V. Roche

The brittle and ductile components of displacement along fault zones

Abstract The total offset across a fault zone may include offsets by discontinuous faulting as well as continuous deformation, including fault-related folding. This study investigates the relationships between these two components during fault growth. We established conceptual models for the distributions of displacement due to faulting (i.e. brittle component or near-field displacement), to folding (i.e. ductile component) and to the sum of both (i.e. far-field displacement) for different mechanisms of fault-related folding. We then compared these theoretical displacement profiles with those measured along mesoscale normal faults cutting carbonate-rich sequences in the Southeast Mesozoic sedimentary basin of France. The near-field and far-field displacement profiles follow either a flat-topped or a triangular shape. Several fold mechanisms were recognized, sometimes occurring together along the same fault and represent either fault-propagation folds, shear folds or coherent drag folds. In the last case, local deficit in the fault slip is balanced by folding so that the brittle and ductile components compose together a coherent fault zone. Common characteristics of these faults are a high folding component that can reach up to 75% of the total fault throw, a high displacement gradient (up to 0.5) and a strong fault sinuosity.

Widening of normal fault zones due to the inhibition of vertical propagation

Abstract In this paper, we document the early stage of fault-zone development based on detailed observations of mesocale faults in layered rocks. The vertical propagation of the studied faults is stopped by layer-parallel faults contained in a weak layer. This restriction involves a flat-topped throw profile along the fault plane and modifications of the fault structures near the restricted tips, with geometries ranging from planar structures to fault zones characterized by abundant parallel fault segments. The ‘far-field’ displacement (i.e. the sum of the displacement accumulated by all the fault segments and the folding) measured along the restricted faults exhibiting this segmentation may have flat-topped shapes or triangular shapes when fault-related folding is observed above the layer-parallel faults. We develop a model from the observations. In this model, during the course of restriction, a fault forms as a simple isolated planar structure, then parallel fault segments successively initiate to accommodate the increasing displacement. We assume that, eventually, the fault propagates beyond the layer-parallel fault. This model implies first that fault widening is controlled by the fault capacity to propagate vertically in the layered section. Likewise, owing to restriction, fault growth occurs with non-linear increases in maximum displacement, length and thickness.

The role of lithological layering and pore pressure on fluid-induced microseismicity: Layering and induced microseismicity

The success of hydraulic fracturing treatments is often judged by the shape and size of the resulting microseismic cloud. However, it is challenging to predict the anticipated microseismic cloud prior to treatment. We use geomechanical modeling to predict the distribution of the microseismicity prior to the hydraulic fracture treatment. We analyze the likelihood of tensile and shear failure due to 1-D variations in local stresses and rock strengths, induced by layering and pore pressure, for two field cases. The deviation in the local stresses from the regional stress field is induced by vertical variations in stiffness. This promotes failure of the stronger layers instead of the weaker ones since the stronger layers can become essentially load bearing. The simulations and field studies show that (1) microseismic events tend to locate preferentially where layers reach tensile failure due to fluid injection, and the number of events tends to decrease in layers that do not reach tensile failure; (2) shear initiation can occur in different layers from those failing in tension, thereby creating additional fluid migration paths; and (3) reactivation of preexisting fractures may occur due to fluid migration, even if their orientations are unfavorable. Numerical modeling is a significant aid in understanding the interplay of regional and local stresses and associated in situ failure due to variations in rock strength, pore pressure, and stiffnesses. In a wider perspective, it gives fundamental insights into the understanding of earthquakes and fault localization, the mechanisms of fracture development, and the role of fractures on fluid circulation and on the in situ stress field.

Widening of Normal Fault Zones During Vertical Propagation

In this paper, we document the early stage of faulting, based on detailed observations on mesocale faults in layered rocks. The vertical propagation of the studied faults is stopped by layer-parallel faults. This restriction involves a modification of the fault structures: far from the restricted tip, fault structures correspond to a simple planar slip surface, near the restricted tips, their structures range from a planar structure to a complex fault zone characterized by abundant parallel fault segments. Based on the observations, we developed a model of fault zone evolution in which fault zone complexity, specifically the number of sub parallel segments, increases to accommodate increasing strain, during restriction. Eventually, the fault should finally propagate beyond the layer-parallel faults with a complex geometry inherited from the period of restriction. This model implies that fault widening is controlled by the host rock and formerly developed fractures. Wide fault zones are expected in layered rocks with strong mechanical heterogeneities and with preexisting joints and layer-parallel faults. Likewise, fault growth occurs with non-linear increasing in maximum displacement, length, and thickness, due to restriction. Such a model of fault impacts on the vertical permeability and the seismic behavior of the rock.

Geomechanical Modelling of Induced Microseismicity

Geomechanical modelling is a powerful tool to help bridge the gap between geophysical data analysis and engineering applications of microseismic data by providing a framework for advanced interpretation strategies. In particular, it helps answering questions such as: Why does brittle failure and thus microseismicity occur in specific locations and not everywhere? What are the most likely failure mechanisms? Where does the input energy go? We use both bonded-particle and distinct-element methods to investigate these questions.