Integration of seismic velocity measurements in the context of the CO2 Storage project in the Bonaparte Basin, offshore NW Australia

Abstract

An accurate seismic velocity model is essential for depth conversion and rock property determination in the context of fluid flow modelling to support site selection for secure storage of carbon dioxide in the study area. Three types of seismic velocity measurements are available within the study area: velocities derived from stacking of multi-channel reflection seismic data; velocities determined in the process of ray tracing modelling of large offset refraction data acquired by the ocean bottom seismographs (OBS) along the coincident reflection/refraction transect, and velocities from well log measurements. Comparison of interval velocities calculated independently from refraction and reflection data for the key geological horizons shows good correlation for a large part of the area. However, local discrepancies in depths to these horizons calculated using these two alternative velocity models can exceed 10%. Large discrepancies are observed in the eastern part of the study area where the OBS velocity model is faster than the stacking velocity model. This region of high velocities in the OBS velocity model is consistent with elevated seismic velocities measured by well logs in the Newby 1 and Flat Top 1 exploration wells in the northeast of the Petrel Sub-basin. Discrepancy between two types of velocity models can be due to geological reasons or to differences in methodology of velocity determination in reflection and refraction data processing and interpretation. Accurate measures of uncertainty of both types of velocity need to be developed. However, time-depth functions constructed for individual horizons on the basis of the OBS derived velocity models are acceptable for a first pass depth conversion for a large part of the study area. The accuracy of the first pass depth conversion is further improved by calibration against wells.