--> Palaeo-Gas Discovery in Pinjarra-1, South Perth Basin, Evidence From the Fluid Inclusion Record

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Palaeo-Gas Discovery in Pinjarra-1, South Perth Basin, Evidence From the Fluid Inclusion Record

Abstract

Pinjarra-1 is an exploration well drilled in 1966 by the West Australian Petroleum Pty. Ltd. in the central part of the onshore Perth Basin. The location of the well was chosen at the crest of the Pinjarra structure to evaluate the hydrocarbon potential but no oil or gas shows were identified. During an investigation of the Upper Triassic Lesueur Sandstone, palaeo-formation water trapped in fluid inclusions revealed a significant amount of methane. Water salinity derivation and quantification of dissolved methane using Raman micro-spectroscopy combined with fluid modelling of the system CH4-H2O-NaCl indicated methane concentrations higher than gas-saturation for the current pressure and temperature (PT) conditions. Those results indicate (1) the existence of a palaeo-gas accumulation, and (2) high palaeo-pore pressures. The high palaeo-pore pressures responsible for high methane concentrations in fluid inclusions likely originate from deeper burial than current day. Increased burial depth is also accountable for the higher palaeo-temperatures recorded by the fluid inclusions, and supported by Vitrinite Reflectance data in the area. Assuming an unchanged geothermal gradient, erosion of 1800 m is calculated from the fluid inclusion data, which is comparable to the 2250 m erosion calculated in the Northern Perth Basin at Walyering 1 (Crostella and Backhouse, 2000). A free gas accumulation was likely present in the Lesueur sandstone at Pinjarra-1 before uplift and, assuming a stable geothermal gradient, overpressure can be inferred at the time when gas was present in the sandstone. The palaeo-pressure and temperature, salinity and overpressure data recorded by the fluid inclusions show a decreasing trend consistent with gas escape potentially related to the uplift. The pressure and gas releases are consistent with petrographic compaction features (abundant concavo-convex grain contacts, stylolites) observed in the sandstone. The salinity of the associated water changed from about 10,000 ppm at the time the sandstone was gas-charged to fresh water at shallower conditions. The absence of gas shows during drilling of the well either indicates a complete loss of the palaeo-gas accumulation, or a lack of gas mobility in tight sandstone. This discovery contributes to a perception change of the hydrocarbon potential of the Triassic in the central part of the onshore Perth Basin.