Regional depth-conversion of mapped seismic two-way-times in the Cooper-Eromanga Basins

Abstract

Mapped seismic two-way-times to the top of the Cadna-owie and Toolachee Formations in part of Petroleum Exploration Licences 5 and 6 of the Cooper-Eromanga Basins were depth-converted using the interval velocity and velocity anomaly methods. Two types of input velocity data were used for each depth-conversion method: (i) based on mapped seismic isochrons combined with thicknesses at well locations, and (ii) based on drift-corrected checkshot/sonic log data and thicknesses at well locations. The sequence was split into three layers for the depth-conversion: surface to top of the Mackunda Formation (layer 1), top of the Mackunda Formation to top of the Cadna-owie Formation (layer 2) and top of the Cadna-owie Formation to top of the Toolachee Formation (layer 3). The interval velocity method used constant layer interval velocities as predicted by interval velocity maps. In the velocity anomaly method, regional velocity/depth functions were fitted to the input layer velocities. Anomalies with respect to these regional functions were then mapped and used in conjunction with the regional functions in order to undertake the depth-conversion. The accuracy of the depth-conversions was assessed with reference to test wells not used in the determination of the input velocity models. The 95.5% confidence limits of the error in depth-conversion for the velocity anomaly method are 7?16% closer to zero than for the interval velocity method at the top of the Cadna-owie Formation, and 21?41% closer to zero at the top of the Toolachee Formation. Hence velocity anomaly can be more accurately mapped and predicted away from the input wells than 'raw' interval velocity. There is no significant difference between the errors in depth-conversion based on seismic isochron and checkshot/sonic log input velocities. Given that the former are more widely available, and more easily determined than the latter, their use is preferred.