Undiscovered (speculative and hypothetical) petroleum reserves and field sizes for sedimentary basins in SA have been estimated using methods that are appropriate to the level of previous exploration (and thus the data available). These methods range from extrapolation of discovery trends in the relatively mature Cooper Basin, to Monte Carlo probability simulation in the recently gas-proven Otway Basin, and in the underexplored Officer Basin. The results indicate that 1.7 TCF of sales gas is yet to be discovered in the SA portion of the Cooper Basin compared to 1.6 TCF of sales gas in the western onshore Otway Basin, and that in the Officer Basin there is a reasonable chance (50% probability) of finding 5 TCF of gas or 1.7 billion barrels of oil. This latter estimate gives considerable encouragement that a further petroleum producing basin may soon be discovered.