The North West Shelf Development Project centers on North Rankin, the largest proven gas field in Australia. The field lies 135 km offshore in water depths of 120 to 150 m. As part of an overall project-definition study, a complete geologic reevaluation of the North Rankin gas field is presently being carried out.

The main gas accumulation occurs within a horst block, formed by late Middle Jurassic tensional tectonics, unconformably overlain by post-tectonic sediments. Only minor variations in the level of the gas-water contact have been observed throughout the block. The main requirements for further refining the evaluation of the field are improved seismic definition at the reservoir level and elucidation of the wide variation in reservoir development observed on well logs.

The structural configuration of the field is currently being reinterpreted, based on a new 500-m seismic grid. Use of the latest processing techniques has led to a considerable improvement in the definition of the horst boundaries and of intra-reservoir markers.

To facilitate detailed geologic studies of the reservoir sequence, 550 m of core were cut in the most recent appraisal well, North Rankin 5, using oriented coring techniques. Sedimentologic analysis has enabled the environment of deposition of the various subunits of the reservoir sequence to be defined, together with the source direction and general orientation of the depositional system. Based on these results, a conceptual model has been formulated which will be used to predict sand percentages and reservoir continuity within the field. Petrologic analysis of the core has revealed a complex diagenetic history. Slight variations in the diagenetic processes have resulted in significant local variations in the petrophysical properties of some of the reservoir units. Further study of t e diagenetic processes should lead to the formulation of a second conceptual model allowing the prediction of overall porosity and permeability trends. The results obtained from the models, combined with the new structural interpretation, will provide the geologic basis for refining the estimate of field reserves and optimizing development planning.