Geoscience Reference
In-Depth Information
the effects of multiples. Ideally, seismic processing should have attenuated multiples
to low levels relative to the primaries. If a better fit to the real seismic is observed
when multiples are added into the well synthetic, then the interpreter needs to be on
his guard; even if multiple energy is not obvious on a section view and does not cause
problems for structural interpretation, it may still corrupt amplitude measurements on
target reflectors. An advantage of having the dense data coverage of the 3-D survey
is that it is possible to observe how correlation varies as a function of trace location.
Ideally one would hope to see a bull's-eye pattern of high correlation centred about the
well location, though the real world is not always so simple.
Sometimes it is easy to obtain a good match between well synthetic and seismic
dataset; sometimes it is very difficult. There are many possible reasons for a mismatch.
Seismic survey problems
incorrect zero-phasing (or other defective processing)
multiples
incorrect
spatial
location
due
to
shot/receiver
mispositioning
or
(more
commonly)
incorrect
migration velocities
Synthetic seismogram
defective logs
hydrocarbon effects
inadequate spatial sampling
defects
In addition, there are the effects of amplitude variation with incidence angle; in principle
this means that synthetic seismograms should be calculated for a range of angles and
added together to simulate the stacked trace of the real data (see
chapter 5
).
It is important to understand the causes of an observed poor well to seismic tie. If
the problem lies with the seismic dataset, similar problems will often be present in all
wells within the survey area. Incorrect spatial location, if the result of mispositioning of
sources and receivers, will probably mean that all ties will be improved by applying a
constant lateral shift to the seismic trace locations. If the problem is incorrect migration
of the surface seismic, then the tie points will be shifted updip or downdip by an
amount related to the steepness of the local dip; thus flat-lying overburden might need
little lateral shift, whereas deeper reflectors with dips of 30
◦
might need lateral shifts
of hundreds of metres.
Problems due to the synthetic seismogram will be different from one well to another.
Various problems can arise with the wireline log data. They may need substantial editing
to remove intervals of incorrect readings. Usually, the petrophysicist is interested mainly
in obtaining good quality logs over the reservoir interval, which he or she will use to
evaluate reservoir quality and hydrocarbon saturation; other intervals, especially shales,
will not have received intense scrutiny for log quality at the time of acquisition. It is
quite common to find data gaps and noisy intervals, owing for example to cycle skips
on the sonic log where the automatic travel-time measurement system triggers on the
wrong part of the signal pulse. Defective log intervals can be edited by removing noise
