The Tirrawarra Sandstone is the primary producing zone at Tirrawarra field in the Cooper Basin of South Australia, where it contains 146 million barrels (MMbbl) of unrecovered oil. Because previous reservoir models had failed to predict reservoir performance accurately and development drilling had yielded poor results, we undertook this study in order to construct a three-dimensional descriptive model of the Tirrawarra Sandstone reservoir, within which we calculated the volumes and residency of original and remaining oil in place.