Diagenesis and formation water evolution in the Browse basin, Australia.

The Caswell Sub-basin, Browse Basin, located offshore on the North West Shelf of Australia, has large reserves of gas and condensate such as Torosa, Brecknock, Calliance and Ichthys fields. Reservoirs are Jurassic sands of the Plover Formation or the Early Cretaceous sand of the Brewster Member. The formation waters in the aquifers in this area have lower salinities than seawater and can be as low as 10000 ppm. Hydrodynamically, it is remarkable and unusual to find such low salinity formation waters several hundreds of kilometre offshore and at depth up to 5000 m; and its origin remains unclear. How salinities in the Plover Formation have evolved with time? Are salinities in the gas zones the same than in aquifers?
In hydrocarbon zones, water saturation and water salinity are related by the Archie equation. A volume fraction of pores in gas reservoir is occupied by water at irreducible saturation. This volume fraction can be challenging to calculate and it impacts on the reserve calculation and the flow rate of production. Water at irreducible saturation does not flow during fluid sampling and therefore cannot be investigated. Interpretation of wireline logs such as electrical resistivity or spontaneous potential measurements are classically used to derive water saturation but require knowledge of the salinity of the irreducible water. Salinity of the aquifer below the gas-water contact may be used, however, samples are frequently contaminated and errors may be introduced if the salinity evolved since gas emplacement. Drainage capillary pressure measurements on core plugs can provide water saturation data, however core samples may be inappropriate or the measurements unreliable.
To provide additional constraints on salinity and its evolution with time, we propose to measures the salinity of water trapped in mineral microcavities known as fluid inclusions in the aquifers and in reservoir samples. It will use conventional microthermometry technique and Raman spectroscopy. The spectroscopy technique will control the polarisation effect of quartz and quantify the concentration of methane. Those data will be used to generate pressure-temperature constraint of entrapment and timing of the gas charge and salinity evolution.
Those data together will contribute to understand the evolution of the formation water salinity and will give indication of the origin of its low salinity characteristic.