Fault zone hydrogeology

[1] Fluid movements, that can be quantified as permeability, can be facilitated or impeded due to the existence of a fault zone.

[1][2] Fluids involved in a fault system generally are groundwater (fresh and marine waters) and hydrocarbons (Oil and Gas).

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) [1] (Figure 4).

[1] However, the void space enlarged by brecciation will lead to further displacement along the fault zone by cementation, resulting in a strong permeability reduction [1] (Figure 5).

[1] 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.

[1][16] Which process takes place depends on geochemical conditions like rock composition, solute concentration, temperature, and so on.

[1] The changes in porosity dominantly control whether the fluid-rock interaction continues or slows down as a strong feedback reaction.

[16] 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.

[20] 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.

[11] Mixing (with low presence of clay) (compared to no fault zone) (- 4 ~ - 5) Porosity (φ) directly reflects the specific storage of rock.

Fracturing, bracciation and initial stage of cataclasis can connect pore spaces by cracks and dilation bands, increasing permeability.

Kaolinite which is altered from potassium feldspar with the presence of water is a common mineral that fills pore spaces.

Lithology has a dominant effect on controlling which mechanisms would take place along a fault zone, hence, changing the porosity and permeability.

Equally important is that identifying the permeability category of the fault zones (barriers, barrier-conduits and conduits) is the main scope of study.

The studies of fault zones are recognised in the discipline of Structural Geology as it involves how rocks were deformed; while investigations of fluid movements are grouped in the field of Hydrology.

(Hydrologists) (Structural Geologist) In situ test includes obtaining data from boreholes, drill cores, and tunnel projects.

[25] Moreover, the presences of breccias and cataclasites, that formed under brittle deformation,[25] suggest that there was an initial stage of permeability increase, promoting an influx of CO2-rich hydrous fluids.

[26] The fluids triggered low-grade metamorphism and dissolution-and-precipitation (i.e. pressure-solution) in mineral scale that shaped a foliated fault core, hence, enhancing the sealing effect significantly.

[26] Underground fluids, particularly groundwater, create anomalies for superconducting gravity data which help study the fault zone at depth.

[28][29] The selection of an appropriate studying approach(es) is essential as there are biases existed when determining the fault zone permeability structure.

For structural geologists, it is very difficult to conduct outcrop study over a vast region; likewise, for hydrologists, it is expensive and ineffective to shorten borehole intervals for testing.

[4] It is economically worth studying the complex system, especially for arid/ semi-arid regions,[30] where freshwater resources are limited, and potential areas with hydrocarbon storages.

However, the presence of a fault zone act as either a seal or a conduit,[21] affecting the efficiency of hydrocarbon formation.

[21] The deformation band blocks the lateral (horizontal) flow of CO2 and the sealing unit keeps the CO2 from vertical migration [21] (Gif 1).

A fault zone that displaces sealing units and reservoir rocks can act as a conduit for hydrocarbon migration.

Two separate seismic events were identified and dated by oxygen isotopic concentrations, followed by episodes of the upward hydrothermal fluid migrations through permeable normal fault zone.

[16] Mineralization started to take place when these hot silica-rich hydrothermal fluids met the cool meteoric water infiltrated along the fault zone until the convective flow system was shut down.

The fluid flow channel along the fault zone will be shut down when the pores are almost occupied by newly precipitated ore minerals.

GIF 1. This GIF shows how fault zones affect fluid migrations in the cross section view. A) The fault zone acts as a barrier that blocks fluid flows across it. B) The fault zone acts as a conduit that allows or facilitates fluid flows across it.
Figure 1. The figure shows the architecture of a fault zone, in which a fault core is enveloped by a damage zone.
Figure 2. The figure shows a cross-section that consists of a fault zone cutting through the sandstone and shale layers. A zoom-in box illustrates the sand-sized grains within the sandstones.
Figure 3. The figure shows a dilation band. Its formation does not involve grain movements and it can facilitate fluid movements.
Figure 4. This shows how a shear band facilitates fluid movements by rotation and sliding.
Figure 5. This shows the brecciation of rocks, thus, enhancing permeability by opening new paths.
Figure 6. The figure shows the formation of a fracture, providing a relatively large aperture for fluid flow.
Figure 7. The figure shows clay smears formed by deformation within a fault zone, providing a sealing effect for fluids.
Figure 8. The schematic diagram shows the approach differences used between Hydrologist and Structural Geologist i.e. subsurface vs surface methods.
Figure 9. This shows the injection of atmospheric carbon by well, encountering a micro-fracture and a fault zone at depth.
Gif 4. This gif shows how fluid (CO 2 ) is facilitated by a fault zone. The fault zone acts as a conduit for fluid and it allows fluid migration to lower layers after it is filled with fluid. [ 6 ] (One-dimensional movement is illustrated for simplicity)
Gif 2. This gif shows how the fluid (CO 2 ) is blocked by the deformation band when fluid travels across but not along with it. [ 21 ] (One-dimensional movement is illustrated for simplicity)
Gif 3. This gif shows how the fluid (CO 2 ) is facilitated by the micro-fracture, which cuts through a sealing unit, within a fault zone. This allows fluid migration to different layers which initially is prohibited by sealing units (shale). [ 21 ] (One-dimensional movement is illustrated for simplicity)
Gif 5. The gif shows how fault, induced by seismic events, propagates into a confined aquifer. [ 31 ] And fluid of the confined aquifer fills the fault zone and precipitates minerals. [ 31 ] Mineralization blocks further fluid movement. Repeated seismic events can deposit economically vulnerable vein structured ore deposit.