Fault zone hydrogeology is the study of how brittlely deformed rocks alter fluid flows in different lithological settings, such as clastic, igneous and carbonate rocks. Fluid movements, that can be quantified as permeability, can be facilitated or impeded due to the existence of a fault zone. This is because different mechanisms that deform rocks can alter porosity and permeability within a fault zone. Fluids involved in a fault system generally are groundwater (fresh and marine waters) and hydrocarbons (Oil and Gas).
Take notice that permeability (k) and hydraulic conductivity (K) are used interchangeably in this article for simplified understanding__TOC__
Architecture
A fault zone can be generally subdivided into two major sections, including a Fault Core (FC) and a Damage Zone (DZ) (Figure 1).
The fault core is surrounded by the damage zone. It has a measurable thickness which increases with fault throw and displacement, i.e. increasing deformations.
The damage zone envelopes the fault core irregularly in a 3D manner which can be meters to few hundred meters wide (perpendicular to the fault zone). Within a large fault system, multiple fault cores and damage zones can be found. Younger fault cores and damage zones can overlap the older ones.
Different processes can alter the permeability of the fault zone in the fault core and the damage zone will be discussed respectively in the next section. In general, the permeability of a damage zone is several orders of magnitude higher than that of a fault core as damage zones typically act as conduits (will be discussed in section 3). Within a damage zone, permeability decreases further away from a fault core.
Permeability classification There are many classifications to group fault zones based on their permeability patterns. Some terms are interchangeable; while some have different subgroups. Most of expressions are listed in the following table for comparison. Dickerson's categorisation is commonly used and easier to understand in a broad range of studies.
The classification of a fault zone can change spatially and temporally. The fault core and damage zone can behave differently to accommodate the deformations. Moreover, the fault zone can be dynamic through time. Thus, the permeability patterns can change for short-term and long-term effects. +Table 1 Common Permeability Classification of Fault ZoneAuthors / DefinitionsKfz < KhrKfz < Khr and Kfz > KhrDynamic SystemKfz > KhrDickerson's (2000) BarriersBarrier-Conduit/ConduitAydin's (2000) TransmittingVertically Transmitting and Lateral SealingTransmitting or Lateral IntermittentlySealingCaine & Foster's (1999) Quantitative mapping and discrete fracture modelsOthers Detailed description (Weak/ Strong); Barrier/ Conduit Permeability Ratio *K = Permeability/ Hydraulic Conductivity
*fz = fault zone
*hr = host rock = Undeformed rock surrounds the fault zones
Mechanisms (permeability)
Fault zone results from brittle deformation. Numerous mechanisms can vary the permeability of a fault zone. Some processes affect the permeability temporarily. These processes enhance the permeability for a certain period, and then reduce it later on: in this case, like seismic events, the permeability is not constant through time. Physical and chemical reactions are the major types of mechanisms. Different mechanisms can occur different in fault core and damage zone since the intensities of deformation they experience are different (Table 3). +Table 2 Occurrences in Fault Core and Damage Zone MechanismsDeformation BandsBrecciationFracturingSediment MixingClay SmearsCataclasisCompactionDissolutionPrecipitationSeismic EventFault Core + + + + + + + + +Damage Zone + + + + *+ = more likely to occur at
Enhancing fault zone permeability
Deformation bands
The formation of a dilation band, in unconsolidated material, is the early result of applying extensional forces. Disaggregation of mineral fabric occurs along with the band, yet no offset is accommodated by the movement of grains (Figure 3). Further deformation causes offsets of mineral grains by rotation and sliding. This is called a shear band. The pore network is rearranged by granular movements (also called particulate flow), hence moderately enhance permeability. However, continuing deformation leads to the cataclasis of mineral grains which will further reduce permeability later on (section 3.2.3) (Figure 4).
Brecciation
Brecciation refers to the formation of angular, coarse-grained fragments embedded in a fine-grained matrix. As breccia (the rock experienced brecciation) is often non-cohesive, thus, permeability can be increased up to four or five orders of magnitude. However, the void space enlarged by brecciation will lead to further displacement along the fault zone by cementation, resulting in a strong permeability reduction (Figure 5).
Fracturing
Fractures propagate along a fault zone in direction responding to the stress applied. The enhancement of permeability is controlled by the density, orientation, length distribution, aperture, and connectivity of fractures. Even fracture with aperture of a 100-250 μm can still greatly influence fluid movement (Figure 6).
Reducing fault zone permeability
Sediment mixing Sediments, typically from distinct formations, with different grain sizes, are mixed physically by deformation, resulting in a more poorly-sorted mixture. The pore space is filled by smaller grains, increasing tortuosity (mineral scale in this case) of fluid flow across the fault system.
Clay smears
Clay minerals are phyllo-silicate, in other words, with sheet-like structure. They are effective agents that block fluid flows across a fault zone. Clay smears, deformed layers of clay, that are developed along fault zone can act as a seal of hydrocarbon reservoir i.e. extremely low permeability that nearly prohibits all fluid flows (Figure 7).
Cataclasis Cataclasis refers to pervasive brittle fracturing and comminution of grains. This mechanism becomes dominant at depth, greater than 1 km, and with larger grains. With increasing intensity of cataclasis, fault gouge, often with the presence of clay, is formed. The largest reduction occurs on the flows that are perpendicular to the band.
Enhancing and reducing fault zone permeability successively
Compaction and cementation Compactions and cementation generally lead to permeability reduction by losing porosity. When a large region, which consist a fault zone, experience compaction and cementation, porosity loss in host rock (undeformed rock surrounding the fault zone) can be greater than that of fault zone rock. Hence, fluids are forced to flow through a fault zone.
Dissolution and precipitation Solute carried by fluids can either enhance or reduce permeability by dissolution or precipitation (cementation). Which process takes place depends on geochemical conditions like rock composition, solute concentration, temperature, and so on. The changes in porosity dominantly control whether the fluid-rock interaction continues or slows down as a strong feedback reaction.
For example, minerals like carbonates, quartz, and feldspars are dissolved by the fluid-rock interactions due to enhanced permeability. Further introduction of fluids can either continuously dissolve or otherwise re-precipitate minerals in the fault core, and thus alters the permeability. Therefore, whether the feedback is positive or negative heavily depends on the geochemical conditions.
Seismic event Earthquakes can either increase or decrease permeability along fault zones, depending on the hydrogeological settings. Recorded hot spring discharges show seismic waves dominantly enhance permeability, but reductions in discharge may also result occasionally. The timescale of the changes can be up to thousands of years. Hydraulic fracturing (fracking) requires increasing the interconnectedness of the pore space (in other words, permeability) of shale to allow the gas to flow through the rock, and very small deliberately induced seismic activity of magnitudes smaller than 1 are applied to enhance rock permeability.
Taking the Chile earthquake in 2017 as an example, the discharge of streamflow temporally increased six times indicates a sixfold enhancement in permeability along the fault zone. Yet, seismic-induced effects are temporary that normally last for months, for Chile's case, it lasted for one and a half months which gradually decreased back to original discharge. + Table 3
Fault Zone Permeability Changes Mechanisms DilationShear BandsBrecciationFracturingSediment MixingClay SmearsCataclasis (with low presence of clay)Changes in Order(s) of Magnitude (compared to no fault zone)1< 14 ~ 5 2 ~ 3 - 2Act as a seal - 1 ~ - 3
(- 4 ~ - 5)
Mechanisms (porosity) Porosity (φ) directly reflects the specific storage of rock. And brittle formation alters the pores by different mechanisms. If the pores are deformed and connected together, the permeability of rock enhances. On the other hand, if the deformed pores disconnect with each other, the permeability of rock in this case reduces.
Pore types + Table 4
Four Major Pore Types (that are related to fault zone)Pore types IllustrationsMechanismsDescriptionsIntergranular poresIntφ refers to intergranular pores.
Image under petrographic microscope is extracted from Farrel & Healy, 2017.Compaction CementationIntergranular pores are the voids formed during incomplete cementation. The pore spaces are not occupied by grains. The main feature of intergranular pores is that they are irregular in shape. Grain dissolution poresDisφ refers to grain dissolution pores.
Image under petrographic microscope is extracted from Farrel & Healy, 2017.Dissolution Grain dissolution pores are formed when minerals are dissolved out. The pores are left as evidence of pre-existing mineral grains. The minerals usually are feldspars, calcites and so on. Intragranular fracture poresFtφ refers to Intragranular fracture pores.
Image under petrographic microscope is extracted from Farrel & Healy, 2017.Fracturing
Brecciation
Cataclasis
????Intragranular fracture pores are the voids created within a mineral grain. The fracture can cut through a grain into two grains. Transgranular fracture poresT-Ftφ refers to Transgranular fracture pores.
Image under petrographic microscope is extracted from Farrel & Healy, 2017.Fracturing
Brecciation
Cataclasis Transgranular fracture pores are the cracks that cut through more than one mineral grain.
Enhancing porosity
Dissolution The mineral grains can be dissolved when there is fluid flow. The spaces originally occupied by the minerals will be spare as voids, increasing the porosity of rock. The minerals that are usually dissolved are feldspar, calcite and quartz. Grain dissolution pores results from this process can enhance porosity.
Reducing porosity
Cataclasis, fracturing and brecciation The mineral grains are broken up into smaller pieces by faulting event. Those smaller fragments will re-organise and further be compacted to form smaller pore spaces. These processes create intragranular fracture pores and transgranular fracture pores.
Important to be aware is that reducing porosity does not equal to reduction in permeability. Fracturing, bracciation and initial stage of cataclasis can connect pore spaces by cracks and dilation bands, increasing permeability.
Precipitation The mineral grains can be precipitated when there is fluid flow. The voids in the rocks can be occupied by precipitation of mineral grains. The minerals fill the voids and hence, reducing the porosity. Overgrowth, precipitation around an existing mineral grain, of quartz are common. And overgrown minerals infill pre-existing pores, reducing porosity.
Clay deposition