* Shenandoah field could be double or triple previousestimates
* Find latest in string of potentially huge fields
* Company aims to continue appraisals
* Anadarko shares add 3.8 pct
By Kristen Hays
HOUSTON, March 20 (Reuters) - Analysts lauded AnadarkoPetroleum Corp's latest well results in its Shenandoahfield in the Gulf of Mexico on Wednesday, calling it apotentially huge find among a string of big discoveries thatillustrate the prolific basin's importance to U.S. crude output.
Results of Anadarko's Shenandoah-2 appraisal well, whichtested how much oil could be in the field, showed the reservoircould be two or three times bigger - or more - than thecompany's estimate of more than 300 million barrels of oil,analysts said.
Pressure and log data from the well also show Anadarko couldpotentially recover a higher percentage of oil from theShenandoah field than in other areas of the vast Lower Tertiarytrend, the Gulf's deepest, most challenging and most promisingplay, estimated to hold up to 15 billion barrels of oil.
"The deepwater Gulf of Mexico is witnessing an astonishingrun of discoveries and hydrocarbon augmentation," Simmons & CoInternational said in a note to investors on Wednesday.
"Quite a renaissance from the depths of Macondo," Simmonssaid, referring to BP Plc's doomed well that ruptured in 2010and spewed more than 4 million barrels of oil into the Gulf.
Other notable well results on discoveries in the LowerTertiary since 2010 include Exxon Mobil Corp's Hadrianfield with an estimated 700 million barrels of oil; ChevronCorp's Moccasin field with an estimated 200 millionbarrels of oil equivalent (boe); and Royal Dutch Shell's Appomattox field with an estimated 500 million boe.
BP also estimates that its Mad Dog field could have up to 4billion boe, up from 1.5 billion boe. Construction on a secondoil and gas platform for the field is slated to start by the endof this year.
Such finds show that deepwater exploration and production,where wells can cost $130 million each or more to drill tens ofthousands of feet through thick salt and rock layers, remainscritical to U.S. production, even alongside booming onshore oiloutput in North America, analysts said.
"The opportunity is vast as deepwater reserves currentlycomprise about 10 percent of global oil reserves," Simmons said.
About 29 percent of U.S. crude output came from the Gulf in2009. Last year it had fallen to 23 percent as onshoreproduction from tight oil and shale oil grew substantially.
'MASSIVE'
Anadarko said its Shenandoah-2 well, about 200 miles southof the Louisiana coastline, showed more than 1,000 feet of netoil pay, meaning the drill pierced 1,000 feet of productiveoil-bearing rock without having tapped any water. Exxon'sHadrian North well had a similar result.
"It's massive. One thousand feet's big, it's world class,"said David Pursell, an analyst with Tudor Pickering Holt & Co inHouston who also is a petroleum engineer.
Anadarko's previous Shenandoah discovery well in 2009 showedabout 300 feet of net oil pay, Capital One Southcoast said in anote to investors on Wednesday. Shenandoah-2's results promptedCapital One to increase its estimate of how much oil could be inthe field to 900 million boe from 225 million boe.
"This is an exciting story and far from over as we awaitresults from two other wells," Capital One said.
Pursell also noted that Anadarko said well results show"reservoir and fluid properties" that indicate it could showbetter recovery rates - or more oil will flow - than in otherareas of the Lower Tertiary.
"Basically, they're saying that relative to the LowerTertiary, not only is there a lot of oil here, but thereservoir's pretty good too," Pursell said.
Anadarko anticipates more appraisal drilling "to advancethis potentially giant project," according to Bob Daniels,senior vice president of deepwater and internationalexploration.
Anadarko shares were up $3.14 to $86.42 in late Wednesdaytrading.