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7th Middle East Geosciences Conference and Exhibition
Manama, Bahrain
March 27-29, 2006
Saudi Aramco, Dhahran Saudi Arabia, phone: 966-3-8736189, [email protected]
Interpretation of an acoustic impedance volume requires an understanding of the relative contributions of seismic
and well
data to the inversion process. In some cases the initial impedance model, which is generated by interpolating and/or
extrapolating the well impedances along the
seismic
event times, can dominate the final impedance inversion, thus
overweighting the well information and underweighting the
seismic
data. In Saudi Arabia, a very strong impedance contrast
occurs between the Permian Khuff carbonates and the Pre-Khuff, Unayzah clastics. Vertically smoothing the well
impedance across such strong contrasts can create a bias in the initial model which impacts the inverted impedance for the
underlying clastic reservoir. In particular, model based inversions which include a constraint requiring that the output
impedance result remain within a specified percent of the initial model are susceptible to this problem.
Seismic
inversion is not sensitive to slowly varying changes in the initial impedance model which correspond to frequencies
outside the
seismic
bandwidth. This is true not only for
vertical
variations, but also for lateral variations. For this reason, it is
important to compare impedance slices from inversions that were generated using multi-well initial models, to impedance
slices that were generated from inversions using either a single well initial model or a constant impedance initial model.
Other approaches include subtracting the initial model from the final inversion to highlight the contribution from
seismic
data, or to use geostatistical methods which allow the user to control the relative contributions of the well information and
the
seismic
to the final inversion results.
Several examples are shown to illustrate problems and solutions related to both
vertical
and lateral variations in the initial
impedance model which are below the
seismic
bandwidth.