Geology Reference
In-Depth Information
with a Class III AVO response may be mistaken for a
hydrocarbon bearing sand. In real seismic data the
problem is further complicated by the fact that inter-
ference effects are not
Reflection hyperbolae, for example from the top and
base of a low impedance sand, are not parallel and
this gives rise to variable interference effects across a
seismic gather as bed thickness changes. This can give
rise to a spiraling effect on the intercept vs gradient
crossplot.
Figure 5.28
shows a modelled example
(incorporating timing stretch) of a prominent
Class III response. When the gradients are low it
is possible to change a Class III response into a
Class IV response simply through offset dependent
interference.
the same at all offsets.
n = 97
6
5.5
5
Clean Arenite
4.5
5.3.5.2 Thin beds, and the non-uniqueness of
composite responses
One consequence of the bandlimited nature of seis-
mic is that below tuning different combinations of
impedance and thickness can give rise to the same
seismic signature (
Fig. 5.29
). Thus, there is uncer-
tainty in thin bed interpretation. Fortunately, in many
sand/shale environments the amplitude of the com-
posite response corresponds in general to the net sand
component (Meckel and Nath,
1977
).
Figure 5.30
illustrates Meckel and Nath
4
Shale
Arenite
3.5
3
Wacke
2.5
0
5
10
15
20
25
30
35
40
Porosity (%)
Figure 5.25
Laboratory data from consolidated sandstones (after
Vernik,
1994
). Note sand-shale categories refer to clay volume
(as percentage of mineral component), clean arenite <2%, arenite
2%-15%, wacke 15%-35%, shales >35%.
s model result with a near
linear relationship between net sand and composite
'
a)
Figure 5.26
A gas sand seismic response with
amplitude signature varying with thickness; (a)
migrated stack section (black ΒΌ soft response),
(b) amplitude map at top reservoir with a high
amplitude (orange) lineament parallel to the
edge of the gas accumulation and related to
tuning between the top sand and gas water
contact. Courtesy of Rashid Petroleum
Company.
top brine sand
top gas sand
GWC
Tuning Thickness (10m)
GDT
b)
GWC
75
