Fault Core Thickness: Insights from Siliciclastic and Carbonate Rocks

Abstract

Fault core accommodates intense deformation in the form of slip surfaces and fault rocks such as fault gouge, cataclasite, breccia, lenses, shale smear, and diagenetic features. The complexity and variation in fault core geometry and thickness affect fluid flow both along and across the fault. In this study, we have investigated a total of 99 faults in siliciclastic and carbonate rocks. This has resulted in two large datasets that include 871 fault core thickness measurements T in siliciclastic rocks and 693 measurements in carbonates, conducted at regular intervals along fault elevations (fault height) on the outcrop or photos of the outcrop. Many of these measurements have been analyzed with respect to fault displacement measurements D in order to study the relationship between displacement and fault core thickness and to further uncover the fault growth process. We found that the fault type and geometry, displacement, type of fault rocks, lithology, and competency contrasts between faulted layers lead to significant variations in the fault core internal structure and thickness. Analysis of average values of fault core thickness-displacement data of this study and of previously published studies shows that the core thickness-displacement relationship follows an overall power law, in which its exponent and intercept change depending on the lithology of the faulted rocks. In general, small faults in carbonate and siliciclastic rocks (D≤5 m) show comparable T/D ratios, with a slightly higher ratio in carbonate rocks. The outcomes of this study contribute to the understanding of the fault core internal structure and variation in fault core thickness as a result of the interplay between fault displacement and host rock in different lithologies. These outcomes have significant implication for characterizing the sealing and conductivity potential of faulted rocks, which is relevant to different applications such as petroleum exploration and development of existing fields, hydrogeology, geothermal energy storage and extraction, and CO2 sequestration.

Structural inheritance and border fault reactivation during active early-stage rifting along the Thyolo fault, Malawi

“…The Thyolo fault has a ~1 km high footwall escarpment, which suggests that the total displacement across the fault is at least 1.2 km (assuming a 60° fault dip) and the damage zone documented here reflects fault-related deformation within a kilometre of the surface. The 15-45 m thick footwall damage zone is within the range of other faults with km-scale displacement in global comparisons, whereas the 0.7 m fault core is relatively narrow (Torabi and Berg, 2011;Savage and Brodsky, 2011;Torabi et al, 2019). However, there is considerable scatter in these global comparisons owing to variations in fault kinematics, lithology and depth of faulting.…”

Review of Modeling Approaches to Groundwater Flow in Deformed Carbonate Aquifers

“…Indeed, the fault throw versus damage zone width curve flattens for throw values greater than approximately 100 m in carbonate rocks. Torabi et al 2019aTorabi et al , 2019b. Cataclastic fault cores are characterized by low-porosity fine-grained matrix generated by processes such as grain fracturing, chipping, and further abrasion, which reduce the grain size of the host rock.…”

“…Damage zone of faults are characterized by fractures, veins, and deformation bands depending on the initial porosity of the carbonate rock under deformation. Porous carbonate rocks are less prone to fracturing and tend to primary develop low-permeability cores with increasing fault throw, while the damage zone is less fractured than that of faults developed within low-porosity carbonates (Antonellini et al 2014;Torabi et al 2019aTorabi et al , 2019bTorabi et al , 2020.…”

Uncertainty assessment for 3D geologic modeling of fault zones based on geologic inputs and prior knowledge

“…While not directly applied in the model shown in Figure 5, when lacking direct observations, fault zone thickness can also be approximated based on prior knowledge of a fault's estimated displacement using an established displacement to thickness (D:T) relationship appropriate for the project's geology (Torabi et al, 2019b;Childs et al, 2009). This approach would inherit uncertainty from the subjective interpretation of the fault's historical displacement and the empirical derivation of the fault's D:T relationship.…”

“…CC BY 4.0 License. function provided by Torabi et al (2019b), with curve-fitting parameters log 10 (b) and m to approximate fault zone thickness (f t ) from displacement (f d ), and a modifier f CoreV sZone used to model different sections of the fault zone based on the work by Childs et al (2009). Uncertainty arises in the parameters of the D:T power-fit relationship, log 10 (b) and m and the assumed fault displacement.…”