146-4 Statistical characterization and modeling of damage/process zones in subseismic normal faults: the role of mechanical stratigraphy

Abstract:

Being able to quantify and model fracturing properties in damage or process zones of small-scale subseismic faults is of huge interest in every kind of project involving the storage and/or migration of fluids in the subsurface. Depending on their architecture, these faults can act as conduits, barriers or combined highly anisotropic conduits/barriers, and being small, with offsets at or under the limit of seismic resolution, but quite often very frequent, they remain very elusive, and yet they can dramatically influence the hydraulic behavior of reservoirs and seals.

Here we present a quantitative outcrop analogue study on spectacular coastal cliff outcrops of the islands of Malta and Gozo, where normal faults with offsets up to a few 10s of meters crosscut a sequence of porous calcarenites with variable petrophysical and geomechanical properties.

Statistical distributions of all measurable fracture parameters have been obtained with rigorous procedures developed by our group in the last 10 years, and include, for each set or for the full network, spatial distribution, kinematics and deformation mechanism, orientation, topology, length and height, density, intensity, representative elementary area (Bistacchi et al., 2020, SE; Martinelli et al., 2020, JSG; Casiraghi et al., 2025, SE).

Petrophysical and geomechanical properties have been characterized in the field with semi-quantitative methods and in the lab with porosimetry, microCT, monoaxial, triaxial, and Brazilian tests, and yield a detailed picture of the behavior of the calcarenites under progressive deformation.

The elastoplastic constitutive models include non-linear effects, such as strain weakening, poroelasticity, strength envelopes with a cap (i.e. porosity collapse in compression), and are used to define material behavior in advanced P-FEM large-deformation non-local numerical simulations where faults emerge spontaneously as plastic shear zones with realistic geometry and topology.

Comparing the statistical mapping of fracture parameters with the evolution of material properties in numerical simulations (i) validates the modelling approach and (ii) provides insight into fracturing processes in the nucleation and early propagation (process zone development) vs. mature slip (damage zone) stages of faults in these layered sequences.

In conclusion, combining quantitative observations on outcrop analogues, petrophysical and geomechanical datasets, and advanced numerical simulations, provides important indications on the development of process and damage zones, and hence on hydraulic properties, for faults that are otherwise almost invisible in seismics.

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