Experimental assessment of the role of natural structures on the mechanical behaviour and deformation processes in quartz-rich carbonates

Carbonate reservoirs commonly contain deformation structures whose reactivation depends on their orientation relative to the prevailing stress field. This study experimentally investigates how the presence and orientation of natural deformation features –primarily deformation bands– affect the mechanical behaviour and deformation processes of quartz-rich (∼10 %) carbonate rocks. Triaxial axisymmetric compression tests were conducted at 0–30 MPa confining pressures on cylindrical samples (10 mm diameter), including cores of pristine material or with deformation structures at varying angles to the maximum principal stress. Mechanical tests were performed both on-beam at Diamond Light Source and off-beam, with Digital Volume Correlation applied to characterise strain evolution and micromechanical processes up to failure. The pristine material exhibits a high friction coefficient (μ = 1.07), and under confining pressures of 10–25 MPa, failure consistently localises along planes oriented ∼22° to the maximum principal stress. Natural deformation bands exert a strong control on macroscopic strength: favourably oriented bands reactivate readily and significantly reduce peak strength, whereas unfavourably oriented bands inhibit reactivation and promote the formation of new Coulomb-oriented fractures. Strain field analyses reveal that both bands remain inactive at low deviatoric stresses. Earlier localisation takes place in favourably banded samples, while pristine and unfavourably banded samples localise only at later loading stages. During macroscopic failure, X-ray images show pristine and unfavourably banded samples to develop macroscopic fractures with higher apertures, whereas favourably banded samples display slip-dominated reactivation. Grain crushing and pore collapse are obvious in both natural deformation bands. These results demonstrate that natural pre-existing deformation structures and their orientation critically influence strain localisation and strength in the tested quartz-rich carbonate samples, with implications for reservoir stability, fault damage zones, and engineered subsurface systems.