Geoscience Reference
In-Depth Information
well control, this process is straightforward; where there are few wells, there is obvi-
ously a danger of imposing an incorrect low-frequency trend on the final result. This is
particularly true if the wells are drilled in updip locations and the target for the inversion
is downdip, with a greater depth of burial causing additional compaction and perhaps
containing additional stratigraphic intervals not sampled at all by the well control. It may
be possible to allow for compaction effects by putting in trends of increasing impedance
with depth, but thick intervals with no well control are bound to cause problems. A
last resort if there is no well control is to create the low-frequency model from seismic
velocities (stacking or migration). The snag with this, apart from the general issues of
the unreliability of seismic velocities as a measure of real rock properties discussed in
to do this, it is necessary to know the lithology, which may be just the point at issue
in an undrilled section.
information is apparent in the progression from low impedance (purple) at the top of the
section to high impedance (yellow/white) at the base. The high-impedance precursor
(green/red) to the target horizon (i.e. above the yellow marker) visible in
fig. 6.2
is not
a feature of
fig. 6.5
; probably it was an artefact produced by minor departure of the
wavelet from the zero-phase assumed in the SAIL processing. However, the definition
simple SAIL approach is adequate, partly because the reflectivity data are close to
zero-phase and partly because the target layer is the right thickness to have a response
within the seismic bandwidth.
Fig. 6.5