Geology Reference
In-Depth Information
a)
b)
1500
Offset (m)
300
2500
A
I
=
3000
Figure 7.29
The Fylla flat spot - related to an opal/Cristobalite transition; (a) full stack section ('flat spot'
is marked as 'd', (b) moveout
corrected gather showing positive AVO associated with the 'flat spot'
(after Isaacson and Neff,
1999
).
A
I
Top chalk
=
Top chalk
Figure 7.30
Amplitude dimming at top chalk associated with development of porosity (after d'Angelo et al.,
1997
).
types, burial history, timing of geological structura-
tion, and fluid migration.
Francis et al. (
1997
) have described a dramatic
relict flat spot in the Triassic Sherwood sandstone in
the East Irish Sea basin.
Fig 7.28
shows models based
on the published data. On full stack models the gas case
is slightly higher amplitude than the palaeo-contact
case (
Fig. 7.28a
,
b
). However, in principle, AVO analy-
sis could help in differentiating the two cases (
Fig.
7.28c
). In terms of the top response the hydrocarbon
case has zero gradient whereas the top water sand has a
strong Class IV response. At the contact, hydrocarbon
gives rise to a strong positive gradient but the palaeo-
contact has little gradient expression.
Other examples of non-hydrocarbon related flat
events
transformations. A well-known example is the Fylla
flat spot described by Isaacson and Neff (
1999
)and
Aram (
1999
)(
Fig. 7.29
). A hydrocarbon interpret-
ation was given to the
as it appears to be
restricted to a distinct stratigraphic unit (
Fig. 7.29a
)
and shows increasing amplitude with offset
(
Fig. 7.29b
). A well drilled into the feature encoun-
tered only mudstones at this level. X-ray diffraction
analysis of sidewall cores suggests that it may be
related to a phase change of amorphous opal
(opal
'
flat spot
'
-
A) converting to cristobalite and tridymite
(opal
CT) (Christiansen et al.,
2001
). The reflection
signature is poorly understood. Christiansen et al.
(
2001
) postulate that there may be a component of
anisotropy, although this has not been verified.
Although the opal
-
144
include
temperature-controlled mineral
-
CT transition gives rise to a
