The vitrinite reflectance technique is considered by the PELs 5 and 6 and ATP259P licensees to be the most reliable means of measuring the maturity of a sedimentary sequence. This subject suite of isoreflectance maps was prepared using vitrinite reflectance analysis results from over 120 wells drilled in the licence areas, in order to illustrate geographical/geological variations in maturity of the main hydrocarbon exploration target zones of the Cooper and Eromanga Basins. Although data are presented for the entire sequence, isoreflectance maps were prepared only for the top of 4 stratigraphically significant formations, due to time constraints. The maturity of these horizons reflect maturity in adjacent source rock intervals which are considered to be major contributors of hydrocarbons for Permian and Jurassic reservoirs. Isoreflectance maps presented in Enclosures 1 to 8 provide a basis for predicting the volume of hydrocarbons likely to be released from suitable source rocks in the area of interest. Areas where little or no accumulations have been found seem to occur in regions of low thermal maturity. The largest fields, although dependent on structural size, are found to occur in thermally mature regions or have access to such areas. A further point of note is that Cooper Basin gas wetness decreases linearly with increasing vitrinite reflectance. A similar trend is also noted with porosity variations, although to a lesser degree. The maps show the influence of past and present thermal regimes and structural configurations when compared to present-day structural maps. Maturation levels have not been significantly affected by Tertiary movements, and in areas of Tertiary uplift, mirror isochron and isopach maps more than present-day structure maps. This indicates that both depth of burial and the local geothermal regime have exerted control on sediment maturation, with the Nappamerri, Arrabury and Windorah Troughs being prime areas of high maturity.