Geoscience Reference
In-Depth Information
that rocks are always in net compression at shear
failure, irrespective of whether the regional tec-
tonic picture is described as 'extensional', 'com-
pressional' or 'strike-slip'. The latter terms
simply describe the relative position of the prin-
cipal stresses at failure.
Faults form as part of a fault network (e.g.
Fig.
6.43
), and a key characteristic of fault
networks is their scale invariance - the same
fault patterns can be observed on a range of
scales. They are fractal.
Faults are formed in three principle settings,
the contrasts between which are important for an
understanding of
6.7
Structurally-Controlled
Reservoirs
All reservoirs are to some extent influenced by
fractures, and there is something slightly artificial
in separating out a group of reservoirs that are
'structurally controlled'. It is truer to say that for
some reservoirs the effects of fracturing are minor
and can be neglected, whereas for other reservoirs
the structural effects are so important that it is the
sedimentary aspects which turn out to play a minor
role. It is also often true that the effects of fractures
are initially assumed to be of minor importance,
but once more detailed reservoir data becomes
available, especially dynamic production data,
the fractures start to reveal themselves.
Modelling fractures requires an underlying
concept, as for all reservoir models. The first
step in forming a concept is the description of
the significant fracture types, the key distinction
being between joints (tensile fractures) and faults
(shear fractures). Although most fractured
reservoirs will contain a mixture of both, the
distinction is important because joint-dominated
systems tend to form high density fracture
systems - these are generally what is being
referred to when the term 'naturally fractured
reservoir' is used - whereas fault-dominated
systems tend to form low density systems.
Joint-dominated systems tend to be open,
whereas is it common to encounter both open
and closed fractures in fault dominated systems.
In this section, high- and low-density fracture
systems will be treated separately because the
modelling workflows required to describe them
are very different.
their
impact on reservoir
performance:
1.
Normal faults
, mainly occur in extensional
tectonic settings and tend to be steeply-
dipping (with fault-plane dips typically in
the range of 60
-90
) and with mainly dip-
slip motion vectors.
2.
Thrust faults
, which occur in compressional
tectonic settings and tend to be shallow-
dipping (with fault-plane dips mainly in the
range of 0
-30
) and with mainly dip-slip
motion vectors.
3.
Strike-slip faults
, which are near-vertical and
created by lateral-slip motions in compres-
sional tectonic settings (e.g. mountain belts)
and at transform plate margins.
All intermediate cases between these three end-
member cases are possible, hence the terms
'oblique-slip', 'transtensional' and 'transpre-
ssional' to cover hybrid cases. Faults also tend to
reactivate, for example normal faults in extensional
basins may subsequently experience reverse fault
motion during later phases of basin compression -
the process of 'structural inversion'.
A founding principle in structural geology is the
Anderson (
1905
) theory of faulting, which relates
the stress system to the style of faulting. The stress
field is summarised by three principle stresses:
6.7.1 Low Density Fractured
Reservoirs (Fault-Dominated)
˃
1
>
˃
2
>
˃
3
6.7.1.1 Terminology
A fault is a zone, either side of which relative
displacement of the host rock has occurred dur-
ing failure, as a result of
shear
when the
deviatoric stress exceeds the rock strength. Note
Anderson showed that (Fig.
6.44
):
Normal (extensional) faulting occurs when
˃
1
is vertical and
˃
2
and
˃
3
are horizontal;
