THE MUTINEER COMPLEX AND EXETER OIL DISCOVERIES: MUTINY IN THE DAMPIER

Abstract

The oil discoveries at Norfolk–1 and Exeter–1 in the Northern Dampier Sub-basin (permit WA-191-P) have realised major commercial potential in an area with a prolonged exploration history. This paper presents the results of the drilling campaign which was undertaken in 2002 comprising two successful exploration discovery and five appraisal wells. The Norfolk–1 (28 m gross oil column), Norfolk–2 (9 m oil column), Exeter–1 (23 m oil column), Mutineer–3 (8 m oil column) and finally Exeter–2 (12 m oil column) confirmed a significant commercial oil volume within the Jurassic Angel Sandstones.

The recent exploration of this area has improved understanding of the geology through the integration of technology with geoscientific understanding. Highs have been revealed from seismic time data using advanced 3D seismic techniques and sophisticated depth conversion processes.

The time structural high of the Mutineer area was first tested by Bounty–1 in 1983 which is now mapped outside the southern limit of closure. Pitcairn–1 was drilled in 1997 discovering three thin oil columns and was followed by a near crestal well at Mutineer–1B drilled 2.6 km northwest of Pitcairn–1 in 1998, discovering an 8m column. The key issue was the understanding of the velocity gradient and depth conversion over the Mutineer Complex which revealed the true structural picture.

This paper summarises results of the exploration and appraisal wells drilled and describes the evolution of the structural/stratigraphic understanding of the area, covering critical components hindering the oil field’s early detection. The first component is a significant seismic velocity gradient which causes true structural closure to be significantly offset from the time closure. The second component is the reservoir pressures within the oil reservoir and older sandstone intervals within the Angel and Legendre Sandstones show differences due to hydrodynamic cells and/or depletion resulting from production from the adjacent NWS Venture oil fields. The final component is the oil is primarily reservoired in the top Angel Sandstone, belonging to the J40 sequence and is sealed by a thin shale from the underlying mainly water bearing sandstones (J35/J30 and Legendre Sandstones).

The combined reserves for the Mutineer Complex and Exeter Oil Fields reservoired in these laterally continuous turbidites are estimated to be 70–160 MMBBL recoverable.