Geoscience Reference
In-Depth Information
bedding reflectors can cause the flat event to be broken up into a series of segments
that may individually appear to be slightly tilted, although the ensemble remains
flat).
(3) There should be apparent thickening of the isochore in the interval above the top
sand if the alternative 'continuous strong loop' interpretation is used.
(4) The flat event at the OWC should run horizontally across inclined bedding, resulting
in apparent reflector terminations below it.
(5) Crucially, the amplitude dimming, the flat spot extent and the apparent isochore
change should be consistent in map view with each other and with a mapped trap,
e.g. a dip closure. The amplitude change should follow a structural contour if it
is indeed caused by a change in fluid type at the downdip edge of a trap. This is
where 3-D seismic can make a big contribution. Both the amplitude map and the
structural map are much more detailed than could be achieved using a grid of 2-D
data, so this test is much more rigorous.
If all the tests are passed then it is possible to have a high degree of confidence
in the interpretation of fluid fill. It is quite usual for the evidence not to be so clear-
cut, however. In particular, a failure of amplitudes to fit structure may be caused by
a stratigraphic element of the trapping mechanism, or by complications due to lateral
changes in rock properties (e.g. porosity). The evidence then needs to be weighed
carefully together with geological understanding. Is there enough well control for us
to be reasonably confident of sand and shale properties? Is the seismic data quality
adequate? For example, flat events may be multiples of sub-horizontal events higher
in the section. If so, they can certainly cut across bedding, as expected for a fluid
contact, but will be likely to continue across the top-seal as well as the reservoir. It
is particularly suspicious if the flat event is one of a whole suite at different TWTs,
which points strongly towards it being a multiple. It is often possible to see very weak
flattish events on a seismic section if they are looked for hard enough, and they are
often multiples that have been reduced in amplitude but not quite eliminated during
processing.
In many cases, lateral amplitude changes are related to changes in porosity rather than
fluid fill. This is particularly true for well-consolidated sands and carbonates.
Figure 5.7
shows how the impedance of the Chalk in the North Sea is strongly affected by porosity,
but relatively little by fluid fill. In this case, the Top Chalk will be an impedance increase
(red trough with the usual North Sea polarity convention) if the porosity is less than
35%, changing to a decrease (blue peak) for higher porosities. At constant porosity, the
difference between the average impedance trend (in blue) and the lower values with
hydrocarbon fill (in dotted black) is quite small; a similar impedance change could be
caused by quite a small change in porosity at constant fluid fill. Conversely, the large
impedance change caused by porosity variation within the usually observed range (say
10-40%) is much greater than would be caused by any change in fluid fill. If there is
enough well control to calibrate the relationship, it may be possible to infer porosity
