Publication Date:
2017-04-04
Description:
It is increasingly apparent that faults are typically not discrete planes but zones of
deformed rock with a complex internal structure and three-dimensional geometry. In the last
decade this has led to renewed interest in the consequences of this complexity for modelling
the impact of fault zones on fluid flow and mechanical behaviour of the Earth’s crust. A
number of processes operate during the development of fault zones, both internally and in the surrounding
host rock, which may encourage or inhibit continuing fault zone growth. The complexity
of the evolution of a faulted system requires changes in the rheological properties of both the fault
zone and the surrounding host rock volume, both of which impact on how the fault zone evolves
with increasing displacement. Models of the permeability structure of fault zones emphasize the
presence of two types of fault rock components: fractured conduits parallel to the fault and granular
core zone barriers to flow. New data presented in this paper on porosity–permeability
relationships of fault rocks during laboratory deformation tests support recently advancing concepts
which have extended these models to show that poro-mechanical approaches (e.g., critical
state soil mechanics, fracture dilatancy) may be applied to predict the fluid flow behaviour of
complex fault zones during the active life of the fault. Predicting the three-dimensional heterogeneity
of fault zone internal structure is important in the hydrocarbon industry for evaluating the
retention capacity of faults in exploration contexts and the hydraulic behaviour in production
contexts. Across-fault reservoir juxtaposition or non-juxtaposition, a key property in predicting
retention or across-fault leakage, is strongly controlled by the three-dimensional complexity of
the fault zone. Although algorithms such as shale gouge ratio greatly help predict capillary
threshold pressures, quantification of the statistical variation in fault zone composition will
allow estimations of uncertainty in fault retention capacity and hence prospect reserve estimations.
Permeability structure in the fault zone is an important issue because bulk fluid flow rates through
or along a fault zone are dependent on permeability variations, anisotropy and tortuosity of flow
paths. A possible way forward is to compare numerical flow models using statistical variations of
permeability in a complex fault zone in a given sandstone/shale context with field-scale estimates
of fault zone permeability. Fault zone internal structure is equally important in understanding the
seismogenic behaviour of faults. Both geometric and compositional complexities can control the
nucleation, propagation and arrest of earthquakes. The presence and complex distribution of
different fault zone materials of contrasting velocity-weakening and velocity-strengthening properties
is an important factor in controlling earthquake nucleation and whether a fault slips seismogenically
or creeps steadily, as illustrated by recent studies of the San Andreas Fault. A synthesis
of laboratory experiments presented in this paper shows that fault zone materials which become
stronger with increasing slip rate, typically then get weaker as slip rate continues to increase to
seismogenic slip rates. Thus the probability that a nucleating rupture can propagate sufficiently
to generate a large earthquake depends upon its success in propagating fast enough through
these materials in order to give them the required velocity kick. This propagation success is
hence controlled by the relative and absolute size distributions of velocity-weakening and velocity-
strengthening rocks within the fault zone. Statistical characterisation of the distribution of
such contrasting properties within complex fault zones may allow for better predictive models
of rupture propagation in the future and provide an additional approach to earthquake size forecasting
and early warnings.
Description:
Published
Description:
5-33
Description:
2.3. TTC - Laboratori di chimica e fisica delle rocce
Description:
N/A or not JCR
Description:
reserved
Keywords:
fault zone
;
earthquake
;
04. Solid Earth::04.06. Seismology::04.06.01. Earthquake faults: properties and evolution
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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