Geology Reference
In-Depth Information
a)
b)
0.2
0.5
0.1
Shale cap rock
0.4
0
Inc. Sw only
0.3
-0.1
Inc. Sw only
Inc. Eff. Press.
and Sw
-0.2
0.2
Inc. Eff. Press. and Sw
Pre-production
-0.3
0.1
Pre-production
-0.4
Inc. Eff Press. only
Inc. Eff Press. only
-0.5
0
-0.04
-0.02
0
0.02
0.04
6400
6600
6800
7000
7200
7400
Intercept
Acoustic Impedance
Figure 5.37
Modelled production effects on an initially overpressured reservoir; (a) intercept vs gradient, (b) acoustic impedance vs Poisson's
ratio
typically have a Class IV AVO response. However, in
many cases the top of geopressure is transitional and
it may not be visible as a distinct reflection. The
difference between reflection responses from nor-
mally pressured and overpressured sands depends
on many factors. It is possible that the overall style
may be similar between the two regions (e.g. strong
Class III gas sand responses), particularly if the
increase in overpressure with depth is gradual, such
as appears to be the case in many parts of the Gulf of
Mexico (Hilterman,
2001
). In other cases there can be
significant differences in AVO response.
Figure 5.36
shows a modelled AVO crossplot
example of three sands (based on a real case study)
in which the middle sand is overpressured. The upper
and lower sands are normally pressured and show
Class I to Class II behaviour whereas the middle sand
has a Class IV signature (
Fig. 5.36a
). The top sand full
stack response changes depending on the strati-
graphic interval and the fluid fill, so that sand 1 shows
hard responses when brine filled but soft responses
when hydrocarbon filled; sand 2 shows a negative
reflection when brine filled and bright spots with
hydrocarbon, and sand 3 shows dimming of a hard
event when hydrocarbon filled. It is interesting that
even though the reflection character changes, oil and
gas responses for each case show increasing absolute
amplitude difference with offset compared to the brine
cases. In practice, however, these types of relative
AVO comparisons are often hampered by uncertainty
in identifying the appropriate water sand signature on
seismic. Dvorkin et al.(
1999
) have noted that the
Poisson
s ratio of gas bearing rocks can dramatically
reduce at low effective pressures whilst water filled
rocks do not show this effect. A dramatic drop in
Poisson
'
s ratio may be a useful indicator of hydrocar-
bons in overpressured sediments.
Pressure changes can have a significant effect
on reflectivity during production. To illustrate this,
Fig. 5.37
shows the modelled top reservoir reflectiv-
ity from an initially overpressured reservoir. Note
that it is assumed in the model that there is no
change in porosity and that the overburden proper-
ties do not change. In practice the shale cap rock
may undergo stress effects that alter physical prop-
erties (e.g. Gray et al.,
2012
). In the model (
Fig. 5.37
)
the pre-production AVO style is Class III but the
AVO style changes with production. Changes in
water saturation and pressure drawdown (i.e.
increase in effective pressure) stiffen the reservoir
rock. Thus, the intercept amplitude becomes more
positive as the reservoir is produced. The largest
effect occurs where there is both pressure and fluid
change. If pressure is maintained during production
the gradient decreases but if the effective pressure
increases the gradient becomes more negative. These
types of dynamic effects are most evident in uncon-
solidated and overpressured reservoirs whilst
'
the
reflectivity associated with
res-
ervoirs is usually less susceptible to pressure change.
There is often an asymmetry to pressure effects, for
example increasing pressure in normally pressured
'
normally pressured
'
82
