GULF OF MEXICO: Record depth well focusing attention on Atwater Foldbelt

Gulf of Mexico geological targets may shift

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BHP's record well and the surrounding Atwater Foldbelt area.
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The Atwater Foldbelt geological region in the Gulf of Mexico is beginning to get a great deal of attention. BHP (Broken Hill Proprietary) Petroleum's record deep-water wildcat well in 8,835 ft water depth is in the Walker Ridge area of the foldbelt. The Atwater Foldbelt, a series of compressional foldbelts that run across the Atwater Valley, is more than 200 miles long, stretching from the southeastern part of the Green Canyon area through the northwest segment of Walker Ridge. Within the foldbelt, which also includes the Neptune, Atlantis, and Mad Dog fields, water depths reach 11,000 ft quite rapidly.

BHP began drilling the record ultra-deep-water well on October 22 on Walker Ridge Block 425. Walker Ridge runs from 6,000 ft water depths to over 9,000 ft, about 250 miles offshore, just north of Mexico's line of demarcation. The Walker Ridge borders the cutoff between the Central and Western Gulf and is at the southernmost point of US lease territory. This well depth represents the greatest reached offshore GOM.

Early this year, BHP specially commissioned a drillship, the C.R. Luigs, from Global Marine. The state-of-the-art ship incorporates recent technology and safety features to meet the demands such a record water depth requires. BHP has retained the ship through a long-term contract that will enable the company to pursue additional deepwater and ultra-deepwater drilling in the coming year. As operator, BHP owns 70% of this operation, while Total is partnering with the remaining 30%.

Nearby action

BHP is not alone in making a move in ultra-deepwater. Three other companies have put down ultra-deepwater wells in neighboring blocks in the Walker Ridge area: Marathon Oil (Block 165), Texaco (Block 456), and Unocal (Block 678). Of these wells, Marathon Oil's 7,997-ft-deep well drilled in February 1999 ranks second in depth.

Only one ultra-deepwater well has been drilled in the Gulf of Mexico outside the Walker Ridge area. That well, drilled by Shell Oil Co. is in Block 557 of the Alaminos Canyon area and is being drilled in 7,790 ft water depth.

These new ultra-deep wells will yield new geologic information that could change the direction of OCS drilling in the Gulf of Mexico and beyond. According to Minerals Manage-ment Service Director Walt Rosenbusch, ultra-deepwater drilling has the potential to uncover new geologic plays. "The geologic information gathered from these ultra-deep wells will be important new data that can have long-term energy consequences."

US Gulf and Brazil

The move to ultra-deepwater is new in the Gulf of Mexico, but ultra-deepwater drilling has been underway for nearly two years offshore Brazil. Petroleo Brasileiro S/A (Petrobras) has the deepest offshore oil well in the world in water depth of 9,111 ft. in the Roncador field, located in the northeast portion of the Campos Basin offshore Brazil. Reserves there are estimated at more than 3 billion bbl.

The success of exploration wells, improved seismic data acquisition, and interpretation of 3D seismic data in the area spurred Petrobras to develop the field immediately after its discovery. Success in the Roncador field may well be a springboard to additional Petrobras ultra-deepwater drilling in the area because it allows the company to intelligently plan future ultra-deepwater drilling activity.

The experience of using a dynamically positioned FPSO has been very positive, and it is likely that the Seillean, the FPSO in service in the Roncador field, will be used elsewhere for the same purpose when drilling activities are concluded.

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