Pluto—a major gas field hidden beneath the continental slope

Abstract

The Pluto gas field, North West Shelf of Australia, was discovered in April 2005. It is located in production licence area WA-34-L some 190 km northwest of Karratha, and is situated beneath the continental slope in water depths of 400–1,000 m. The seabed topography initially hindered the recognition of this field. The presence of large seabed channels and steep dips in the overburden, together with the variable water depth, result in seismic ray-bending effects which reduce the quality of imaging and attenuate amplitudes over the southern and eastern parts of the field. Consequently the depth conversion of seismic data over the Pluto structure has been a key uncertainty for the field definition. Structurally, the trap is a tilted fault block, bounded on the west and north by major bounding faults, and sealed by overlying regionally extensive shales. The Triassic reservoir sequence dips gently to the east, and subcrops against the regional Jurassic Unconformity. Pluto–1 encountered a gross gas column of around 209 m in Triassic sands of the Mungaroo Formation and Tithonian sands, sealed by Cretaceous shales of the Forestier and Muderong formations. Petrophysical analyses of the conventional wireline dataset confirmed an average net porosity of 28% and average gas saturation of 93% for the Mungaroo Formation (Lower E Unit). Production tests proved the high deliverability of the Mungaroo E Unit (46.5 MMscf/d) but showed the Tithonian section to be of poor deliverability (9.5 MMscf/d with possible in-wellbore leakage from the deeper Mungaroo DST). Since the discovery, five appraisal wells (excluding sidetracks) have been drilled to delineate the accumulation, and to target areas of higher quality sand development for optimisation of development well locations. In addition, the Pluto 3D data has been twice processed to a pre-stack depth migration—once, immediately following the Pluto–1 discovery to aid in the appraisal campaign and then again following final investment decision (FID) to take advantage of new and improved techniques for seismic processing which has led to increased confidence in the proposed development well locations. The Xena gas field, a satellite field adjacent to Pluto, was discovered by Xena-1ST1 in September 2006. Since then a further two appraisal wells have been drilled to delineate the structure and define the accumulation. The Triassic reservoir section has been extensively cored, logged and analysed in detail for reservoir characterisation and correlation. The reservoir is composed of thickly developed, amalgamated fluvial multi-valley sandstones of the Mungaroo Formation, and coastal plain sediments (tidal bars and channels, estuarine beach deposits) of the more tidally influenced Brigadier Formation.