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

Abstract

AbstractPalaeoformation 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 homogenization, salinity and gas content of water inclusions show that the salinity of the palaeoformation 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 8500 and 24 000 ppm eq. NaCl (0.8–2.4 wt%). An influx of meteoric water from the Ashmore Platform in the north is presented as a hypothesis on the origin of the low salinities of the formation water in the Plover Formation in the Browse Basin, supported by the distribution of the lowest palaeowater salinities, but still remains problematic.