Deformation mechanisms and geometries of superposed fault zones in dolostones

Studying fault zone properties is crucial in addressing key challenges in subsurface exploration, resource management, and seismic risk evaluation. As global interest in geothermal energy, hydrogen, and carbon storage intensifies, the mechanical and structural characterization of faults, as well as their impact on fluid migration, reservoir integrity, and fault sealing analysis, is becoming increasingly important.

The presented study focuses on the structural and mechanical properties of superposed fault zones in dolostones, an issue that has been poorly investigated. In particular, we addressed how the architecture of an earlier, large-scale normal fault (F1) influences the geometry and deformation mechanisms of younger, superposed strike-slip faults (F2). The F1 fault consists of four sub-parallel fault rock units, each several tens of meters thick: (i) a cataclastic core (Cu), bounded in the hanging wall by (ii) cemented micro-mosaic breccia (MB), and in the footwall by (iii) high-strained (HS) and (iv) low-strained (LS) fault rocks. To achieve this aim, we performed some geotechnical and morphometric analyses. Uniaxial compressive strength (UCS) tests revealed mean values of 83 MPa and 120 MPa for MB and LS, respectively, whereas Cu and HS exhibit lower UCS values of 58 MPa and 62 MPa, respectively. MB and Cu exhibit heterogeneous particle size distributions (PSD) and porosities of 4.02% and 4.96%, respectively, while HS and LS show more homogeneous PSDs with porosities of 2.87% and 2.19%, respectively.

The F2 faults developed a spectrum of structural facies such as cataclastic shear bands (CSBs) in the LS and HS, and as compaction bands (CBs) in the MB and Cu. In the LS, cataclasis is highly localized within widely spaced, thick, tabular, and cemented CSBs. In the HS, deformation occurs through anastomosing CSBs, accommodating diffuse cataclasis and dissolution-precipitation mechanisms. In the Cu, anastomosing CBs develop through pore collapse and dissolution-precipitation processes. In the MB, compaction is driven by pore collapse and grain crushing, forming well-localized, widely spaced CBs.

Overall, the microstructural properties (PSD and porosity) and mechanical strength of the F1 fault rocks are key factors influencing the deformation mechanisms (cataclasis vs. compaction) and the geometry (localized vs. anastomosing) of the F2 faults. These findings contribute to the understanding of fault permeability and fluid flow dynamics in multi-faulted dolomite reservoirs.