Abstract
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. 10.1144/SP299.2
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