A seismic survey conducted in 1961/62 for Beach Petroleum NL indicated a large structural high at basal Tertiary centred at Beachport. The southern culmination of this general high runs through the Geltwood Beach area, where beach strandings of bitumen and oil occur. It was thought that stratigraphic drilling to penetrate the Dilwyn Formation (Knight Group) might give a more positive picture of the structure for the purpose of siting a deep petroleum well. Nine holes were completed, to depths up to 488m, and all encountered a similar sequence of Quaternary and Tertiary sediments. Electrical surveys consisting of SP and resistivity were run in each hole. The results enabled construction of structure contour maps which formed the basis for siting Geltwood Beach 1 oil exploration well. Coastal bitumen is shown to be natural crude oil of local origin in the continental shelf. The crudes are asphaltic and mixed-based waxy oils with moderately high sulphur content. The oils are not far travelled. At Geltwood Beach, strong submarine fresh water spring activity in the surf zone, flows enormous volumes of water to the surface carrying brown humic or lignic matter in suspension, and small amounts of bitumen and crude oil. The waters are artesian flows from the Lower Tertiary Knight formation. The oil may be from marine Cretaceous sediments wedging in from the continental shelf. Evidence favours diapirism of bituminous plugs to account for the periodic stranding of bitumen; but erosion of fault fissure infillings are also possibilities. Increased stranding of bitumen after the 1948 Beachport earthquake is in keeping with a theory of invasion of the crude oils into fault planes, possibly the projected Tartwaup Fault.