Adaptation of fluid inclusion techniques for investigating gas charge: examples from the Caswell Sub-basin, Browse Basin, Australia

Palaeo-formation water trapped in quartz cements in sandstone during diagenesis is typically of interest for constraining the temperature history, cementation and timing of hydrocarbon charge. Recent progresses in developing methods for salinity measurement, gas detection (CH₄, CO₂, N₂, H₂S) and fluid modelling of the CH₄–H2O–NaCl system by combining conventional microthermometry techniques with Raman spectroscopy provide powerful tools for investigating formation water and its evolution in gas-bearing basins. Samples from the aquifer, in the Plover Formation and in the Brewster Member in the Upper Vulcan Formation, underlying large gas accumulations in the Caswell Sub-Basin provided an opportunity to test these new techniques and generate data on formation water evolution. Temperature of homogenisation, salinity and gas content of water inclusions show that the salinity of the palaeo-formation waters decreased with increasing methane content and temperature. Detection of CO₂ shows, however, that water inclusions with dissolved CO₂, often in association with CH₄, do not follow the same salinity trend. These inclusions are often associated with higher trapping temperatures. The salinities associated with water reaching methane saturation (coeval with free-gas) are between 8,500 to 24,000 ppm eq. NaCl (0.8 to 2.4 wt%). An influx of meteoric water from the Ashmore Platform in the north is presented as an hypothesis of the origin of the formation water low salinities of the Plover Formation in the Browse Basin, supported by the distribution of the lowest palaeo-water salinities, but still remaining problematic.