Page 114 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 101

ductile
yield point
elastic limit

stress

elastic brittle

strain

Figure 6.4 The stress^strain diagram for a reservoir rock.

two modes of deformation. The composition of the material, confining pressure,
rate of deformation and temperature determine which type of deformation will be
initiated.
If a rock is sufficiently stressed, the yield point will eventually be reached. If
a brittle failure is initiated, a plane of failure will develop which we describe as a
fault. Figure 6.5 shows the terminology used to describe normal, reverse and wrench
faults.
Since faults are zones of inherent weakness they may be reactivated over geologic
time. Usually, faulting occurs well after the sediments have been deposited. An
exception to this is a growth fault (also termed a syn-sedimentary fault), shown in
Figure 6.6. They are extensional structures and can frequently be observed on seismic
sections through deltaic sequences. The fault plane is curved and in a three-
dimensional view has the shape of a spoon. This type of plane is called listric. Growth
faults can be visualised as submarine landslides caused by rapid deposition of large
quantities of water-saturated sediments and subsequent slope failure. The process is
continuous and concurrent with sediment supply, hence the sediment thickness on
the downthrown (continuously downward moving) block is expanded compared to
the upthrown block.
A secondary feature is the development of rollover anticlines which form as a result
of the downward movement close to the fault plane which decreases with increasing
distance from the plane. Rollover anticlines may trap considerable amounts of
hydrocarbons.
Growth faulted deltaic areas are highly prospective since they comprise thick
sections of good-quality reservoir sands. Deltas usually overlay organic-rich marine
clays which can source the structures on maturation. Examples are the Niger,
Baram or Mississippi Deltas. Clays, deposited within deltaic sequences may restrict
the water expulsion during the rapid sedimentation/compaction. This can lead to the
generation of overpressures.