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A companion volume to 'The geology and petroleum potential of the Clarernce-Moreton Basin, New South Wales and Queensland' compiled and edited by A.T. Wells and P.E. O'Brien, Australian Geological Survey Organisation bulletin 241(1994).

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Thangoo No. lA well was drilled as a result of mechanical difficulties encountered at the drilled depth of 3 ,475 feet in Thangoo No. 1 well which were insurmountable and prevented further operations on this well. Total loss of circulation at 3,475 feet in Thangoo No. 1 well resulted in unexpected total collapse of the hole below the 10 3/4" casing in a section of friable sandstone and conglomerate (Grant Formation). In Thangoo No. lA well these formations were cased off. A lost circulation zone was tested for potential hydrocarbon production and found to contain water only. The well was drilled to 5,429 feet into basement rocks (programmed depth 4,500 feet). Excellent hole control was maintained to total depth by drilling below the 16" conductor shoe with high pH, low shear and waterloss, freshwater mud. The 1,366 feet of Mesozoic and 1,377 feet of Permian section in Thangoo No. lA closely resemble that of Thangoo No. 1. The Ordovician section is 2,318 feet thick, consisting of 695 feet of Goldwyer Formation (new name) and 1,623 feet of Thangoo Limestone (amended name). Basement, consisting of Precambrian phyllite , was encountered at 5,100 feet in Thangoo No. lA. Traces of oil were observed throughout the Ordovician section. Oil shows are confined predominantly to vuggy veins of coarse crystalline dolomite. A minor show was also observed in the sandstone of the basal unit of the Thangoo Limestone. A porous zone within the Thangoo Limestone, causing some lost circulation, appeared at about 3,670 feet in Thangoo No. lA. A formation test of this zone recovered only brackish water (11,999 ppm. total salts) with no signs of oil or gas. It is possible that the small shows of oil in the Thangoo Limestone are retained by the impervious Goldwyer Formation, as no shows were present in the Roebuck Bay and Dampier Downs wells, where porous Permian rocks directly overlie the Thangoo Limestone. The correlation of the sections of the Thangoo Limestone in all the exploratory wells on the Broome Platform indicates that the Thangoo No. lA well occupies a low structural position. Consequently the objectives of testing the structure in the Ordovician and Permian Grant Formation were not achieved. The chances of finding commercial oil accumulations in the Ordovician section of the South Canning Basin in the vicinity of Thangoo Nos. 1 and lA are very small at this time, because of the low source rock potential and flushing by meteoric waters of the Thangoo Limestone, the lack of porosity in the Goldwyer Formation, and the difficulties experienced in resolVing the structure of the Ordovician with present seismic methods.

Union-Kern-A.O.G. Cabawin No. 1 spudded on 6th October, 1960, was drilled as a structural test of the petroleum potential of sediments underlying a major unconformity within the Surat Basin. The age and nature of the section below the unconformity was questionable prior to drilling the well, but it was presumed to be partly Triassic and partly Permian in age and related to the sedimentation of the Bowen Geosyncline. The drilling of the well confirmed this prognosis. In addition to providing stratigraphic information related to the petroleum potential of the Bowen Group, the well also provided information of regional significance on the Mesozoic rocks of the Great Artesian Group. Under a thin veneer of Tertiary rocks the drill penetrated the Roma, Blythesdale, Walloon and Bundamba Formations of the Great Artesian Group, and the Cabawin, Kianga, and Back Creek Formations of the Bowen Group, before bottoming in volcanic rocks of (?)Permo-Carboniferous age at a depth of 12,035 feet. The drilling of the well confirmed the presence of hydrocarbons in the Surat Basin. During the drilling operation a blowout occurred at a depth of 9938 feet, surfacing free gas and condensate. The blowout was caused by formation pressures, in excess of the mud weight, within an eight-foot sand body of the Permian Kianga Formation. In addition significant gas shows and a flow of salt water were encountered in weathered andesite of the Cracow Formation. Minor gas shows were recorded in the Permian Back Creek Formation, and in the Lower Triassic Cabawin Formation, and Significant shows of hydrocarbons were also present in porous and permeable sandstones of the lower member of the Triassic-Jurassic Bundamba Formation. The number of occurrences of oil and gas showings indicated that only a sustained testing programme through casing would properly evaluate the potential of these hydrocarbons. After reaching total depth, 5 1/2" casing was cemented at 12,033 feet, and there followed a series of production tests of all significant shOWings of oil and gas encountered during the drilling operation. Results were negative with the exception of the one zone in the Permian Kianga Formation. This zone proved to be the only producing horizon. After 22 days of continuous production, this zone was producing 62 barrels per day of 490 API gravity crude through 22/64" choke, with casing and tubing well-head pressures at 550 p.s.i. and 300 p.s.i. respectively, and 534 Mcf/D of gas. A total of 1761 barrels of crude oil and 13,744 Mcf of gas were produced during the testing period. Pressure and temperature surveys were conducted, and the well was shut in and suspended on 17th June, 1961, retained in a condition for production at any future time.