Diagenesis-dependence of cataclastic thrust fault zone sealing in sandstones. Example from the Bolivian Sub-Andean Zone

Microstructural analysis of deformed sandstones in thrust fault zones of the Bolivian Sub-Andean Zone (SAZ) indicates that the porosity of fractures and cataclasites is primarily related to the occurrence or absence of quartz sealing resulting from local pressure solution and diffusive silica transport during deformation. Temperature was most probably the main factor that controlled quartz sealing and therefore the distribution of quartz sealing must be a function of the depth of burial at the time of deformation. Consequently, fault zones were sealed at depths exceeding about 3 km (T>70–90°C), whereas shallower faults remain unsealed. In the SAZ, due to foothill uplift during fault activity, erosion reduced burial and allowed non-quartz-sealed fracturing to post-date quartz-sealed fracturing in critically buried fault segments. Most of the non-quartz-sealed faults were permeable to longitudinal carbonate-rich fluid flow. This distribution of quartz sealing is compatible with the current role of faults in oil migration in the SAZ and the foredeep. Therefore, we emphasise that the diagenetic conditions of deformation can play a major role in the variability of fault zone permeability in sandstones, this effect being independent of the fault type (normal or reverse) or fault offset.