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Fig. 6.43 Extensional fault network in Somerset - comparable geometries can also be observed on a seismic scale
˃ 3 is vertical and
￿ Thrust faulting occurs when
modelling and simulation because faults then
become 3D model elements, and all model
elements must have petrophysical properties.
The best way to understand faults is to visit
some. Essential fault terminology began in the
mining industry, such that a mining geologist
standing within a fault found the footwall at his
feet and the hangingwall overhead (Fig. 6.45 ).
With normal faults the hangingwall has moved
downwards with respect to the footwall (in
reverse or thrust faults the hangingwall has
moved upwards). This basic terminology has
developed into a wide set of vocabulary (e.g.
Sibson 1977 ; Wise et al. 1984 ). In reservoir
modelling it is convenient to treat fault zones in
terms of two key components: the thin, very
highly deformed fault core around the main slip
surface (centimetres or 10's of centimetres
across) and a wider fault damage zone, which is
can be several metres or 10's of metres across.
˃ 2 are horizontal;
￿ Strike-slip faulting occurs when
˃ 1 and
˃ 2 is vertical
˃ 3 are horizontal.
This simple theory is founded in rock mechani-
cal principles, where brittle failure occurs along
surfaces of maximum shear. Failure occurs on one
preferred slip plane, often accompanied by smaller
movements on conjugate planes oriented approxi-
mately 60 from the main plane of shear failure.
For a fuller understanding of the processes
involved we refer you to texts such as Twiss
and Moores ( 1992 ) and Mandl ( 2000 ).
In practice, faults in reservoirs are not single
2D planes, but form zones in which many
fractures coalesce. The result is a highly deformed
fault core surrounded by a wider, less deformed
damage zone. Faults are therefore volumetric .
This is important to appreciate because on seis-
mic, faults are usually interpreted as 2D planes,
and are typically represented as 2D surfaces in
reservoir and simulation models with little more
analysis. The acknowledgment that fault zones
represent volumes has a bearing on the world of
˃ 1 and Handling the Effects of Faulting
In reservoir modelling we are mainly concerned
with the effects which faults have on fluid flow.
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