THE PERSEUS FIELD, NORTH WEST SHELF, AUSTRALIA: A GIANT GAS ACCUMULATION IN A COMPLEX STRUCTURAL/STRATIGRAPHIC TRAP

Abstract

The Perseus Field contains reported expectation (proven plus probable) dry gas and condensate reserves of 6.48 TCF (183 × 109 m3) and 165.7 MMBBL (26.3 × 106 m3), respectively, in a complex structural/stratigraphic trap. Gas is predominantly reservoired in Bathonian shallow marine sandstones of the Legendre Formation which subcrop the Upper Jurassic-Lower Cretaceous 'Main Unconformity' in a graben separating the Goodwyn and North Rankin horsts. The Lower Cretaceous Muderong Shale forms the main regional top seal. The Lower-Middle Jurassic Athol Formation forms the seat seal for the Perseus trap and subcrops the Main Unconformity northwest of North Rankin. The maximum gross gas column for the field is approximately 360 m. The Athol Formation and underlying Murat Siltstone also form the topseal for the small Searipple Field, which underlies Perseus.

The Perseus trap is dip-closed to the northwest. A steep fault scarp forms the southern margin of the trap. Trap integrity is dependent upon fault seal along the western flank of the field, where the reservoir section is juxtaposed against Athol Formation claystones in a long narrow fault block downthrown from the Goodwyn block. At the southwestern corner of the field this fault block is absent and the Perseus reservoir is juxtaposed against Triassic reservoir section in the Goodwyn block.

The NRA22 well, drilled from the North Rankin A platform, has been producing gas from the Perseus Field since mid-1991. Reservoir pressure measurements and production history data have been of immense value in the exploration and appraisal of this field, both in driving further drilling and in understanding the results. Integration of geoscientific and engineering data and expertise within a multi-disciplinary team was essential for the efficient appraisal and evaluation of the field.