Constraining the timing and evolution of hydrocarbon migration in the Bight Basin

Abstract

Abstract Micron-sized fluid inclusions within quartz grains from Late Cretaceous sandstones, recovered from drill cuttings from four wells in the Great Australian Bight, contain palaeo-fluids such as brine, black oil, light oil, gas condensate, and N2 and CO2-rich hydrocarbon gas, providing evidence of multiple palaeo-hydrocarbon migration in the Bight Basin. A series of microscopic, spectroscopic and thermometric characterisation techniques were applied to the petroleum fluid inclusions in Gnarlyknots-1A, Potoroo-1, Greenly-1 and Duntroon-1 in order to estimate fluid types, palaeo-pressures and palaeo-temperatures (PT) of hydrocarbon entrapment, and to provide constraints on the age of petroleum migration in the central and eastern parts of the Ceduna Sub-basin. The Coniacian interval (Tiger Supersequence) in Gnarlyknots-1A trapped a variety of petroleum fluid types (black oil, light oil and gas-condensate) over an extended period. The earliest phase of oil entrapment recorded in quartz grains took place at a minimum of 58 °C as light oil and, together with pressure, constraints this to the end of the Cretaceous (circa 75 Ma). Source rocks capable of light oil generation at this time occur in the Blue Whale Supersequence. Petroleum compositions evolved toward more gas-rich fluids during the Cenozoic. This, together with some late oil constrained to Miocene age migration, implies generation from at least two source rocks. Source rocks capable of generating gas at this time are interpreted to be in the Blue Whale and White Pointer supersequences, while late oil may have been expelled from the Tiger Supersequence in the central basin depocentre. The palaeo-PT evolution recorded by the fluid inclusions in Gnarlyknots-1A closely follows the modelled burial curve, suggesting that the petroleum fluids were in thermal equilibrium with the rock. In contrast, the oil and gas inclusion assemblages found at Potoroo-1, Greenly-1 and Duntroon-1 were trapped later, with some at temperatures higher than the formation temperatures predicted from burial models. Whilst there are several potential mechanisms that explain these observations, we suggest that these wells are located near major extensional faults that have the potential for conducting fluids upwards. Additionally, gases such as CO2 and N2 associated with hydrocarbons possibly relate to mixing with mantle-derived fluids, which may also have contributed to the aforementioned thermal anomalies.