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Stones discovery looking better Shell, partners say more drilling planned to gauge ‘ultra-deepwater’ field’s size, commerciality, in lower tertiary GOM trend Ray Tyson Petroleum News Houston Correspondent
Shell and partners BP and Marathon Oil, with information gleaned from their announced Stones discovery, appear to have further confirmed the vast reach and magnitude of the lower tertiary trend, one of the hottest plays in the “ultra-deepwater” Gulf of Mexico.
The seven-block Stones prospect in Walker Ridge, drilled to a true vertical depth of 28,560 feet, including 9,576 of water, “confirms that hydrocarbons were encountered,” Shell said May 23.
However, Shell provided scant detail on the Walker Ridge block 508 discovery, saying only that “further drilling is necessary to determine the extent and commerciality of the discovery.”
The Stones discovery well was drilled to roughly the same depth as other lower tertiary wells in the region.
Stones happens to be located between two significant lower tertiary discoveries at St. Malo and Cascade, along a geological trend thought to extend some 140 miles from Walker Ridge southwest into Keathley Canyon. Other lower tertiary finds along the trend include Jack on Walker Ridge 759 and Chinook on Walker Ridge 468.
Shell holds a 26 percent working interest in the Stones prospect, while BP operates the prospect with a 59 percent stake. Marathon has a 15 percent interest.
The Horizon, owned by deepwater contractor Transocean, began drilling operations at Stones on Dec. 31 and reached its final depth on March 10. The prospect includes Walker Ridge blocks 507, 508, 509, 551, 552, 553, 596 and 597.
Stones is roughly 20 miles southwest of Cascade and about a dozen miles southwest of St. Malo, thus far the largest announced discovery along the lower tertiary trend in Walker Ridge. The Unocal-operated St. Malo discovery well logged a robust 450 feet of net oil pay, while appraisal drilling encountered 400 feet of net. That places St. Malo among the largest discoveries in the deepwater Gulf based on pay thickness alone.
The St. Malo discovery well, located in 7,032 feet of water and drilled to a total depth of 28,903 feet, re-entered and deepened the Dana Point dry hole drilled in 2001 that stopped roughly 2,000 feet shy of the prize. Aside from expanding the discovery, the well provided further evidence of the enormous potential and extent of the deeper Eocene and Paleocene horizons of the lower tertiary trend.
The Chevron-operated Jack discovery, drilled to a depth of 29,000 feet in nearly 7,000 feet of water, is 20 miles southwest of St. Malo and was drilled to help establish the boundaries of the vast lower tertiary trend. Instead, the well ended up penetrating a respectable 350 feet of net oil pay and could be included in future development. Cascade discovery got ball rolling However, it was the 2002 Cascade discovery on Walker Ridge Block 206, located more than 30 miles northeast of St. Malo and over 50 miles northeast of Jack, which got the exploration ball rolling on the potential of the lower tertiary play. Cascade uncovered an “encouraging hydrocarbon bearing column,” according to operator BHP Billiton.
BHP’s 2003 Chinook discovery, also a geologically deep find, is about 19 miles south of Cascade and about 10 miles west of Stones. The well exceeded a total depth of 27,650 feet and was said to have encountered a gross hydrocarbon column of 620 feet with 260 feet of net oil pay.
In an effort to establish the western boundary of the lower tertiary trend, Unocal last year drilled a wildcat well on Keathley Canyon block 681, more than 100 miles from major lower tertiary discoveries in Walker Ridge. Although officially declared a dry hole, Sardinia did encounter more than 1,100 feet of porous sandstones, an encouraging sign because of its thickness and the fact it contained some hydrocarbons, according to Unocal.
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