RESERVOIR CHARACTERISATION OF THE TOOLACHEE UNIT 'C' IN THE MOOMBA/BIG LAKE AREA: FOCUSSING ON MINIMISING RISK

摘要

In the Moomba/Big Lake area, the Toolachee Unit 'C' contains approximately 25 per cent of the southern Cooper Basin gas reserves. To optimally exploit this resource, there was a need to expand and modify previous geological models to better characterise the reservoirs and minimise the risk associated with future activity. Remapping of at least 18 vintages of seismic data resulted in, for the first time, a consistent set of structure maps over the entire area. Detailed core facies analysis of 580 m of core, together with log motif analysis of 135 wells, resulted in a new predictive model for sand distribution. Coals previously thought to be laterally extensive were shown to be largely of local stratigraphic significance. The study identified 16 separate zones within the Toolachee Unit 'C' which had been deposited in a fluvial flood plain by multiple, relatively shallow, small-scale, medium- to high-sinuosity channels. The new model indicated that channel sand bodies were much more limited in lateral extent than previously thought, leading to a better understanding of the risk associated with appraisal and development drilling. Seismic attribute analysis was undertaken which involved acquiring new seismic data, reprocessing older data and then testing correlation of seismic attributes and reservoir petrophysics. The resultant reservoir seismic characterisation, although only qualitative in nature, allowed greater confidence to be placed on future appraisal well locations. The three-dimensional geometry of the deposi-tional model suggested that reservoir heterogeneity was high to very high. This indicated reservoir compartmentalisation but also significant additional reserves potential. Potential targets include new in-field reservoirs, untapped or incompletely drained zones and other bypassed reservoirs. The proposed depositional model has been tested with several appraisal wells. Risks previously in- volved in addressing potential reserves have been significantly reduced by this integration of geological, petrophysical, geophysical and engineering disciplines.