Geology Reference
In-Depth Information
gather (
Fig. 7.27b
) from an oil field discovery. The
model is fairly sophisticated, including the effects of
acquisition geometry and applying the same process-
ing sequence as that applied to the seismic. The seis-
mic shows a brightening of the contact at mid offsets
but the strong modelled AVO effect at far angles is
not replicated on the real gather probably because of
poor imaging.
Flat-lying reflectors that cross-cut stratigraphy
are not always related to the presence of hydrocar-
bon. In some cases the flat-lying event is related to
the position of a previous contact with the trap
having been filled but subsequently breached, with
the hydrocarbon escaping. The most common
explanation for the flat event in these situations is
a porosity variation related to differences in diagen-
esis between the porous units in the trap and the
rocks below the contact. Porosity in the relict trap is
commonly higher by a few porosity units perhaps
as a result of hydrocarbons inhibiting cementation
(e.g. Gluyas et al.,
1993
) or other diagenetic pro-
cesses such as bituminization around the level of
the relict contact (e.g. Yaliz and McKim,
2003
).
Clearly, it is important for the interpreter to
be aware of the interplay between the reservoir
a)
Top reservoir
A
I
b)
=
Top reservoir
OWC
Figure 7.27
Real vs modelled seismic gather. The far angle
brightening predicted by the model (b) is not replicated by the real
gather (a) (G. Drivenes, personal communication).
a)
A
I
c)
=
0.3
GWC
Palaeo-contact
0.2
0.1
Top water sand
Top water sand
(Palaeo)
0
-0.1
b)
Top gas sand
-0.2
-0.3
0
0.1
0.2
0.3
0.4
sin
2
θ
Figure 7.28
Hydrocarbon and relict contact models; (a) full stack hydrocarbon (gas) contact model, (b) full stack relict contact model
(difference in porosity across the relict contact is 2%-3%), (c) AVO responses of different reflecting boundaries (based on data from Francis
et al.,
1997
).
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