ABSTRACT: Fortescue Field, Gippsland Basin: Flank Potential Realized

J. H. Hendrich, D. A. Schwebel, I. D. Palmer

Fortescue field was the last major oil field to be discovered in the offshore Gippsland basin, southeastern Australia. The discovery well, 1 West Halibut, was drilled in 1978 on the basis of a l-km seismic grid as a follow up to the dry 1 Fortescue wildcat. Data from this well were interpreted to indicate that there was a high probability of a stratigraphic trap occurring on the western flank of the giant Halibut-Cobia structure. The 2, 3, and 4 Fortescue wells were drilled by early 1979 to determine the limits of the field, delineate the stratigraphy, and define the hydrocarbon contacts. Additional seismic data were acquired and interpreted before two 21-conductor platforms, Fortescue "A" and Cobia "A," were commissioned in 1983. Cobia "A" had the dual purpose of develop ng the Cobia field (the southern extension of the Halibut field) and the southern extent of the Fortescue reservoirs that were inaccessible to the Fortescue "A" platform. At the conclusion of development drilling in early 1986, eight Cobia "A" wells and 20 Fortescue "A" wells were capable of producing from Fortescue reservoirs.

The Fortescue reservoirs are Eocene sandstones that were deposited in coastal plain, upper shoreface, and lower shoreface environments. Integration of well log correlations, stratigraphic interpretations, reservoir pressure data, and seismic data indicates that these Fortescue reservoirs are stratigraphically younger than, and are hydraulically separated from, the underlying Halibut-Cobia fields. Pressure data acquired during development drilling and while monitoring subsequent production performance have conclusively demonstrated that there are at least three separate hydraulic systems active within the Fortescue field.

Fortescue field dimensions are approximately 11 km × 4 km with a maximum relief of 100 m above the original oil-water contact. Reserves are estimated at 280,000 STB, based on original oil in place estimates of 415,000 STB and recovery factors in the 65-70% range. Production rate peaked in 1984 at 100 K BOPD from the combined development facilities and was sustained until late 1986. More than two-thirds of the recoverable reserves have been produced to date.

AAPG Search and Discovery Article #91000©1990 AAPG Conference-Giant Oil and Gas Fields of the Decade 1978-1988 Conference, Stavanger, Norway, September 9-12, 1990