The West Erregulla gas discovery. Implications for an extensive Permian play fairway across the onshore northern Perth Basin, Western Australia

Abstract

The West Erregulla field is a significant new discovery in the northern Perth Basin that expands the play fairways for the basal Triassic/late Permian sandstones of the Dongara/Wagina formations and early Permian sandstones of the Kingia/High Cliff formations. The 2019 discovery well, West Erregulla-2, targeted three stacked seismic amplitude anomalies interpreted to be gas-charged conventional sandstones at depths between 4100m and 5000m. Gas charge is confirmed in all three units. Gas is hosted in linked, reactivated Permian-aged fault blocks located in the axial part of the Dandaragan Trough. They represent a down-dip analogue to the Waitsia gas field NW of West Erregulla. Only the Kingia sandstone was tested in West Erregulla-2. It contains good to excellent quality reservoir with >55m of pay averaging 12.6% porosity and gas saturations of 65%. Despite deep burial, porosity of the reservoirs was retained by a combination of syndepositional clay coatings and early burial gas charge. Testing of this zone achieved a maximum sustained flow rate of 69mmcf/day. Wireline logs and seismic mapping suggest the presence of a large gas field with gross gas column height of >200m over an area of ~40km2. Scoping volumetric estimates using a range of possible gas water contact (GWC) suggest a P50 in-place original gas in place (OGIP) of ~1182 Bcf for the Kingia formation (informal name). The West Erregulla, Waitsia and Beharra springs deep fields contain significant gas resources. Their spatial distribution suggests the existence of a deep, regional Permian fairway that could cover a large portion of the Perth Basin.