Originally discovered in 1993, the Stag oil field, on the North West Shelf of Western Australia, has proven to be both a prolific and complicated reservoir since production began in 1998. To date, more than 56 million barrels have been produced, in addition to significant volumes of water and glauconite, which has resulted in complex changes to the reservoir over time as the field has produced. Because the reservoir is shallow, at 680 m true vertical depth subsea (TVDSS) with shallow carbonates causing noise issues and imaging challenges, Stag Field remained largely invisible on seismic data. To address these issues, a 220-[Formula: see text] 4C-3D ocean-bottom cable survey (OBC) was acquired in early 2008. Subsequent processing and interpretation of these data, including a joint multicomponent inversion, have resulted in drilling six successful new wells with additional wells currently underway and planned in the near future.

The Halyard—1 exploration well discovered gas in a new and separate reservoir unit at the top of the Barrow Group along the outer margin of the Barrow Delta. The well is located in WA—13—L, approximately 15 km northwest of the East Spar Field and 4 km east-northeast of the Spar—1 well. It was drilled in February 2008 and intersected 27.8 m of net gas pay in a Valanginian age Barrow Group reservoir interpreted to be a wave-dominated shoreface sandstone. The nearby Spar—1 well was drilled in 1976 and intersected 18.4 m of net gas pay in an older Barrow Group Sandstone within a distinctly separate, delta-front turbidite setting. The Halyard and Spar fields are located within the overall confines of a structural closure at the Top Barrow Formation level. Earlier reservoir models dating back to the 1970s assumed connectivity between all permeable sandstones within the Top Barrow closure. Critical analysis and integration of all subsurface and engineering data sets inclusive of modular formation dynamics tester (MDT) pressures, condensate gas-ratios, production test data, post-drill amplitude variation with offset (AVO) modelling and seismic facies mapping, however, indicate these reservoirs are distinct and separate stratigraphic traps formed by successive, sequentially sealed depositional units within a larger delta slope edge sequence. Collection of this data has yielded considerable insight into what historically have been viewed as simple traps. In our view and experience, the Barrow Group geology is much more complex with many traps typified by critical stratigraphic components in their trapping mechanism. The Halyard discovery is an example of the type of trap we expect to host much of the remaining reserve potential in the proximal Barrow Delta.

Located on the North West Shelf of Western Australia, the Stag Oil field has proven to be a prolific reservoir, having produced more than 55 million barrels (MMbbls) of oil since 1998. This has not been without its challenges, however; with premature water breakthrough from injection wells occuring in several wells, potentially stranding large volumes of oil in the ground. Using the multicomponent processing and joint amplitude-versus-offset (AVO) inversion of an ocean bottom cable (OBC) seismic survey acquired in late 2007, new light has been shed on the distribution of unswept oil. This data has led to the succesful drilling of six wells and a marked increase in field production. Additionally, the seismic data has also been used to minimise drilling risks by using seismic coherency to steer the well around potential problems with a significant impact on well costs due to reduction of wellbore problems associated with horizontal drilling in the Muderong shale. To date, four wells have been drilled using this technique, resulting in a significant decrease in non-productive time while drilling during the most recent drilling campaign, which has a significant impact upon the profitability of these late-stage development wells.

The John Brookes gas field was discovered by the drilling of John Brookes–1 in October 1998 and appraisal drilling was completed in 2003. The field is located about 40 km northwest of Barrow Island on the North West Shelf, offshore West Australia. The John Brookes structure is a large (>90 km2) anticline with >100 m closure mapped at the base of the regional seal. Recoverable sales gas in the John Brookes reservoir is about 1 Tcf.Joint venture approval to fast track the development was gained in January 2004 with a target of first gas production in June 2005. The short development time frame required parallel workflows and use of a flexible/low cost development approach proven by Apache in the area.The John Brookes development is sized for off-take rates up to 240 TJ/d of sales gas with the development costing A$229 million. The initial development will consist of three production wells tied into an unmanned, minimal facility wellhead platform. The platform will be connected to the existing East Spar gas processing facilities on Varanus Island by an 18-inch multi-phase trunkline. Increasing the output of the existing East Spar facility and installation of a new gas sweetening facility are required. From Varanus Island, the gas will be exported to the mainland by existing sales gas pipelines. Condensate will be exported from Varanus Island by tanker.