Publication Name: The Sedimentary Basins of WA
Authors: D.J. Hearty & M.A. Battrick
Publication Volume: 3
Date Published: December 2002
Number of Pages: 43
Reference Type: Book Section
Abstract:
The Woollybutt Oil Field is a small, under-saturated oil pool, located within the offshore Barrow Sub-basin of theNorthern Carnarvon Basin. It is approximately 40-50 km from the nearest existing production facilities and over 80 km off the coast of Western Australia. The maiu oil-filled, Barrow Group reservoir is trapped within an un faulted anticline on the West Barrow Terrace, with the greatest gross oil column intersected being 16m. The reservoir in the field consists of deltaic to shallow-marine sandstones at the top of the Valanginian/Berriasian-aged Barrow Group.
West Barrow 1A intersected the field in 1982 while drilling for deeper Lower Barrow objectives, hut was not tested due to operational problems. Hydrocarbon shows were recorded at the top of the Barrow Group and
subsequent log interpretations by other companies suggested that a hydrocarbon column was present at this
level, ranging in height from 8-14 m. It was not until 1997 that the field was confirmed when Woollybutt 1 penetrated the top of the Barrow Group reservoir, 2 m up-dip and 2 km south-southwest ofWest Barrow 1A.
The present field interpretation is based on a reprocessed multi-client 3D seismic survey and a 3D velocity field derived from Pre-Stack Depth Migration (PSDM) processing. Depth conversion was completed using 3 layers and a combination of PSDM velocities and the mid-point depth technique. The best estimate depth map shows that the Woollybutt Oil Field is separated into a north lobe and a southwest lobe, with the spill point for both lobes being to the south.
Reservoir studies have defined the top-most section of the Flacourt Formation of the Barrow Group in the
Woollybutt Field to he a predomiuandy sand-prone, tidal, estuarine, valley-fill sequence within a broad incised-valley. This sequence was deposited in response to a fourth or fifth order sea level fall. Chaunel, sand bar and hay-fill fades are interpreted, each haviug its own distinctive lithology and definitive rock properties. The estuary was eventually drowned by the encroaching ocean during the ensuiug second-order flooding event and was covered by thin, transgressive, offshore marine, glauconitic sandstones of the 'basal' Mardie Greensand Member of the Muderong Shale. This layer is highly porous hut has poor permeability and may he a distal equivalent of the
Birdrong Sandstone. The top of the 'basal' Mardie Greensand Member represents the final hack stepping of
the deep marine environment and defines the boundary between reservoir and seal.