Anadarko find, technology gains renew subsalt hopes

G. Alan Petzet
Exploration Editor

Anadarko gulf subsalt prospects [173,107 bytes]

Subsalt exploration ambitions in the Gulf of Mexico got a shot in the arm from the recent Tanzanite discovery on Eugene Island Block 346.

The operator, Anadarko Petroleum Corp., Houston, said 13 of industry's 43 Gulf of Mexico wells deliberately targeted to prospects below salt have found oil and gas. The company believes that eight are clearly commercial and that Tanzanite is the ninth.

Three commercial subsalt fields have started production: Mahogany, Enchilada, and Agate.

The main players with subsalt exploration interests are Shell Offshore, Texaco, Exxon, Marathon, BP, Amerada Hess, and Phillips. Phillips said it has drilled the majority of its subsalt portfolio on the continental shelf and is using technologies gained there to expand its deepwater program.

Anadarko holds 41 lease blocks that contain 20 subsalt prospects in shallow gulf waters. The 20 prospects are in various stages of geophysical evaluation, and the company plans to drill several more next year. Some have already been bypassed because they did not "shoot out" (survive seismic processing).

Anadarko spent about a year in 1996-97 working up its best two prospects and became determined to drill Tanzanite and Hickory. It has two partners in Hickory on Grand Isle Block 116, but 28 declined to join in drilling Tanzanite, reflecting individual companies' prioritization of subsalt prospect risk. They include Phillips, which initially acquired blocks 345 and 346 with Anadarko 50-50 and retained only a 4% overriding royalty interest in 345.

Seismic evolution

Anadarko acquired some subsalt leases in 1992-94 based on its interpretation of 2D seismic data. It tried initially to depth migrate the 2D data.

"We have been through as many as five iterations of seismic from different vendors to try to crack the code of seeing through salt," an Anadarko official said. He traced the attempts for one prospect:

A 3D prestack depth migration with one seismic contractor, a 3D poststack depth migration with another seismic company, and a larger 3D poststack depth migration with a third seismic firm. Anadarko still feels it needs to acquire more seismic data in that prospect area.

Seismic images show the salt bodies to have highly irregular surfaces. Portions of the design parameters of the 3D surveys remain proprietary to Anadarko. Long offsets are vital so that streamers pick up energy that steeply dipping beds reflect long lateral distances from the feature being shot.

Defining the top of salt has proved challenging and time consuming (OGJ, July 14, 1997, p. 19).

"Several years ago we thought 1 month on a supercomputer was a long processing time. Now it can be 9 months" to complete a prestack depth migration on a 9 mile square block.

Anadarko drilled Tanzanite as a gas prospect. Its 3D seismic data affords a great view of the gas amplitude but does not reveal the underlying oil leg or the oil leg's extent below salt.

Subsalt production

The Phillips operated Mahogany field, with about 100 million BOE of reserves by industry estimates, began producing in December 1996.

An eighth well is to be drilled shortly. A7, the seventh producing well, came on a few weeks ago. Three of the other six wells are producing from the main field pay sand, called the P sand.

The other three produce from thinner zones above and below the main field pay, and the P sand has not been opened up yet in those wells.

The A5 well has nearly depleted in the thin zone and will soon be recompleted in P sand.

Mahogany output averaged 15,000 b/d of oil and 35 MMcfd of gas in early August.

The Mahogany structure is a four way dip closure that more than likely would have been found 20 years ago except for the lack of seismic technology to image prospects beneath salt. Geologically the producing Miocene/Pliocene sands that are the targets of subsalt exploration are similar to those found in the deepwater play.

Phillips and Anadarko on July 26 started up Agate field, two blocks west of and producing into Mahogany platform. Flow is 25 MMcfd of gas and 2,500 b/d of condensate from one subsea completion.

Agate field size is roughly one fifth that of Mahogany. Agate's condensate provides a blending advantage for Mahogany's heavier crude.

Mahogany is a 20 slot platform with capacity of 45,000 b/d of oil and 100 MMcfd of gas.

Shell expects Enchilada field production to peak ultimately at 200 MMcfd of gas and 10,000 b/d of condensate from two platforms. First production came in early June 1998 from one subsalt well in which Amerada Hess and Pennzoil also hold stakes.

About half of Enchilada's reserves are below salt. The field is on parts of Garden Banks blocks 83, 84, 127, 128, and 172. Enchilada platform has processing capacity of 400 MMcfd of gas and 60,000 b/d of liquids and is expected to serve Enchilada and other nearby developments.

Tanzanite discovery

Anadarko calls Tanzanite an "extraordinary sand body" with more than 450 ft of continuous hydrocarbon pay. Log and pressure data indicate the field contains at least 140 million BOE of reserves. The feature has an areal extent of about 1,000 acres. Above it are three other zones totaling about 70 ft of net pay. Gross sand thickness is nearly 800 ft.

Anadarko cored near the oil-water contact and also took sidewall cores that show better than average sand quality. TD is 14,350 ft.

Porosity is above 30% in the main reservoir. Oil gravity is in the low to mid 20s, similar to Mahogany. Some pressures are higher than at Mahogany.

Flow tests are being planned and carried out during August-September. Anadarko obtained a second jack-up to conduct the tests. The Rowan Odessa will then drill a delineation well. Water depth is 314 ft.

Further drilling is needed to determine the areal extent of the four pay zones. Drilling will resume in 6 weeks and continue during platform construction. The discovery well, being saved for future production, is near the center of the northwest quarter of Block 346, and part of the field extends north onto Block 345.

Gas dominates oil slightly in the reservoir as presently understood. Development will probably involve producing the oil first, with first oil in late 2000. Pipelines are common near the field.

Drilling advances

Anadarko should have results from Hickory prospect drilling in September.

The Hickory well, projected to about 16,500 ft, was spudded Apr. 9. It is to evaluate two targets in a large rollover structure below salt.

Drilling has gone almost entirely according to prognosis on both Tanzanite and Hickory.

Tanzanite costs were $14 million to top of the pay and $3 million more to TD. Including tests, cost will total about $20 million.

Industry subsalt drilling misconceptions were the existence of a 200-1,000 ft zone of overpressured gumbo rock immediately below salt and the need for drillers to speed up rotation and weight up mud to drill the zone.

The reality is that the subsalt zone is underpressured, brittle rock that can be penetrated successfully with slower rotation and mud weight in balance.

At Hickory, Anadarko and partners have drilled through 8,000 ft of salt and cemented double casing strings from about 50 ft above to 500 ft below the salt. Water depth is 320 ft.

The only problem encountered at Tanzanite was an "unintentional sidetrack" that occurred in a salt withdrawal zone just below 11,000 ft. Nevertheless, the company believes it can drill a subsalt well to TD 16,500 ft for $15 million with a mobile rig and about $5 million with a platform rig.

Matagorda 622/623

Anadarko said its discoveries off Louisiana and Texas show that giant fields with reserves of 100 million bbl of oil equivalent or more can still be found in the U.S. Other Gulf of Mexico discoveries provide further evidence of this.

Anadarko compared Tanzanite's size to another giant field it discovered off Texas in 1980.

Though not related to the subsalt activity, that field, Amoco operated Matagorda Island 622/623, has estimated ultimate recovery of about 1.3 tcf of gas. Cumulative production is about 800 bcf of gas and 8 million bbl of condensate.

The field, which started up in 1984, is producing about 300 MMcfd through six platforms and two single well jackets in about 70 ft of water. It is one of the largest fields in the shallow Gulf of Mexico.

A pipeline looping project under construction will soon afford capacity expansion to 400 MMcfd from 350 MMcfd. Recent wells have flowed 60-70 MMcfd plus condensate with 10,000 psi FTP from Siphonina davisi below 10,000 ft.

Main interest holders are Amoco and Anadarko 37.5% each and Union Pacific Resources 19%.