Unlocking Atlantis: An Integrated Approach to Reservoir Management of a Giant Subsalt Field
Authors: Bartlett Rick M. , Singh Pramod K. , Howie John M. , Humphrey Kathryn , Kurtz Jessica A. , Croft Mike D. 
Published: July 2010

Abstract
Atlantis is a giant oil field with challenges that require a multi-disciplinary and phased approach to reservoir development. Initial development focused on the better-imaged extra-salt segment of the structure after eight appraisal penetrations revealed over a 1,500 ft oil column contained in several high quality thick pay sands. Signals of the reservoir complexity that would be associated with Atlantis were seen from the beginning. The early wells found variable fluid contacts, fluid compositional variations, large previously undetected faults, and what turned out to be perched water. Development drilling continued while the facilities were being fabricated. The drilling results strengthened confidence that there were large in-place resources but heightened concerns about complexity. In response, the development plan was modified to include ocean-bottom-seismic, downhole-flow-control completions, and a second production drill center. Initial reservoir performance revealed sub-seismic baffles that resulted in lower stabilized well productivity and more complex compartmentalization than expected. Integrated analyses of static and dynamic data enabled the integrated team to demonstrate that recovery per well was similar to pre-drill expectation and modification to the development plan would allow sustained production growth. Challenges going forward include appraisal and development of remaining segments of the field to effectively grow production and extend production plateau, and efficient operation of the wells, subsea architecture and production facilities.

Introduction
This paper focuses on the subsurface challenges involved in appraising and developing Atlantis, a giant subsalt deepwater oil field in the Gulf of Mexico. Significant uncertainty about the resource in less appraised sectors, the level of aquifer support, and root cause of the observed static complexities indicated the development scheme required flexibility.
A phased approach focused initially on development of the extra salt area of the reservoir while providing the flexibility to develop the subsalt sector and expand the topside capacity based on the actual reservoir outcome.
The results to date indicate this flexible phased approach was an efficient way to manage uncertainty while realizing value from a large resource base. Even after two years of production and continuous rig activity, there are sectors requiring further appraisal and significant uncertainty still remains. This paper will touch on how the learnings from available data were used to make development decisions while still retaining some flexibility to deal with remaining uncertainty.

Background
The Atlantis Field, discovered in 1998, is located approximately 190 miles south of New Orleans in the Gulf of Mexico (Figure 1). Atlantis is one of several deepwater hydrocarbon discoveries located in the southern Green Canyon protraction area. The Atlantis hydrocarbon accumulation is contained within a large salt cored anticlinal fold. Atlantis is situated beneath the Sigsbee Escarpment, a major bathymetric feature associated with the overlying edge of allochthonous salt as it approaches the seafloor. Complex sea-floor topography plunges from 4,500 ft to 7,000 ft water depth at slopes up to 30 deg. Two thirds of the Atlantis structure is located subsalt, and is therefore seismically obscured. Several Miocene hydrocarbon bearing reservoirs are present over the structure at depths of 16,000 to 19,000 ft subsea (Figure 2). The main subsea drill center for Atlantis is positioned at the base of the Sigsbee Escarpment.