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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