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In-Depth Information
a)
b)
Gathers 1-12
2
Water
Oil
1.0
1
0
0
25
1.5
Trace number
Target
c)
Gathers 13-24
2
2.0
1
Reference
0
2.5
0
25
Trace number
Figure 7.35
technique discriminating hydrocarbon sands in platform carbonate sequences; (a) full stack showing
target and reference reflectors, (b) target/reference AVO ratio showing positive AVO associated with oil in calcarenites, (c) target/reference AVO
ratio showing negative AVO in wet calcarenites. Increasing AVO ratio with offset defines the presence of the hydrocarbon (after Chiburis,
1993
).
Chiburis
''
relative AVO
'
a)
b)
azimuthal AVO (e.g. Macbeth et al.,
1999
;Gray
et al.,
2002
; Neves et al.,
2003
;Johnset al.,
2008
).
Typical amplitude effects are shown in
Fig. 7.36
,
where Macbeth et al. (
1999
) model and interpret
thepresenceoffracturesonthebasisofBase
Chalk dimming on seismic lines that are parallel
to the strike of the fractures. As noted in
Chapter 5
,
other workers have interpreted similar far offset
dimming to be related to the direction normal to
fracture strike.
Increasingly, workers are attempting to make
quantitative predictions based on anisotropic param-
eter inversion (effectively deriving the coefficients in
Rüger
Inside fracture zone
Outside fracture zone
Offset (
m)
Offset (m
)
121
871 1621 2371
121
871 1621 2371
b)
2.0
2.2
2.4
s(
1998
) equations (
Chapter 5
)) and combin-
ing amplitude measures with other geophysical attri-
butes such as coherence (e.g. Hunt et al.,
2010
). The
AVO response through fractured zones is a complex
function of fracture density, azimuth, porosity,
aspect ratio, and target impedance contrast, so frac-
ture description from seismic is clearly a significant
challenge. Most studies focus on integrating azi-
muthal amplitude observations with other seismic
attributes and incorporating all relevant geological
knowledge to arrive at an interpretation.
Fracture detection on land can make use of shear
wave sources and the phenomenon of shear wave
'
2.6
2.8
3.0
Figure 7.36
Gathers froma 2D line parallel to a fracture swarm in the
Chalk of the Fife Field, UK North Sea (after Macbeth et al.,
1999
);
(a) gather from inside the fracture zone showing dimming at far offsets at
the Base Chalk, (b) gather from outside the fracture zone showing high
amplitude reflectivity at far offsets. Gather (b) is typical of gathers from
seismic lines perpendicular to fractures and from areas of no fractures.
148
