The portion of OEL 12 covered by the geological reconnaissance is approximately 2800 square miles in area. Of this area, about half was examined over the period from 13 October to 1 November 1958 by making fairly close spaced traverses along fencelines, roads and tracks, and by visiting most of the obvious outcrops. The relatively closely investigated area is bounded by Bon Bon Homestead, McDouall Peak, Billakalina Homestead and Parakylia Homestead. The aim of the survey was to delineate the southern boundary of the marine Cretaceous rocks in the licence area, and to examine their stratigraphy with a view to assessing the general prospects for the occurrence and accumulation of petroleum in them. Copies of bore log information pertaining to the area investigated were also assembled from the SA Department of Mines and pastoral leaseholders' records, however, all such bores had been sunk to search for groundwater, and they rarely exceed 300 feet in depth. The area's geology is as follows: Pleistocene-Recent: alluvium (unconsolidated sand), derived from Blythesdale (Group). Some ferruginous and silty cappings contain fossil leaves (?Tertiary). Tertiary: Highly siliceous laterite up to 40 feet thick overlies calcareous shale (probable same order of thickness), silica-impregnated. Contains Planorbis and Lymnaea. Unconformable on Cretaceous. Cretaceous: (1) Roma Formation. (a) Sanderson Shale Member (blue to black, weathers white; 300 feet thick; 503 feet at ?Coorie Appa). (b) Millers Creek Sandstone Member (fine-medium grained, some pebbles, useful aquifer, variable thickness up to 190 feet in area investigated; possibly 265 feet at Coorie Appa). (c) Paisley Shale Member (contains lower Cretaceous forams; bluey-grey, weathers chalky white; somewhat sandy; up to 342 feet thick; 320 feet at ?Coorie Appa). (d) Mount Eba Sandstone Member (medium-grained, reasonable aquifer; siliceous at surface; up to 285 feet thick, averages about 200 feet, thins to north; 62 feet at Coorie Appa). (e) Vivian Shale Member (brown to blue; up to 254 feet thick; minor sandstone intercalations; probable constitutes top 100 feet of Lake Phillipson Bore; 314 feet at Coorie Appa). (2) Blythesdale Group. Gritty sandstone and shale; smoky blue-grey quartz; variable thickness (due to proximity to floor of basin); possible sub-basins extend south of railway; thickness unknown (18+ feet at Coorie Appa). Cambrian and/or Proterozoic: Probably consists of Purple Downs Limestone, Lake Richardson Sandstone and Lake Hart Shale [see Miles, K.R., 1956, Woomera 1 bore drill core descriptions (Envelopes 51-53)]. The position of the Cambrian / Cretaceous boundary as mapped is only approximate. Palaeontology: 4 specimens examined by Dr. Ludbrook. Structure: Basement highs along line NW from Koolymilka Lake. Minor syncline in Millers Creek Homestead - Mount Paisley area, anticline to north. Oil prospects rated poor.