Stephen P.J. Cossey
Cossey & Associates Inc.
July 2005 will mark the 30th anniversary of the first field discovered in the deepwater (greater than 600 ft) Gulf of Mexico. This province is one of the most prolific petroleum areas in the world and has been active since the mid 1970s. A look back at the exploration and development achievements in the deepwater Gulf of Mexico from 1975 to 2003 can help define the future of the Gulf.
The Flex Trend play, the Mini-Basin play, and the Fold Belt play are three major exploration trends in the deepwater GoM. The Flex Trend play began in the early 1970s with Shell's discovery of the Cognac field. Development of this area reaches just beyond the present-day shelf edge, where there is a flex in the sea floor profile. Most discoveries in the play were fields with small reserves, discontinuous sands, and wells with fairly low flow rates.
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The Mini-Basin play began in 1983 when Shell discovered Bullwinkle. This play targeted the flanks of structural intraslope basins where reservoir sands pinched out and formed combination structural/stratigraphic traps. The first mini-basin fields online – Bullwinkle in 1989 and Auger in 1994 – produced at much higher than expected rates, had better than expected aquifer support, and needed fewer wells to develop them.
The Fold Belt play began in 1995 when BHP Billiton discovered Neptune in Atwater Valley block 575 in the Mississippi Fan fold belt, one of at least three fold belt trends. Shell made the first Perdido fold belt discovery in 1996 with BAHA, and there has not been a discovery in the Port Isabel fold belt.
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Approximately 21.2 Bboe have been discovered in the deepwater Gulf of Mexico. Most reserves were added in 2001 when 32 fields were discovered.
Mars, discovered in 1989, has 750 MMboe, and was often thought to be the largest field in the province until BP discovered Thunder Horse with 1,000 MMboe, in 1999. The sizes of these fields suggest that it is possible to find more fields in the 750-1,000 MMboe range.
Cognac started production in 1979 and the first gas field followed in 1980 at Garden Banks block 236. In 1984, almost 10 years after Shell discovered Cognac, Lena, the third field, started production.
Initial per-well production rates from the early producing fields were very disappointing. Peak well rates at Lena were only 500 to 2,200 b/d. It was clear from these early producing fields that higher per-well flow rates had to be achieved if the deepwater province was to be economically viable.
Fields that started production in the late 1980s achieved slightly higher per well rates. The Green Canyon block 18 A-8 well flowed at 7,400 b/d during January 1989, but that rate was short-lived. Bullwinkle wells achieved 3,000-6,000 b/d, but these rates were reached years after the startup of the field.
The Auger field achieved higher per well rates, which came online in 1994, but it was not until 1995 that the first well at Auger achieved a rate over 10,000 b/d. It was then realized that high per-well rates from the deepwater province were possible. Three years later, well rates at Troika and Ram Powell exceeded 20,000 b/d. Virtually no testing was done prior to field sanction and development, so potential flow rates were not known until fields came online. The Mars discovery was developed without any flow test being run in any of the reservoirs.
At the end of 2003, 136 fields were producing in the deepwater GoM and 16 of those started production that year.
Past, present, and future
Over 21.2 Bboe have been discovered in 232 fields in the deepwater Gulf of Mexico from 1975 to the end of 2003. Of the 44 operators in the deepwater GoM, Shell is the most successful with 52 discoveries. The 1995 Royalty Relief Act gave a boost to the deepwater GoM, as the number of operators, fields discovered, and reserves added all increased. Reserve additions continue to climb, but the reserve additions show a slight tendency to flatten this year.
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