Inversion - 3-D Seismic Interpretation - page 164

6.3 Benefits of inversion

Several benefits of inversion have been identified, for example by Buiting & Bacon
( 1999 ) . These include the following.
(1) Impedance displays are layer-oriented, unlike reflectivity displays which are
interface-oriented. This greatly facilitates interaction of seismic interpreters with
geologists, petrophysicists and reservoir engineers.
(2) Impedance can often be related to reservoir properties such as porosity or net to
gross ratio. The inverted dataset can therefore be used directly to constrain reservoir
models, to improve volumetric estimates and to target wells more precisely.
(3) Inversion lends itself to stochastic approaches that permit exploration of the range
of reservoir models that are consistent with the observed seismic data.
Some examples will help to explain what can be achieved. Although they will be
illustrated by means of 2-D sections taken from the 3-D dataset, it is important to
bear in mind that the value is much enhanced by the possibility of generating a full
3-D volume of reservoir attributes. This is a requirement for reservoir modelling, but
is also a good QC tool. Inversion is interpretive processing, requiring decisions on
parameter settings which may rest on geological intuition. A 3-D volume that can be
interpreted in a geologically consistent way is good evidence of correct choices; a few
2-D lines may not offer much scope to check whether the parameters give sensible
results beyond the traces they were designed on.

6.3.1 Inferring reservoir quality

Figure 6.6 shows a section from an inverted 3-D dataset in the UK Central North Sea.
A well was drilled at the location marked by the vertical differently coloured strip
in the centre of the figure. The coloured strip shows well impedance values derived
from sonic and density logs, and gives an impression of the degree of agreement be-
tween well and seismic data. The target for the well was the Fulmar sand, a Jurassic
reservoir of variable quality. The other picked events are the Base Cretaceous (Xunc)
and Top Zechstein. There is clearly a patch of low-impedance (green) Upper Fulmar
reservoir around the well, but most of the Fulmar is harder (red/yellow). These dif-
ferences can be turned into an estimate of the porosity of the Fulmar sand, using the
calibration shown in fig. 6.7 . This is a plot of acoustic impedance versus porosity,
based on data from several wells; trends can be derived for the prediction of poros-
ity and its uncertainty, as shown for the case of the aggradational facies that forms
the low-impedance reservoir in fig. 6.6 . In this way, the Fulmar reservoir porosity
can be determined from the inversion dataset throughout the 3-D volume. This will
improve the estimates of oil in place after a discovery; it will also help the reser-
voir engineer to assess the quality of reservoir interconnection, which is important for