Geology Reference
In-Depth Information
a)
b)
c)
Figure 5.32
Transitional boundary
effects on seismic signatures; (a-c)
amplitude decreasing with increasing
thickness of transitional zone, (d) thin bed
with sharp boundaries, (e)-(f) assymetric
responses related to coarsening upward
and fining upward layering.
Impedan
ce
20
40
60
80
100
120
d)
e)
f)
20
40
60
80
100
120
with various combinations of internal pore pressure
and external confining pressure. A key observation is
that the compressional velocity of a rock varies
according to the difference between the two pressures
(
Fig. 5.33
) (i.e. the higher the differential or effective
pressure, the higher the velocity).
Figure 5.33
also
shows a high degree of variability in the rate of change
of velocity with pressure between different sand-
stones. This is discussed in
Chapter 8
.
In sedimentary basins the differential or effective
pressure is the difference between the overburden
pressure (related to the weight of rock above the
reservoir) and the pore pressure (
Fig. 5.34
). Provided
that a reservoir is in porous communication with the
surface (i.e. water expelled as the rock compacts is free
to flow to surface) the pore pressure is hydrostatic (i.e.
the pressure is equivalent to the weight of a column
of water above the reservoir). Typically, hydrostatic
gradients are around 0.45 psi/ft but may vary
depending on salinity of formation waters. The over-
burden pressure gradient is typically about 1 psi/ft but
can vary depending on average rock density. If all
rocks are hydrostatically pressured (a situation com-
monly referred to as normally pressured), the effective
pressure (and rock velocity) will
increase with
increasing depth.
Pore pressure can depart from hydrostatic for a
number of reasons (e.g. Mouchet and Mitchell,
1989
;
Swarbrick and Osborne,
1998
; Mukerji et al.,
2002
),
the two most important being related to (1) the
entrapment of pore waters as the rocks are buried
(i.e. the rate of burial exceeds the rate at which pore
water can be expelled), which is typically referred to as
disequilibrium compaction, and (2) the generation and
movement of fluids related to hydrocarbon matur-
ation at depth. In each case the pore pressure will be
abnormally high (i.e. significantly greater than hydro-
static). In the first case, compaction is retarded and
rocks will have porosity greater than expected for a
given depth whilst in the second case the rocks have
already undergone some degree of compaction before
being overpressured.
Given the effect of pressure on rock velocity, the
sonic log is commonly a good indicator of pressure.
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