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Use Of Stacking Velocity For Depth Prediction And Lithological Indication In The Challum Field Of The Cooper/Eromanga Basin, Queensland.

Joraandstad, Susann

Honours Degree, 1999

University of Adelaide

Abstract

With decreasing reserves and smaller findings, optimum development of existing fields has become of greater importance in mature hydrocarbon provinces such as the Eromanga Basin. One way to do this is to obtain greater value from seismic data by using the stacking velocities, which are generated as a byproduct of processing. The use of seismically-derived velocities is potentially of great value in determining depth to target reservoirs, estimating reservoir volume and reservoir characterization. However, there are many sources of errors which affect these velocities and must be taken into consideration if they are to be used successfully.

This project investigated the use of average velocities derived from stacking velocities for time to depth conversion, and Dix interval velocities for reservoir characterization. The Challum field in the Queensland sector of the Cooper/Eromanga Basin was used as the test area, as a dense grid of stacking velocities from 3D survey, as well as data from 15 wells were available. The depth conversion targets were the top of the Hutton Sandstone and the top of the Nappamerri Group. The intervening Hutton Sandstone represented the reservoir used for the interval velocity work.

The depth conversion method tested in this research requires the approximate conversion of stacking velocity to average velocity and finding the smoothing for this velocity. The resulting velocities are then calibrated to the well velocities to give the final product. A procedure of excluding subsets of the wells from the calibration step, and then using the result to predict the depths at the hidden wells was conducted to check the utility of the depth predictions. The results of these tests, using ten wells for each calibration, and predicting depths at the remaining five, showed a standard deviation of 20 feet in the depth errors for the Hutton Sandstone and 40 feet for the Nappamerri event. The Hutton result is acceptable, while the Nappamerri result is poor.

Dix interval velocities were computed for the Hutton Sandstone interval and investigated to determine if they could be used to predict reservoir properties such as net-to-gross ratio, net sand and average density. However, there is no significant statistical correlation between interval velocities calculated from well data and these reservoir properties, so the method cannot be used to predict these particular properties for the Hutton Sandstone in the Challum Field. This does not mean that it will not work for other properties or in other reservoirs or fields. Of more concern is that the Dix interval velocities do not correlate significantly with the well interval velocities. This indicates the high noise level in Dix interval velocities and the need for high quality stacking velocity data if useful Dix interval velocities are to be calculated.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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