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Marianne Rauch1,
Phil Woods2
(1) CGG Americas Inc, Houston, TX
(2) Cultus Petroleum, Gordon, Australia
Abstract: Combining inversion to acoustic impedance and AVO to
predict fluid and 
porosity
The study area is located
offshore north/west Western Australia. The well Turtle-2 intersected oil-bearing
sands in the Tanmurra and Milligan Formations with a porosity of 12% and poor
permeability. Before drilling a subsequent well a detailed geoscience study was
performed.
1D AVO modeling was carried out,
using the rock properties obtained from logs of Turtle-2. The different
synthetic AVO response of oil saturated and water-wet sand indicated that AVO
could be used to predict the fluid content. The fluid factor and product stack
displays of the seismic data showed the same AVO response at the proposed new well
location as the oil saturated sandstone.
The seismic data were also
inverted to acoustic impedance. The logs of Turtle-2 were used to derive a
linear relationship between the acoustic impedance and porosity. A porosity of
24% was predicted.
The new well encountered oil,
which confirmed the AVO results. However, the porosity of the sand did not
exceed 12%. A post-mortem revealed that the incorrect porosity prediction was
caused by using stack data for the inversion. To calculate the porosity log
data was used, which represents only the zero offsets and does not contain any
AVO effects. It is recommended to use a near offset stack or intercept data as
input for an inversion to acoustic impedance if the output is being used for
porosity prediction
.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana