Application of the Saturation Water Gradient (SWGRAD) Method to Well-Log Analysis for Gas-Bearing Zone Identification and Perforation Interval Recommendation in the Poseidon Field, Browse Basin, Western Australia
DOI:
https://doi.org/10.25299/jgeet.2026.11.3.25775Keywords:
SWGRAD, water saturation, formation-water salinity, gas reservoir, Plover Formation, Browse Basin, well-log analysisAbstract
Accurate identification of gas-bearing intervals in heterogeneous sandstone reservoirs remains challenging when formation-water salinity is poorly constrained. This study evaluates the applicability of the Saturation Water Gradient (SWGRAD) method for identifying potential gas-bearing intervals in the Plover Formation of the Poseidon Field, Browse Basin, Western Australia. Well-log, mudlog, well-report, and available DST/MDT data from five wells, namely Torosa-1, Kronos-1, Boreas-1, Poseidon-1, and Poseidon-2, were analyzed. Gamma Ray, neutron–density porosity, and resistivity logs were integrated to characterize reservoir intervals prior to SWGRAD analysis. Water saturation was calculated using Archie’s equation for formation-water salinities ranging from 1 to 25 kppm, and SWGRAD was determined from the slope of the least-squares linear regression between water saturation and salinity at each depth sample. The resulting SWGRAD profiles were interpreted using well-specific cutoff criteria and evaluated against available DST/MDT and production-test evidence. The results indicate that well-specific SWGRAD cutoffs ranging from ≤0.00002 to ≤0.000045 provide a more appropriate discrimination criterion than a universal threshold. Independent validation is strongest in Boreas-1, where SWGRAD intervals overlap approximately 61 m of the 70 m DST interval, corresponding to approximately 87.1% agreement, and in Kronos-1, where the interpreted intervals correspond to reported gas-show and production-test evidence. In Poseidon-2, the identified low-SWGRAD intervals are considered prospective rather than independently validated because the available DST results were inconclusive. For Poseidon-1, where direct well-test evidence is unavailable, the ≤0.00002 criterion was adopted as a working cutoff based on comparison with Boreas-1 and was used to identify prospective intervals and recommend perforation targets. The results demonstrate that SWGRAD can provide a quantitative supporting indicator for gas-zone screening under uncertain formation-water salinity, but its interpretation requires well-specific calibration and integration with conventional petrophysical, geological, and independent well-test information.
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