The properties and role of carbonate cementation in fault zones

Abstract

The interaction between fluids and rock is recognised as being highly important in the seismic cycle. Both the mechanical and chemical processes induced by fluid-rock interactions can alter various rock physical properties, which in turn has implications ranging from altering slip behaviour on faults to affecting hydrocarbon reservoir longevity. Therefore, the study of physical manifestations of fluid-rock interactions in fault zones, specifically the cements present, is of both societal and economic value. Due to its reactivity with even weak acidic fluids, carbonate rocks, composed mostly of the mineral calcite (CaCO3), are very susceptible to dissolution-precipitation processes. Sedimentary rocks are frequently cemented by carbonate and they are also hosts of many hydrocarbon reservoirs (50% of the world’s hydrocarbon reservoirs are in carbonates). Perhaps most importantly, carbonate-bearing terrains are host to a multitude of destructive earthquakes, from the 1981 sequence in the Gulf of Corinth, to the 2008 Wenchuan earthquake in China and the numerous earthquakes in the Apennines, Italy over the last ten years (L’Aquila 2009, central Italy 2016, 2017). The effect of the interaction between carbonates and fluids on the physical properties of fault zones has been questioned in recent years, but detailed studies are few. The focus of this study is to better constrain the properties and role of carbonate cementation in fault zones. A multidisciplinary approach is taken in this project, in which methods of isotope geochemistry, microscopy of cements in fault zones, and rock deformation experiments are combined to gain insight in the processes concerned with cementation over the course of the earthquake cycle, and the subsequent effect on rock properties. Stable and clumped carbon and oxygen isotope ratios in cement phases are determined to derive information on the fluid source, the migration of fluids, and the associated interactions with the surrounding rock. Field descriptions and microscopy (conventional, cathodoluminescence (CL), scanning electron microscopy (SEM) and transmitted electron microscopy (TEM)) permit the analysis of the textures and cement generations. Rock deformation experiments inducing field-relevant conditions in carbonate-saturated fault zones permit the characterisation of changes in rock properties. The multidisciplinary approach used has permitted me to link together the geochemistry of cements with the alteration of fault physical properties. I have identified that textures within a fault zone fingerprint the chemical and mechanical processes which have occurred during fault zone evolution. Such heterogeneities produced by fluid-rock interactions can influence future fracture propagation, non-optimal fault activation and enigmatic events, such as slow-slip. I have found that fault properties, including strength, permeability and fault slip behaviour are strongly dependent upon the degree of cohesion and compaction achieved within the fault zone, which in turn is dependent upon temperature, the presence of gouge, host rock type, the presence of fluid (and fluid type) and fault roughness. This thesis documents the use of cements in studying fault zone evolution, and how the processes leading to, and characteristics induced by, cementation in fault zones exert profound changes in rock physical properties across all scales.