Multi-scale analysis of faults in reservoir systems:

Abstract

Faults are complex shear zones within the Earth’s lithosphere with a crucial role in plate tectonics and are of important interest for society. They define the boundaries of tectonic plates and accommodate their relative motions, shaping mountain belts and basins. Faults are conduits for fluid migration and control the precipitation of economically valuable minerals, and they often bound geo reservoirs crucial for energy production and storage. Faults also host earthquakes, among nature’s most devastating phenomena. Understanding fault zones and their evolution is therefore of societal and economic importance. The induced seismicity associated with gas production from the Groningen gas field in the northeastern Netherlands highlights the importance of understanding fault zone properties and architecture and their implications for fault mechanical behaviour. Despite extensive research, critical geological observations and interpretations of fault zone architecture and kinematic evolution in reservoir systems like Groningen are still lacking. The central aim of this thesis is to advance our multi-scale understanding of fault zone evolution as a function of fault displacement, lithological variability, and fluid-rock interaction processes. In Chapter 2, I address the processes governing fault zone formation and internal structural evolution, applicable to strike-slip settings and brittle shear zones in general. Using physical analogue modelling with integrated monitoring techniques, I explore how fault zones evolve from distributed deformation and incipient localization to the formation of a mature, throughgoing fault, and how this evolution relates to fault mechanical behaviour. This approach couples short time scale observations of stick-slip cycles to long-term fault evolution within a single experimental framework. Chapters 3–5 focus on fault zone processes and properties in the Groningen reservoir. In Chapter 3, I conducted friction experiments on simulated, compositionally and spatially heterogeneous gouges derived from the Slochteren Sandstone and Ten Boer claystone. With these experiments, I investigated how mechanically contrasting lithologies are juxtaposed along faults and how this affects sliding friction and its velocity dependence. My results show that fault friction and its rate dependence are not simply governed by the weakest lithology nor by a homogeneous mixture of juxtaposing lithologies; instead, detailed knowledge of stratigraphic layering combined with fault offset is required to predict macroscopic frictional behaviour. In Chapter 4, I explore the interaction between fluids and fault rocks (structural diagenesis) by quantifying diagenetic mineral distributions across fault zone domains within the Groningen Rotliegend reservoir. I show that authigenic quartz is widespread in fault zones and precipitates preferentially in cataclastic zones. Authigenic siderite and anhydrite are also more abundant in fault zones, and possible precipitated from percolating extrinsic fluids. In Chapter 5, I present an integrated dataset synthesizing fault gouge microstructural observations, experimental properties, and meso-scale interpretations from reflection seismics and well data. I identify two genetically different fault rock types: quartzofeldspathic gouges formed by cataclasis and healed by cementation and pressure solution, and clay-supported gouges formed by mixing sandstone and claystone materials. Importantly, these fault rocks show mechanically contrasting behaviour. The chapters in this thesis provide geological and mechanical constraints for interpreting induced seismicity in reservoirs like the Groningen gas field.