Cambro-Ordovician Warburton Basin strata and intrusive granites are conventionally regarded as the economic basement beneath the Cooper-Eromanga oil and gas province in northeastern South Australia and southwestern Queensland. This study, the first of its kind, has identified and mapped a regional fracture system extending throughout the upper levels of the Warburton Basin, and because such fractures contain crude oil and natural gas derived from Cooper Basin source rocks, the reality of effective secondary hydrocarbon migration into fractured 'basement' traps is confirmed. Examples include Sturt 6 and 7, where mobile oil has been recovered in commercial volumes from fractured Cambrian tuffs, and Moolalla 1 and Lycosa 1, where strong natural gas flows have been tested from Ordovician sandstone and fractured siltstone lithologies respectively. Considerable information about fracture properties has been gathered from a detailed core study of 91 wells, in conjunction with the use of dipmeter and formation micro-scanner (FMS) logs from 27 wells. Absolute orientation data have been obtained from measurements of cores and logs from 23 wells, and additional relative orientation data have been collected from 24 wells. Two systems of orthogonal fracture sets have been identified for the first time passing across local structures, and these are thought to extend widely beneath the Cooper Basin in South Australia. System I involves a pair of orthogonal fracture sets striking NNE-SSW (20-200°) and ESE-WNW (110-290°). This system is similar in direction to low frequency lineaments NNE8 and WNW9. System II has a pair of orthogonal fracture sets striking NE-SW (60-240°) and NW-SE (150-330°). The NE-SW strike orientation is similar to lineament NE1. Open fractures found in Lycosa 1 have azimuths striking WNW and NW within Systems I and II, and dip SW. In this well a set of open, steeply SW dipping fractures are interpreted from FMS logging in the production zone over the depth interval 8653-8670 feet KB, while incompletely filled fractures were observed in core 1. Other sets of open fractures striking NNE and NW are interpreted from FMS logging in Malgoona 4. Based on data from Lycosa 1, Gidgealpa 5 and Tilparee-A 1, System I is possibly younger than System II. These results indicate that an optimum production well trajectory designed to maximise intersections with open natural fractures should be led off in the directions 200-210° and 240-250°, and possibly 270-290°, True. The complementary hole deviation angle should be approximately 30° from horizontal in the fracture zone, due to the high angle to sub-vertical fracture dips. A semi-quantitative estimate of fracture density has been determined for 91 wells, and is summarised in the form of an interpretive map. The greatest fracture density is located in major fault zones or structural culminations, as expected. Generally speaking, fractures are mainly developed within brittle rather than ductile rock types if they are frequently interbedded. Open fractures or incompletely filled fractures are associated with brittle dolomites such as in Gidgealpa 1, 5 and 7, and are also seen in brittle sandstones such as those in Beanbush 1, Meranji 2, Jennet 1, and Merrimelia 6 and 7, or else are seen in brittle ignimbrites and tuffaceous sandstones such as those in Boxwood 1, Gidgealpa 4, and Sturt 7 and 8. These fractures occur mostly at high angles. Microfractures have been found, and are believed to connect secondary porosity in the ignimbrites of Sturt 7 and 8. Fracture fairways thus appear to exist in brittle sandstones, dolomites and volcanics, and probably also exist in any rock types with the above suggested orientations of the two systems of orthogonal fracture sets.