--> Gas, Salinity and Temperature Evolution of the Formation Water in the Plover Formation and Brewster Member, Browse Basin, Australia

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Gas, Salinity and Temperature Evolution of the Formation Water in the Plover Formation and Brewster Member, Browse Basin, Australia

Abstract

Investigating the nature of palaeo-fluids in fluid inclusions can deliver critical information about the petroleum system in a basin. Palaeo-formation water trapped in cements in sandstone is typically of interest. Recent progress in developing methods for salinity measurement, gas detection (CH4, CO2, N2, H2S) and fluid modelling of the CH2-H2O-NaCl system, combining conventional microthermometry techniques with Raman spectroscopy, provides a new understanding of formation water evolution in gas-bearing basins. Samples from the aquifer underlying large gas accumulations in the Browse Basin provided an opportunity to test these new techniques and generate data on formation water evolution. Palaeo-temperature, salinity and gas content of water inclusions from core samples in the Plover Formation and Brewster Member, below the Torosa, Calliance, Ichtys and Poseidon gas fields, and from dry wells at Buffon-1, Yampi-2 and Caswell-2, were measured. The fluid inclusion data show that the salinity of the palaeo-formation waters decrease with increasing methane content, and that the salinities associated with water reaching methane saturation (contemporaneous with free-gas) are between 11,000 to 24,000 ppm (1.1 to 2.4 wt%). Detection of CO2, however shows that water inclusions with dissolved CO2, often in association with CH4, do not follow the same trend with salinity, and in some cases show an opposite trend. They also are often associated with higher temperatures. Water inclusions from wells located in the inboard side of the Caswell sub-basin (including Dinichthys North-1, Caswell-1 and Yampi-2) have characteristics in salinity, gas content and temperature that differ from those in the outboard side of the Caswell Sub-basin (Torosa-5, Kronos-1, Calliance-1, Buffon-1), and suggests a difference in fluid circulation. While the origin of inferred low-salinity formation water, at distances of about 200 km from the coast, remains enigmatic the influx of meteoric water is not supported by the location of the lowest palaeo-water salinities. The common association of methane and low salinity water might suggest that they migrated together and could have a same origin.