A reservoir geology study has been undertaken for all gas and oil fields in the South Australian Cooper Basin except Moomba, Gidgealpa and Packsaddle. The fields studied are those which will supply the Sydney gas market. Moomba and Gidgealpa supply Adelaide. Packsaddle is considered to have reserves too small to be economic. The aim of this project is to outline the geology of the gas-bearing Permian formations, and to obtain estimates of the Proven and Probable gas reserves contained within the individual structures. From individual isopach maps of producing formations the SADM Petroleum Engineering Branch has calculated the hydrocarbon pore volume and deliverable gas and oil for each of the fourteen fields. Three main criteria were used to determine the field limits. If a gas-water interface was recognised in one or more of the wells, it was used to define the pay sand boundaries. In all such cases, with the exception of the Toolachee Field, a horizontal or dipping planar interface was assumed. In the case of no gas-water interface being present within a field, a horizontal plane was drawn through the base of the lowest producing sand and used as an assumed gas-water interface. In many cases this would have given a conservative reserve estimate. In the absence of any recoveries of formation water during drill stem testing, it was assumed that permeability barriers were affecting production, so these were placed halfway between producing and dry wells wherever possible. A special study of permeabilities and their relation to structure and formation thicknesses was made for use only in the Tirrawarra Field.