Over the last 15 years, several giant petroleum accumulations have been discovered in the Mississippi Fan foldbelt, deepwater Gulf of Mexico. In the mid 1980s, initial work was driven by interest in the nature of sections at the edge of, and immediately inboard of, the Sigsbee Escarpment. Outboard and counter-regional dip beneath an allochthonous salt canopy was recognized in early speculative 2D seismic transects.

In the first area-wide sales in 1984 and 1985, all but the exclusively subsalt features of the foldbelt were leased. Progress in assessing the foldbelt slowed with the oil price collapse of 1986. In 1987, Shell drilled the first foldbelt structure in Atwater Valley Block 471. The well mitigated the chief concern, a lack of reservoir, but dampened hopes due to a lack of petroleum. By mid-1994 only four of the trend's features remained leased.

In 1993, BP's regional work suggested that the potential of these very large structures merited another look. In addition to Neptune, which appeared to be of substantial size, several other features were leased, including Atlantis and Mad Dog.

Rumors of the 1995 Neptune discovery re-ignited interest in the area. Pre-merger Amoco had established a position in the very competitive 1996 and 1997 sales and further leveraged into the trend in 1998 by bringing a rig to expiring acreage. In 1998, three wells were drilled in the trend. The Neptune appraisal well was drilled in Atwater Block 574, the Atlantis discovery was drilled in Green Canyon Block 699 and the Mad Dog discovery was drilled in Green Canyon Block 826.

Seismic imaging was key in exploring the area. A combination of shallow salt sheets, significant seabed topography, and steep dip panels within the folds themselves created a significant imaging challenge. Integrated teams seismic contractor and oil company personnel addressed the issue aggressively, working with both proprietary 3D post-stack and pre-stack depth seismic volumes for the initial discovery at Neptune. Subsequent studies have utilized proprietary 3D seismic volumes.

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