SANDSTONE RESERVOIR GEOMETRY OF NON-MARINE SEDIMENTS IN THE TOOLACHEE GAS FIELD

Abstract

The concept of Genetic Increments of Strata (GIS) has been applied to correlations of the non-marine rocks of the Early Permian Patchawarra Formation of the Cooper Basin. Boundaries of GIS in the non-marine rocks are related to a dynamic model of non-marine deposition in which discrete sandstone bodies result from channel activity.In the 9 wells in the Toolachee Gas Field area of about 150 sq. miles, log correlations based on these principles determined eight major sandstone bodies, six of which hold gas reserves. The sandstone bodies are elongate and sinuous. They have cross-sectional dimensions of about 5 miles wide by up to 60 ft thick. Faulting and differential compaction have influenced the locations of the axes of the channel sandstone bodies.The value of mapping the geometry of the channel sandstones in the Cooper Basin lies in establishing a possible trapping mechanism which is independent of structural closure and which requires only a structural dip nonperpendicular to the channel sandstone direction; and providing a geologic basis for gas reserve estimates, the positioning of future appraisal and development wells and the prediction of field extensions and nearby new fields.An estimate of the proven-probable gas in place in the Toolachee Gas Field (560 BCF) based on the channel sandstone mapping is comparable with estimates based on lumping all pay intervals together in each well and drawing geometric pay isoliths. Estimates of possible reserves are increased by the mapping because of the introduction of the trapping mechanism of sandstone margins. The Toolachee Field has the potential to be perhaps doubled in size.