Geology Reference
In-Depth Information
fault throw is less than reservoir thickness over
part of the hydrocarbon bearing section across the
fault,
a)
the seismic anomaly is spatially continuous across
the fault.
b)
The PRMS gives criteria for the use of seismic ampli-
tudes anomalies and flat spots in increasing confidence
in fluid contacts which can be summarised as:
the flat spot or seismic amplitude anomaly is
clearly visible in 3D seismic and not related to
imaging issues,
c)
1.8
2.9
well data (logs, pressures, production test data)
demonstrate a strong tie between the calculated
hydrocarbon-water contact (not necessarily
drilled) and the seismic flat spot or downdip edge
of the seismic anomaly,
2.0
d)
1.875
Original top Ty interpretation
1.900
the flat spot or downdip edge of the amplitude
anomaly fits a structural contour within the
reservoir fairway.
1.925
Figure 10.41
Ringhorne Field, Norwegian North Sea. Top: (a)
geological section, (b) seismic section. The Ty reservoir sand is
almost invisible on the seismic section, where the response is
dominated by the high-impedance Chalk. (c) Cross-section from the
4D difference volume, phase rotated by 90° to pseudo-impedance,
showing a strong response from the Ty sand body where oil has
been replaced by water in the course of production, (d) detail of
difference section showing the original top reservoir pick (after
Johnston and Laugier,
2012
).
Evidence on the reliability of the seismic data can
come from assessment of data quality, modelling of
response in different fluid and reservoir scenarios,
and the track record of prediction accuracy.
Figure
10.40
shows an example from Kloosterman and
Pichon (
2012
) where high seismic amplitudes could
be directly linked to stacked channel belts on the basis
of 16 wells penetrating 40 sands. In this case, the
prognosis of hydrocarbon sand in the C2 and
H zones but not in the D zone was in accordance
with the actual well results, demonstrating a reason-
able level of certainty in the interpretation of seismic
amplitudes.
The PRMS also recognises the uncertainty in pre-
dicting reservoir properties from seismic data, noting
in particular that if seismic inversion is used then
there will be a low-frequency component that is not
uniquely constrained by the data and that this should
be taken into account in the uncertainty analysis.
Time-lapse (4D) seismic can also be a useful tool
in reserves estimation.
Figure 10.41
shows an example
where the reservoir is almost invisible on conven-
tional seismic (
Figs. 10.41a,b
) but is clearly apparent
on a 4D difference section (
Figs. 10.41c,d
). In this case
the 4D data led to revision of the extent of the reser-
voir sand and hence an increase in the reserves
volumes.
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