A 50 square mile area centred immediately south-southeast of Lake Cootabarlow, and lying ~12 miles south of the Moolawatana 1 water bore, was taken up by Australian - based holding company Frome - Broken Hill Co. Pty Ltd as an Oil Prospecting Licence in order to conduct stratigraphic test drilling as part of a search for possible natural gas accumulations within Mesozoic reservoir rocks of the Frome Embayment portion of the Great Artesian Basin sedimentary sequence. The drill site on OPL 4, located ~17 km north-northeast of Lake Frome, was chosen following the conduct in 1947 on OEL 2 of ground gravity and ground magnetic surveys operated by the Standard-Vacuum Oil Co. of New York, which had utilised its experienced Maatschappij geophysical field parties (brought in ex-Indonesia). These regional geophysical potential field surveys had attempted to remotely map the subsurface Mesozoic/Palaeozoic stratigraphic boundary and associated structuring. It was proposed to drill through the entire petroleum prospective sedimentary section to reach the crystalline basement rocks, which were estimated to lie at a depth of ~4200 feet in that portion of the basin, a prognosis based on modelling of the surveys' data. Before the program of drilling was begun, Zinc Corp.'s joint venture partners Standard-Vacuum Oil Co. and D'Arcy Exploration decided to withdraw from further participation in OPL 4 because they felt that the area did not offer petroleum prospects that were as enticing as those they had identified in New Guinea and their other search properties overseas. Zinc Corp. decided to continue the proposed work alone, and so between May and October 1949 it drilled three vertical rotary mud holes for a total penetration of 4424 feet 2 inches over a strong gravity gradient (N.B. the first hole was drilled as a water well, accessing the aquifer closest to the surface to supply drilling water for the eventual other two, deeper stratigraphic test holes). The sedimentary rock column drilled at Lake Cootabarlow prospect was found to be much thinner than anticipated, because presumed Mid-Palaeozoic, hard and strongly metamorphosed red sandstones were encountered at around 1650 feet depth; in consequence of meeting with this barrier to petroleum migration, the drilling was abandoned. It was concluded that the gravity anomaly which had been seen there might relate to effects of a steep syncline present in underlying Cambrian strata. No indications of crude oil or of natural gas were recorded from the drilled section. The Jurassic Walloon Formation porous reservoir sands, lying beneath a relatively thin marine Cretaceous interval, proved to be merely fresh water - bearing. The sequence of drilling-related activities conducted at the Lake Cootabarlow prospect was as follows : - between 28/5/1949 and 7/6/1949, an artesian water supply well, designated as (Lake) Cootabarlow 1, was rotary mud drilled vertically to a total depth of 1434 feet before being cased and cemented down to 1386 feet depth, and completed with a standard bore head. The top of the artesian aquifer had been cut at 1383 feet, and this aquifer was found to extend down to 1393 feet. It yielded a flow rate of 821,760 gallons per day at a discharged water temperature of 130 degrees Fahrenheit. The static pressure at the bore head was 55 pounds per square inch, as measured on 10/7/1949. Following its use later in 1949 during Zinc Corp's subsequent petroleum exploratory drilling program, this bore served as an additional water supply well for use by the lessee of Wertaloona Station to meet pastoral needs; - the second hole, and first intended exploratory well, (Lake) Cootabarlow 2, was spudded on 12/7/1949, in a collar position not very far removed from that of the first hole. Many short runs of core were cut over various intervals during the progress of the hole, starting from depths where organic material was noted in the sediments. A tiny amount of natural gas was observed to flow from this bore and the one drilled prior, both of which flows burned with a blue flame when ignited. Drilling was terminated at a depth of 1615 feet on 20/9/1949, because below 1471 feet 6 inches depth the hole had entered a sequence of indurated barren quartzite and silty tight sandstones. The hole was subsequently plugged and abandoned (in October 1949); - the third hole, (Lake) Cootabarlow 3, was spudded on 8/10/1949 at a location removed by about 11 miles distance to the south-southeast of Lake Cootabarlow 2. It was taken to a total depth of 1375 feet 3 inches, being then within a hard grey ?pre-Mesozoic mudstone, and drilling was terminated on 19/10/1949. After bailing the drilling mud out of the finished hole, a inflow of artesian groundwater commenced which was estimated to be 315,000 gallons per day at a discharged water temperature of 123 degrees Fahrenheit. The static pressure at the bore head was 57 pounds per square inch. Because the wildcat drilling undertaken had failed to disclose any indications of oil or of gas occurring in commercial quantities, as was evidenced in the drillhole logs and samples that it had submitted to the SA Department of Mines [Note: cf. DEM's PEPS database to access the now separately held drillhole data], Zinc Corp. decided in November 1949 to fully surrender OPL 4 and was granted permission to do so, having spent over 124,000 pounds in local currency on the campaign. The company stated that it had been warned prior to doing its drilling that magnetic and gravity surveys alone could not define a petroleum trap structure, and that a seismic survey would be necessary to delineate any such structure in detail, to optimise the chances of drilling success. However, it was informed that the Australian Commonwealth Government's Bureau of Mineral Resources was the only body in the country with the requisite seismic surveying equipment, only recently purchased, and it was already fully committed to doing other work for some time into the future.