A Comparison of AVO Techniques for Reservoir Characterisation in the Presence of Coals in the Barrolka Field, Cooper Basin, QLD
Anthony P. Jumeau
Honours Degree of Bachelor of Science (Petroleum Geology & Geophysics)
Australian School of Petroleum
The University of Adelaide
Coals in the Toolachee and Patchawarra Formations can cause significant problems for seismic reservoir characterisation. The Cooper Basin coals have anomalously low density and seismic velocity compared to the interbedded shales and sandstone. Variable coal bed thicknesses (most are below tuning thickness) in addition to relatively large impedance contrasts (when adjacent to shales and sandstones) have made underlying reservoir sands difficult to interpret using conventional seismic reflectivity methods. The use of Amplitude versus Offset (AVO) techniques to derive interval properties such as impedance, velocity, density and Poisson's ratio, can greatly enhance reservoir characterisation in the presence of these coals. This thesis compares two AVO inversion techniques; Generalised Linear Inversion (GLI) and Extended Elastic Impedance (EEI); that are used to transform pre-stack seismic volume into acoustic impedance and Poisson's ratio volumes, from which to predict the location of reservoir sands within the Toolachee Formation. Furthermore, this thesis provides a basic understanding into the workings of each technique whilst also delivering a workflow for the GLI technique.
Results for both the GLI and EEI inversion techniques have proven to be successful in delineating the PC30 coal and PC35 sand and in correlation with well log data. In both techniques there is the problem of non-uniqueness, however due to the comparison of results with well data and between the two techniques, the problem of non-uniqueness can be minimised and it's possible to validate the interpretation. Furthermore, it is shown that an increase in resolution can be achieved with the inclusion of low frequency well data when inverting for AVO attributes in the case of the GLI, though special care must be taken when preparing the well log data in this model-based inversion technique. Additionally, determining the optimal chi angle for Poisson's ratio in the correlation coefficient study is a major step in the application of the EEI technique in order to provide the best possible inversion results. Finally, results for both techniques demonstrate how acoustic impedance and Poisson's ratio can be used to predict the location of several potential reservoir sands and channels within Toolachee Formation. From the results it may be possible to determine the best location for new wells to be drilled intersecting these reservoir sands.