THE JOHN BROOKES GAS DISCOVERY—AN EVOLVING STORY

Abstract

John Brookes–1 was drilled as part of a work commitment for the WA-214-P Joint Venture in 1998 and discovered an 85 m gross dry gas column. The objective of the well was to test a structural closure at the base of the Muderong Shale regional seal on the Tryal Rocks anticline, up-dip from Tryal Rocks–1, drilled in 1970. Tryal Rocks–1, the 31st offshore well to be drilled within the Carnarvon Basin, WA, was initially considered a dry hole. However, a review of the well data in 1997–98 suggested that Tryal Rocks–1 might contain a hydrocarbon column. The mapping of the structure using initially 2D seismic data acquired post Tryal Rocks–1, then 3D data, indicated that Tryal Rocks–1 was drilled within closure, but off crest, and that significant closure existed up-dip. The John Brookes–1 location was selected to test this updip potential. The John Brookes–1 discovery confirmed the validity of the structural mapping. However, the unexpected nature of the reservoir, interpreted as a well developed turbiditic channel of Birdrong Sandstone age, changed the emphasis from purely structural to a play with structural/stratigraphic potential. An amalgamated turbidite complex model was invoked which infers that the John Brookes–1 reservoir represents a confined channel system cut into the underlying substrate. This model explains the results to date, with the John Brookes–1 gas reservoir being in direct continuity with the sandstones at Tryal Rocks–1. A review of the 3D seismic data across the field and seismic modelling support the stratigraphic model developed from the palynological interpretation.