fractures in rock across which the two sides have moved relative to each other. The movement is driven by tectonic stress and can range from centimetres to hundreds of kilometres. Faults are classified by the direction of motion: normal faults form when rock is pulled apart (extension) and the hanging wall drops down relative to the footwall; reverse and thrust faults form when rock is compressed and the hanging wall is pushed up; strike-slip faults accommodate horizontal motion along the fault plane. In petroleum geology, faults are important in two competing ways: they create traps by juxtaposing reservoir rock against impermeable rock (fault traps) and they can threaten reservoir integrity by providing migration pathways that leak hydrocarbons out of a trap.

Key Takeaways

Faults are described by their dip (the angle of the fault plane from horizontal), their strike (the compass direction of the fault trace at surface), and their throw (the vertical component of displacement across the fault). A normal fault with 50 metres of throw means one side has dropped 50 metres relative to the other. In reservoir characterization, the throw across a fault determines whether reservoir rock on one side is juxtaposed against reservoir or shale on the other, which controls whether the fault can seal hydrocarbons.
Fault sealing occurs when the fault zone itself or the juxtaposition of shale against reservoir prevents fluid from crossing the fault. A shale smear seal forms when ductile shale is dragged along the fault plane during movement, creating a continuous low-permeability layer. Cataclastic seals form when the fault grinds reservoir rock into fine-grained gouge that has much lower permeability than the undeformed rock. Fault seal analysis is a critical step in risking any fault-bounded prospect.
The Alberta Foothills contain some of the world's most studied thrust fault systems. The Foothills fold-and-thrust belt formed as North America's western margin was compressed during the Laramide orogeny (roughly 75 to 55 million years ago). Thrust faults carried sheets of carbonate and clastic rock eastward over younger sediments, creating structural traps in the folded and faulted rock beneath the thrust sheets. Fields like Turner Valley, Jumping Pound, and Waterton are all thrust fault-related traps.
Normal faults in extensional basins (such as the North Sea Graben system) are the dominant trap type for many major oil fields. The Brent, Forties, and Ninian fields of the North Sea are all in fault-bounded or fault-assisted structural traps where normal faults created horst blocks or tilted blocks with four-way or three-way closure against fault planes.
Fault complexity increases with distance from the master fault surface. In the damage zone surrounding a major fault, rock is cut by multiple smaller faults and fractures. This damage zone can have dramatically different permeability than the intact rock (higher if fractures are open, lower if sealed by cement or clay), which creates heterogeneous flow paths that complicate reservoir simulation and EOR planning.

What Is a Fault and How Does It Form?

Take a stack of paper and push on both ends. The sheets slide past each other. Now take a block of brittle material (a dry bar of soap works well) and push from both ends. Rather than bending smoothly, it snaps along a diagonal plane. That snap is a fault: the two pieces have moved relative to each other along a planar surface. In rock, this happens at depths where the temperature and pressure are within the brittle failure range, typically in the upper 10 to 15 kilometres of the crust.

The stress that causes faulting comes from the movement of tectonic plates. Where plates pull apart (at mid-ocean ridges and continental rift zones), the crust stretches and normal faults form. Where plates collide, the crust is compressed and reverse or thrust faults form. Where plates slide past each other (like California's San Andreas Fault), strike-slip faults form. Most geological basins experienced several phases of different stress regimes over geological time, meaning they commonly contain multiple generations of faults in different orientations.

In the Western Canada Sedimentary Basin, the dominant structural style changes from west to east. In the Foothills, Laramide compression produced the fold-and-thrust belt with major northeast-dipping thrust faults. Moving east into the Alberta Plains, the stress was largely extensional and transpressional, creating smaller normal and strike-slip faults. The Peace River Arch in northern Alberta was a Paleozoic basement high with bounding faults that influenced deposition and trap formation throughout the Mesozoic.

Fast Facts

The Turner Valley oil and gas field, discovered in 1914 and brought to major production in 1924, is located 40 kilometres southwest of Calgary and was Canada's first major oil field. The field produces from Mississippian carbonates in the hanging wall of the Turner Valley thrust fault, with overlying Mesozoic shales providing the seal. At peak production in 1942, Turner Valley was producing 25 percent of all Commonwealth oil during World War II. The field also provided the first recognition in North America that a gas cap could overlie an oil zone in a fault-bounded anticlinal trap, a geological relationship now understood worldwide.

Fault Traps in Petroleum Geology

A fault trap requires two things: a structural closure created by the fault geometry, and a seal at the fault plane that prevents hydrocarbons from leaking across. The structural closure can be a simple fault-bounded block (the fault dips one way and the reservoir dips the other, creating a closure against the fault) or a more complex pattern involving multiple faults and tilted blocks.

Fault seal evaluation asks: is the fault plane sealed or open? This depends on what rock is juxtaposed across the fault and what happened to the fault plane during movement. The shale gouge ratio (SGR) is the most widely used quantitative method. SGR calculates the proportion of shale in the rock interval that passed through a given point on the fault during its motion. A SGR above about 0.18 to 0.20 is generally considered capable of sealing against a hydrocarbon column, though the threshold depends on the capillary pressure and the hydrocarbon type (gas leaks more easily than oil).

Open faults (those that are not sealed) can leak hydrocarbons out of a trap to shallower formations or to surface. Natural seeps at surface in many petroleum basins are the result of hydrocarbons migrating up open fault planes from deep petroleum systems. In the Alberta Foothills, natural seeps in the mountains mark the surface expressions of deeply rooted thrust faults that have acted as migration conduits for millions of years.

Faults and Reservoir Heterogeneity

Inside a producing reservoir, faults create barriers and baffles to fluid flow that are not present in unfaulted rock. A fault with a clay-rich gouge zone acts as a low-permeability barrier: fluid (water injection, gas, or produced oil) will not easily cross it. A fault with open fractures in the damage zone acts as a high-permeability channel that preferentially carries fluid along the fault trend rather than through the matrix.

Both effects cause problems for reservoir management. Fault barriers segment the reservoir into isolated pressure compartments that must be managed separately. Fault-related fracture channels cause early breakthrough of injected water at producers near the fault, bypassing unswept oil in the matrix between the fractures. Identifying fault geometry, orientation, and the sealing or open character of each fault in a producing reservoir is one of the key tasks of the reservoir geologist during field development.

4D seismic (time-lapse seismic surveys repeated during field life) is the primary tool for monitoring fluid fronts crossing (or being blocked by) faults in producing fields. The Gullfaks field in the Norwegian North Sea has been extensively monitored with 4D seismic, and the data clearly shows hydrocarbon saturation changes being blocked along certain fault planes while passing freely across others.

Faults are classified as normal, reverse, thrust, strike-slip, and oblique-slip depending on the kinematics. A thrust fault is a low-angle (less than 45 degrees dip) reverse fault. Related terms include normal fault (a fault where the hanging wall has moved down relative to the footwall; forms in extensional tectonic settings; the dominant fault type in the North Sea and Gulf of Mexico rift basins), thrust fault (a low-angle reverse fault where the hanging wall has moved up and over the footwall; forms in compressional settings; the dominant structural style in the Alberta Foothills fold-and-thrust belt), fault seal (the ability of a fault plane to prevent hydrocarbon migration across it; evaluated using shale gouge ratio, juxtaposition diagrams, and capillary pressure analysis), throw (the vertical component of displacement across a fault; determines the juxtaposition of reservoir and non-reservoir units across the fault plane), and graben (a down-dropped crustal block bounded on both sides by normal faults; major petroleum provinces including the North Sea are graben basins flanked by fault-bounded horst blocks).

How an Unrecognized Fault Cut Production from a Viking Sand Field by 40 Percent

An operator was developing a Viking Formation oil pool in central Alberta. The field had been mapped from 2D seismic acquired in the 1980s, and the development plan called for five production wells and two water injection wells arranged in a line-drive pattern to push oil from the injectors toward the producers.

The first two producer-injector pairs responded as expected: oil rates at the producers increased after water injection started and stabilized at plateau rates. The third producer-injector pair, located at the southern end of the pool, did not respond. The producer maintained its natural depletion decline with no response to injection. Tracer tests (chemical tracers pumped with the injection water) confirmed that tracers were appearing at the nearby producers but not at the unresponsive third producer, despite the wells being only 800 metres apart.

A newly acquired 3D seismic survey over the pool revealed a northeast-striking normal fault with approximately 6 metres of throw cutting across the injection-production line between the third pair. Six metres of throw in the Viking sand (which is 8 to 12 metres thick) was enough to juxtapose the sand completely against the overlying Joli Fou shale across the fault plane. The fault was sealing. The injected water was staying in the northern compartment.

Two remediation wells were drilled in the southern compartment, one producer and one injector, at a combined cost of CAD 3.2 million. The southern compartment then responded to injection and recovered an additional 180,000 cubic metres of oil that would otherwise have been left in the isolated southern block. The unrecognized fault had hidden roughly 30 percent of the pool's recoverable reserves from the original field model. The 3D seismic cost CAD 620,000. The 2D-only development plan had been directing injection water into the wrong compartment for 22 months before the fault was identified.