The mapping of the Birdsville 4-mile Sheet area and examination of water bore logs established a sequence of at least 3,000 feet of Mesozoic sediments, unconformably overlain by Tertiary rocks which have a maximum thickness of 100 feet. The Mesozoic rocks form the main aquifer and the impermeable cap in the western portion of the Great Artesian Basin. The marine shales of the Wilgunya Formation are possible source rocks for petroleum. Source rocks occur in the Lower Palaeozoic sequence to the north, in the Boulia 4-mile area. The only known reservoir rocks are the sandstones of the Lower Cretaceous Longsight Sandstone. Neither oil nor gas has been reported from any of the water bores in the area. Showings were encountered in the Bedourie Bore, 120 miles north of Birdsville, and in the Delhi Frome Santos Betoota No. 1 Well, 100 miles east of Birdsville. No important structures or economic mineral deposits are present in the area.