The Development of Sub-seismic Fractures around a Fault – The Causes and the Implications for Fluid Flow (original) (raw)
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Journal of Structural Geology, 2018
Whatever the processes involved in the natural fracture development in the subsurface, fracture patterns are often affected by the local stress field during propagation. This homogeneous or heterogeneous local stress field can be of mechanical and/or tectonic origin. In this contribution, we focus on the fracture-pattern development where active faults perturb the stress field, and are affected by fluid pressure and sliding friction along the faults. We analyse and geomechanically model two fractured outcrops in UK (Nash Point) and in France (Les Matelles). We demonstrate that the observed local radial joint pattern is best explained by local fluid pressure along the faults and that observed fracture pattern can only be reproduced when fault friction is very low (µ < 0.2). Additionally, in the case of sub-vertical faults, we emphasize that the far field horizontal stress ratio does not affect stress trajectories, or fracture patterns, unless fault normal displacement (dilation or contraction) is relatively large.
Journal of Structural Geology, 2011
In this paper, we describe an outcrop to characterize the effect of fracture spacing and type on larger scale effective elasticity, which is measured for the first time in-situ with a Schmidt hammer. The outcrop is dominated by lime mudstones and belongs to the deformation zone of the St Clément fault, in southern France. Our results suggest that small spacing of faults, open fractures and styolites leads to lesser effective Young's modulus, whereas small sealed fracture spacing leads to greater effective Young's modulus. These relationships are compatible with theoretical models of effective elasticity. Using Amadei and Savage (1993) approach, we define a non-linear model that relates Schmidt hammer rebound to spacing by fracture type. A hemisphere with a radius of 40 to w200 cm is the rheological volume characterized by the Schmidt hammer. Results of model inversion demonstrate that variations of Schmidt hammer rebound over the outcrop can be used to estimate fracture type and stiffness. Stiffness of sealed fractures is 2e3 orders of magnitude greater than the stiffness of faults, stylolites and open fractures. This result is consistent with an increase of the rate of interseismic stress build-up of major faults with sealing of fractures in their damage zone.
The way that faults transport crustal fluids is important in many fields of earth sciences such 15 as petroleum geology, geothermal research, volcanology, seismology, and hydrogeology. For 16 understanding the permeability evolution and maintenance in a fault zone, its internal 17 structure and associated local stresses and mechanical properties much be known. This 18 follows because the permeability is primarily related to fracture propagation and their linking 19 up into interconnected clusters in the fault zone. Here we show that a fault zone can be 20 regarded as an elastic inclusion with mechanical properties that differ from those of the host 21 rock. As a consequence, the fault zone develops its own local stresses which differ from the 22 associated regional stresses. The local stresses, together with fault-rock heterogeneities and 23 interfaces (discontinuities; fractures, contacts), determine fracture propagation, deflection 24 (along discontinuities/interfaces), and arrest in the fault zone and, thereby, its permeability 25 development. We provide new data on the internal structure of fault zones, in particular the 26 fracture frequency in the damage zone as a function of distance from the fault core. New 27 numerical models show that the local stress field inside a fault zone, modelled as an 28 inclusion, differ significantly from those of the host rock, both as regards the magnitude and 29 the directions of the principal stresses. Also, when the mechanical layering of the damage 30 zone, due to its variation in fracture frequency, is considered, the numerical models show 31 Manuscript Click here to view linked References abrupt changes in local stresses not only between the core and the damage zone but also 32 within the damage zone itself. Abrupt changes in local stresses within the fault zone generate 33 barriers to fracture propagation and contribute to fracture arrest. Also, analytical solutions on 34 the effects of material toughnesses (critical energy release rates) of layers and their interfaces 35
Geomechanical simulation to predict open subsurface fractures
Geophysical Prospecting, 2009
Geomechanical simulation of the evolution of a geological structure can play an important role in predicting open fracture development for all stages in that structure's development. In this work, three such geomechanical simulations are used to predict the evolving stress and strain fields, including dilational and compactional changes in the rock fabric in developing fault and fold systems. Their consequences for open fracture development and flow are addressed. These simulated stress and strain fields show considerable spatial and temporal heterogeneity that is consistent with deformation patterns observed in both natural examples and in laboratory-deformed analogues. But the stress and strain states that develop are neither co-axial nor do they bear a simple relationship to one another. The dilational and compactional strains, manifest as open fracturing or sealing, represent some significantly increased or significantly decreased flow rates. However, open-fracture predictions based on such geomechanical simulations are extremely difficult to validate with any degree of confidence as there is little direct evidence of sub-surface fracture distributions. In this context we also discuss possible integration of seismic anisotropy measurements, as an independent measure of open fracture alignment, to support the geomechanically derived fracture predictions. The focus of this work is on volumetric strains in fault zone evolution, though folding is also addressed.
Effects of local stress perturbation on secondary fault development
Journal of Structural Geology, 2002
The complex patterns of normal faults in sedimentary basins are commonly attributed to a complex geological history with varying directions of tectonic extension. However, we show an example of normal faulting from a North Sea hydrocarbon reservoir where the variability in secondary fault orientations can be attributed to stress perturbations that developed around the larger faults during a single phase of extension. This is demonstrated by comparing attributes of the stress ®elds computed around largest faults from detailed threedimensional (3D) geomechanical models, with fault data such as discretized fault orientation and density observed from a high quality 3D seismic re¯ection survey. The modeling results show the strong in¯uence of the irregular geometry (bends and intersections) of larger faults on the development of smaller faults. Methods developed in this study can be applied to predict likely locations and orientations of subseismic faults.
Journal of Structural Geology, 1992
Abstraet--A plane strain model for a fault is presented that takes into account the inelastic deformation involved in fault growth. The model requires that the stresses at the tip of the fault never exceed the shear strength of the surrounding rock. This is achieved by taking into account a zone, around the perimeter of the fault surface, where the fault is not well developed, and in which sliding involves frictional work in excess of that required for sliding on the fully developed fault. The displacement profiles predicted by the fault model taper out gradually towards the tip of the fault and compare well with observed displacement profiles on faults. Using this model it is found that both (1) the shape of the displacement profile, and (2) the ratio of maximum displacement to fault length are a function of the shear strength of the rock in which the fault forms. For the case of a fault loaded by a constant remote stress, the displacement is linearly related to the length of the fault and the constant of proportionality depends on the shear strength of the surrounding rock normalized by its shear modulus. Using data from faults in different tectonic regions and rock types, the in situ strength of intact rock surrounding a fault is calculated to be on the order of 100 MPa (or a few kilobars). These estimates exceed, by perhaps a factor of 10, the strength of a well developed fault and thus provide an upper bound for the shear strength of the crust. It is also shown that the work required to propagate a fault scales with fault length. This result can explain the observation that the fracture energy calculated for earthquake ruptures and natural faults are several orders of magnitude greater than that for fractures in laboratory experiments.
AAPG Bulletin, 2008
In addition to seismically mapped fault structures, a large number of faults below the limit of seismic resolution contribute to sub-surface deformation. However, a correlation between large-and small-scale faults is difficult because of their strong variation in orientation. A workflow to analyse deformation over different scales is described here. Based on the combination of seismic interpretation, coherency analysis, geostatistical analysis, kinematic modelling, and well data analysis, we constrained the density and orientation of sub-seismic faults, and made predictions about reactivation and opening of fractures. We interpreted faults in seismic and coherency volumes at scales between several km and a few tens of meters. 3D retro-deformation was performed on a detailed interpreted 3D structural model to simulate strain in the hanging wall at the time of faulting, at a scale below seismic resolution. The modelling results show that (1) considerable strain is observed more than 1 km away from the fault trace, and (2) deformation around the fault causes strain variations, depending on the fault morphology. This strain variation is responsible for the heterogeneous sub-seismic fracture distribution observed in wells. We linked the fracture density from well data with the modelled strain magnitude, and used the strain magnitude as a proxy for fracture density. With this method we can predict the relative density of small-scale fractures in areas without well data. Furthermore, knowing the orientation of the local strain axis we predict fault strike, and opening or reactivation of fractures during a particular deformation event.
Journal of Structural …, 2010
The way that faults transport crustal fluids is important in many fields of earth sciences such 15 as petroleum geology, geothermal research, volcanology, seismology, and hydrogeology. For 16 understanding the permeability evolution and maintenance in a fault zone, its internal 17 structure and associated local stresses and mechanical properties much be known. This 18 follows because the permeability is primarily related to fracture propagation and their linking 19 up into interconnected clusters in the fault zone. Here we show that a fault zone can be 20 regarded as an elastic inclusion with mechanical properties that differ from those of the host 21 rock. As a consequence, the fault zone develops its own local stresses which differ from the 22 associated regional stresses. The local stresses, together with fault-rock heterogeneities and 23 interfaces (discontinuities; fractures, contacts), determine fracture propagation, deflection 24 (along discontinuities/interfaces), and arrest in the fault zone and, thereby, its permeability 25 development. We provide new data on the internal structure of fault zones, in particular the 26 fracture frequency in the damage zone as a function of distance from the fault core. New 27 numerical models show that the local stress field inside a fault zone, modelled as an 28 inclusion, differ significantly from those of the host rock, both as regards the magnitude and 29 the directions of the principal stresses. Also, when the mechanical layering of the damage 30 zone, due to its variation in fracture frequency, is considered, the numerical models show 31 Manuscript Click here to view linked References 2 abrupt changes in local stresses not only between the core and the damage zone but also 32 within the damage zone itself. Abrupt changes in local stresses within the fault zone generate 33 barriers to fracture propagation and contribute to fracture arrest. Also, analytical solutions on 34 the effects of material toughnesses (critical energy release rates) of layers and their interfaces 35