The Western Atwater Foldbelt (WAFB) (Figure 1) in the deep water Gulf of Mexico has yielded several large oil fields that have a total resource potential of more than two billion barrels. Oil has accumulated in Middle and Lower Miocene age submarine fan reservoirs draped over large, salt-cored, faulted compressional anticlines and 3-way closures against salt /welds. The WAFB produces from six fields at the rate of about 400,000 BOPD with a cumulative production of more than 300 million barrels of oil. It has developed into an important oil-producing province that contributed about a quarter of the total oil produced in the Gulf of Mexico during 2009 (Figure 2).

Figure 1.

Figure 2.

Early exploration in the WAFB focused on large compressional structures partially visible beneath the southern edge of shallow salt. BHP Billiton and BP formed a partnership leading to the trend-opening discovery on the Neptune structure in 1995 and major discoveries at Atlantis and Mad Dog in 1998. Subsequent discoveries have been made at K2 (1999), Shenzi (2002), Tahiti (2002), Puma (2003), Knotty Head (2005), Pony (2006), Friesian (2006) and Heidelberg (2009). As seismic imaging has continued to improve, exploration has focused on deeper subsalt targets. The WAFB is still actively being explored with four recent exploratory wells and continued leasing activity in the 2010 GOM lease sale. The exploration success rate in the WAFB has been about 60%.

Exploration success in the WAFB has led to an extensive appraisal program to confirm the presence of economic resources. This has proved challenging due to the effect of shallow salt on seismic imaging leading to uncertainties with regard to fault location and density. As a result, multiple appraisal penetrations are required for each structure, with an average of ten per producing field. Operators have had to drill additional appraisal wells after the start of production in most WAFB fields. Appraisal drilling has added significant resources on the north side of Atlantis and the west and south sides of Mad Dog.

Initial production from the WAFB began at the Mad Dog Field in January 2005. Other fields that have come on production include K2 (May 2005), Atlantis (October 2007), Neptune (July 2008), Shenzi (March 2009), and Tahiti (May 2009). Early production data suggest the following:

1. Good initial production rates with about half of the development wells flowing at a rate of more than 15,000 BOPD.
2. It will be a challenge to maintain production rates due to structural complexity, reservoir energy issues, and stratigraphic complexity in some areas.
3. Areas of poor subsalt seismic imaging result in increases in the difficulty of predicting well results. As a result, some production wells have been sidetracked, but fewer than planned.

Development wells are very expensive in the WAFB, so it is important to use lessons learned from early production to optimally locate future wells in order to efficiently drain the remaining resources in each field.