Exploration Discoveries fuel Gulf of Mexico subsalt play

A.D. Koen
Senior Editor-News

A Sample of Significant Subsalt Wildcats [182926 bytes]

The 1990s are shaping up as a decade of discovery for subsalt exploration in the Gulf of Mexico as evidence mounts of the play's high potential.

Gulf operators early this month were drilling five subsalt wildcats. Phillips Petroleum Co. and partners were drilling an appraisal well in Mahogany field, the gulf's first commercial subsalt discovery (OGJ, Oct. 11, 1995, p. 30). Groups led by Shell Offshore Inc. and Texaco U.S.A. were evaluating data from Enchilada and Gemini subsalt finds, respectively, both disclosed in second half 1995.

Scant data released so far by companies testing gulf subsalt prospects confirm that sandstone reservoirs with high porosity, permeability, and pressure underlie the region's vast subsurface salt sheets. In addition, thick strata of subsalt clastic sandstones encountered in deep water-including Pliocene, Pleistocene, and Miocene formations-are helping advance deepwater depositional models guiding the play.

Taken together, such data indicate potential is great for production from future subsalt strikes. Subsalt activity will remain strong in the gulf because of the play's great profit potential. So conclude officials of two key subsalt companies in an update of subsalt activity in the gulf's northwest portion.

Clint Moore of Anadarko Petroleum Corp. and Rob Brooks of TGS-Calibre Geophysical Co., both of Houston, say implied subsalt oil and gas reserves in relatively shallow water, the pace of advancing technology, and existing offshore production facilities and pipeline systems will encourage gulf operators to aggressively pursue subsalt prospects during the rest of the century.

Over time, Moore and Brooks believe, integration of better subsalt seismic and well control data will enable explorationists to discover giant oil and gas reservoirs beneath the gulf's horizontal salt sheets.

Based on results of activity to date, it might be only a matter of time before a giant subsalt discovery is announced.

Production to climb

Gulf operators have declared three subsalt discoveries commercial, and subsalt production is to begin in the region by yearend.

Mahogany partners Phillips, Anadarko, and Amoco Production Co. plan to kick off the region's first subsalt production in December. Mahogany is expected to flow about 22,000 b/d of oil equivalent (BOE/day) and 30 MMcfd of gas. Production equipment on the Mahogany platform is being designed to handle as much as 45,000 BOE/day of oil and 100 MMcfd of gas.

Information released by Mahogany partners indicates the field is a three way dipping anticline on trend with conventional discoveries at deepwater sites to the south: Bullwinkle, Boxer, and Green Canyon Block 18 fields. Recoverable Mahogany reserves, according to independent estimates, amount to about 100 million bbl of oil equivalent (BOE).

Independent sources also estimate recoverable reserves at about 100 million BOE for Shell's Enchilada subsalt prospect on Garden Banks Blocks 127, 128, and 172. However, when combined with above-salt discoveries on adjoining tracts-Garden Banks Blocks 83 and 84-Shell estimates Enchilada reserves at 400 bcf of gas and 25 million bbl of oil (OGJ, Dec. 4, 1995, p. 42).

Enchilada partners are laying a $240 million development plan for the five tract area that would allow production to begin early next year. When fully developed, project production could amount to as much as 300 MMcfd of gas and 40,000 b/d of oil and condensate.

In addition, Texaco and partner Chevron U.S.A. continue to evaluate their Gemini subsalt field. Lying under a tabular salt formation more than 2,900 ft thick at a site in 3,393 ft of water on Mississippi Canyon Block 292, Gemini could mark the gulf's first subsalt production in deep water (OGJ, Sept. 25, 1995, p. 40). Partners are planning more well tests this year to further appraise Gemini.

Phillips and partners have not released more information since summer 1994 about the status of their Teak subsalt prospect on South Timbalier Block 260. At last report, their 1 South Timbalier 260 wildcat flowed a combined 4,431 b/d of oil and 7.76 MMcfd of gas on tests of three zones above 16,610 ft total depth. Production came from intervals beneath a salt sheet at about 10,500 ft.

Teak partners Phillips and Anadarko later plugged the discovery well because of mechanical problems. However, Minerals Management Service granted Phillips suspension of production status on the lease before it expired in April 1995, so further activity is expected on the tract.

Subsalt drilling

With more subsalt discoveries apparently bound for development, subsalt drilling activity in the gulf is sustaining a steady pace and starting to diversify.

Phillips, Anadarko, and Amoco Production Co. at the end of January still were drilling the fourth appraisal well in Mahogany field-3 Ship Shoal South Addition Block 359-aiming for a bottomhole location under Block 259 from a six well template on Ship Shoal South Addition Block 249 (OGJ, Jan. 22, p. 16). Partners also were drilling the 1 Ship Shoal South Addition Block 337 wildcat to test the Alexandrite subsalt prospect.

In addition, operator Phillips and Anadarko were drilling the 1 Ship Shoal South Addition Block 361 wildcat to test their Agate subsalt prospect, and operator Anadarko, Phillips, and BHP Petroleum (Americas) Inc. were drilling a wildcat on Vermilion Block 375 to test their Monazite subsalt prospect.

In other subsalt exploration:

Marathon Oil Co. in early February was continuing to drill a 16,200 ft subsalt wildcat on South Timbalier Block 308.
Pennzoil Exploration & Production Co. was drilling a 21,000 ft wildcat well on South Marsh Island Block 97.

Each subsalt well adds to the growing body of knowledge describing subsalt geology in the gulf. One key finding of companies leading the subsalt play has been discovery of noncompetent zones in strata below many salt formations.

Nearly a dozen wells aiming for subsalt targets failed to drill deep enough to make it all the way through such intervals. In some wells that penetrated the top and bottom of salt, companies cut Tertiary, Eocene, Oligocene, and some mixed Miocene intervals in discrete zones within the salt sheets.

Moore and Brooks say it is extremely important that explorationists in the future drill subsalt wells well beyond the base of the so-called gumbo zones. Penetrating several thousand feet below such intervals would significantly increase the chances to encounter reservoir sands through predictive cycles of sequence stratigraphy, they say.

Moore and Brooks expect subsalt activity in the gulf this year to continue revealing even more potential for the play.

They say, "As advanced seismic acquisition and processing techniques provide improvements in seismic image resolution and subsalt well control continues to refine geologic concepts, geoscientific integration will lead to giant discoveries in multiple style traps beneath the horizontal salt sheets of the Gulf of Mexico."