UNDERSTANDING SALT MOVEMENT KEY TO HOT GULF OF MEXICO PLAY

Bob Tippee
Managing Editor-Economics and Exploration

A. D. Koen
Senior Editor-News

The Gulf of Mexico's hottest exploration play in years hinges on drastically changing concepts about shapes and movements of underground salt.

The old, standard view of salt as an upward thrusting dome rooted to a deep source bed has expanded to encompass a range of salt shapes and behaviors.

Although piercement domes of the old model exist, many salt bodies prove to have more width than depth, often detached from source beds.

These new concepts about salt emerge from advances in seismic imaging technologies combined with recently released information from wells drilled in the past decade.

In particular, a complex seismic technique called 3D prestack depth migration, made practical only recently by gains in computer processing capability, has enabled geophysicists to create images of salt bodies with unprecedented accuracy (OGJ, Sept. 27, 1993, p. 41).

It, 2D and 3D poststack depth migration, and related procedures also bring into focus-often for the first time-seismic images of structures in sedimentary layers beneath salt.

Armed with growing knowledge of salt and subsalt geology and the gulf's first apparently commercial subsalt discovery, operators in the Gulf of Mexico are stepping up subsalt activity

In addition to two subsalt wildcats and a subsalt delineation well in progress, operators this year and next could spud more than a dozen subsalt tests in the gulf.

Competition for offshore acreage with subsalt potential dominated Outer Continental Shelf Lease Sale 147 in March for tracts in the Central Gulf of Mexico, accounting for about $100 million of apparent high bids totaling $277 million. In addition, availability of 3D seismic data coverage in the gulf-so important to the subsalt play-is increasing rapidly.

Anadarko Petroleum Corp., Houston, and Phillips Petroleum Co. are pacing the gulf's subsalt drilling activity, and Anadarko leads recent leasing. Amoco Production Co. also has a leading role in the play.

Phillips and Anadarko are participating in all three subsalt tests under way in the gulf. Phillips operates the 2 Mahogany delineation well on Ship Shoal South Addition Block 349 and the 1 Teak wildcat on South Timbalier Block 260. Anadarko operates the Mesquite wildcat on Vermilion South Addition Block 349. In addition, each company will hold interests in as many as four subsalt wells to spud later this year.

The only active Gulf of Mexico subsalt well in which Amoco holds an interest is 2 Mahogany.

MAHOGANY STRIKE

A structure identified with the new wealth of seismic technology . yielded the landmark Mahogany, discovery disclosed by operator Phillips and partners Anadarko and Amoco last October. The discovery well, on Ship Shoal South Addition Block 349 off Louisiana, flowed 7,260 b/d of oil and 10 MMcfd of gas (OGJ, Oct. 11, 1993, p. 30).

Mahogany, was the first potentially commercial strike disclosed in a subsalt play that had been under way for several years. Its validation of subsalt production potential and imaging technologies was the main reason that selected blocks in Outer Continental Shelf Sale 147 last March set OCS leasing records for bids per acre (OGJ, Apr. 11, p. 36).

But the play is young. Much about it remains a technological and geological puzzle. And the ability to image salt bodies and structures beneath them, important and challenging as it is, is just one piece.

While explorationists know much more about salt shapes than they did just a few years ago, they are only beginning to understand how and why salt bodies moved to their present locations-and what those movements mean about nearby sediments.

The implied question have been discussed at several recent meetings in Houston. Answers will be crucial to mapping structures below salt, assessing potential for reservoir development, and determining timing of hydrocarbon migrations-all keys to future discoveries.

CHANGING CONCEPTS

Robert 0. Brooks, vice-president of interpretive products for TGS-Calibre Geophysical Co., sans seismic data have indicated the presence of horizontal salt bodies in the Gulf of Mexico since the early 1970s.

It wasn't until the mid-1980s, however, that allochthonous salt-sheets detached from or attached only by thin stems to the Jurassic Louann salt formation-came to be associated with exploration potential. Such sheets occur in the gulf at depths of 30,000-40,000 ft.

Computer power was then beginning to make 3D seismic processing and interpretation practical on a large scale, and significant drilling results were being revealed.

An especially important well was Diamond Shamrock Corp. 1 South Marsh Island 200, a 1985 dry hole that logged 1,000 ft of sand nearly 3,000 ft below salt. The log repudiated a theory that reservoir sands did not exist beneath horizontal salt.

Before then, Gulf of Mexico salt was thought to be mainly vertical piercements with overhangs on the shelf, extensive salt ridges and walls on the slope, and allochthonous tongues on the Sigsbee Escarpment.

Results of the South Marsh Island well and others triggered new seismic studies, which showed that the southern Louisiana shelf-out to water depths of 600 ft-and upper slope had a significant amount of horizontally injected salt in addition to vertical piercements.

In 1992, Exxon Co. U.S.A. released results of a deepwater well on Mississippi Canyon Block 211 that cut 90 ft of hydrocarbon pay beneath a salt tongue. The well showed that hydrocarbons, not just sands, can exist beneath horizontal salt. It set the stage for the Mahogany strike reported late last year.

SALT SHAPES

The round of intense study that has occurred since the mid-1980s, which includes reprocessing of 2D seismic data and massive acquisition of 3D data, has identified several generalized salt shapes (Fig. 1).

A study of nearly 200,000 line miles of regional 2D seismic data defined four regions of the gulf in terms of dominant salt shapes (Fig. 2).

In the relatively nearshore area dominated by piercements there are some overhangs and associated shale sheaths.

The area of horizontal injection, where most subsalt drilling has occurred, has a southern limit 10-15 miles south of the shelf edge. Seismic data provide evidence of horizontal movement and shale flowage. Shapes in this area include winged secondary piercements; thick salt pillows; thick, shallow, faulted salt sheets; deep salt along glide planes; and deep, thin weld zones.

Subsalt drilling so far has focused on structures below thick salt pillows. That type of feature is shallower than some of the others, more visible on seismic data, and thus less expensive and risky to drill.

The area dominated by ridges and walls shows evidence o(shallow salt tops and vertical and horizontal movement. Seismic complications-probably flowed shale, thick salt, undercompacted sediments, and complex salt surfaces-obscure the sides and bases of salt.

The area of salt tongues lies just north of the Sigsbee Escarpment. Shallow horizontal salt bodies show evidence of reverse faults and compressed folds associated with the southward creep of salt tongues or sheets. Exxon made its Mississippi Canyon Block 211 discovery in this area.

Brooks says the next phase of exploration probably will concentrate on targets below the decollement where salt is thin or nonexistent. The decollement is the boundary between earth masses of different Horizontal movement, generally delineated in the gulf play be, the base of salt. Deformation of strata below the decollement can differ greatly from that of strata above it.

HOW IT MOVES

An open question for explorationists is whether salt motion reflects mainly buoyancy under sediment loading or movement along fault planes-or perhaps a combination of both.

And there may be more than one way for salt to move through sedimentary layers.

Howard J. Yorston, of Interpretation Consultants Inc., says salt might move interstitially through sands and shales, finger through them in layers, or displace them by pushing through in mass.

Determining the type of movement can improve estimates of sound velocities through salt bodies, Yorston notes. Sound moves more slowly through a layered salt body, for example, than it does in salt that moved interstitially, or by displacing sediments.

It's the high speed of sound through salt-usually much higher than surrounding sediments-that makes seismic imaging of the base of salt and subsalt sediments difficult.

SALT TECTONICS

On a broader scale, studies of Gulf of Mexico salt tectonics are producing important information about salt migration and deformation.

The geologic history of what is now the continental slope tells much about allochthonous salt on the shelf, where the slope was located when the salt was set in place.

Shengyu Wu of Total Minatome Corp. and A. W. Bally of Rice University's department of geology and geophysics describe that history as a complex mixture of extensional and compressional phenomena since deposition of the Louann salt in middle Jurassic time.

Wu attributes salt formations to pushing by sediments, with differential loading of sediments and buoyancy forcing the salt to rise and spread.

Deformation before the middle Cretaceous resulted in salt massifs, small allochthonous salt bodies, pseudoclinoforms, and turtle structures (Fig. 3). From then until the middle Oligocene, sedimentation was slight, and salt and related structures remained stable.

During rapid middle Miocene sedimentation, large diapiric salt walls and stocks developed, fed from the salt massifs formed before the middle Cretaceous. The rising salt structures displaced the downslope section of the rapidly deposited overburden.

The Mississippi Fan Fold Belt formed as a result of the downdip displacement, focused near the frictional boundary along the basinward limit of the middle Jurassic salt in the middle Miocene. The fold belt quit shortening when allochthonous salt began to spread near the seafloor within sediments during late Miocene.

Since then, large primary allochthonous salt sheets updip from the fold belt continued to form and spread downslope. As the massive salt withdrew from the mother salt, large primary withdrawal basins, regional and counterregional growth fault systems, tension faults, primary and secondary welds, and turtle structures formed.

Increasing sediment loads deformed the primary allochthonous salt. Higher allochthonous salt and related structures developed. Wu speaks of a "second generation" of allochthonous salt.

Bally of Rice University says a focus for exploration should be causes of folding beneath salt.

WHAT SEISMIC SHOWS

William M. House and John A. Pritchett of Amoco Production Co. note differences between salt masses on the shelf and some in the eastern and central deepwater areas of the gulf.

In many deepwater locations, they say, original salt massifs are relatively undeformed, with tops near the seafloor affecting seabottom relief. Shelf salt sheets are more deformed as a result of Pliocene and Pleistocene sedimentary loading, with many secondary salt features having formed.

Most widespread salt sheets on the shelf are significantly above the seafloor, buried beneath late Neogene and Quaternary sediments. In the south central shelf, much Pliocene and Pleistocene structuring and deposition have been controlled by original emplacement and later mobilization of the salt.

From evaluation of 3D depth-migrated seismic data, House and Pritchett postulate a two part process for salt deformation (Fig. 4).

In the first phase, wedge shaped salt massifs move into Miocene and older sections, remaining near the seafloor and creating seabottom relief. Sedimentation occurs at first adjacent to the massifs, with thin, condensed sections on their tops.

The first phase ends when the salt pinches off from its deeper source.

In the second phase of the salt deformation process, the detached salt massif collapses as a result either of regional extension or of lateral movement of salt along massif edges through gravity spreading. Either process removes salt from the central parts of the massif and causes a depression in the upper surface.

Sediment loading can occur in the depression, eventually deflating the salt massif and moving salt into sheets and secondary salt domes. Salt sheets can coalesce to form an extensive salt canopy.

The transition between these phases involves collapse of the original salt feeder stock. Salt evacuation creates a void that must be filled by surrounding sediments.

Movement of the surrounding section into the evacuated space may occur through discrete faulting or through the formation of zones of pervasive shear. Collapse of the feeder stock creates structural disruption that produces an area of increased permeability.

If the source rock lies above the primary or mother salt layer, faulting and shearing associated with the structurally disrupted collapse zone provide a permeability conduit from source to reservoir rocks.

House and Pritchett use a model built from 3D poststack depth-migrated seismic data to show how a salt sheet was emplaced and deformed into what is now a partially collapsed salt massif with adjacent, thin secondary salt sheets (Fig. 5).

SUBSALT DRILLING

If recent reports are an indication, Phillips and Anadarko's subsalt drilling plans at three gulf sites could firm up considerably in the next 4-6 weeks:

• At the 2 Mahogany delineation well, the companies have penetrated the salt underlying Ship Shoal 349, set casing through the formation, and are preparing to test.

• Well tests are under way at the 1 Teak wildcat on South Timbalier 260.

• Crews on 1 Mesquite in Vermilion South 349 have drilled through the salt and are preparing to set casing.

Phillips and Anadarko plan to spud the 3 Mahogany delineation well on Ship Shoal 349 immediately after completing 2 Mahogany. Also this year the companies expect to jointly spud wildcats on the Ebony prospect on Vermilion South Addition Block 306 and possibly the Corundum prospect on East Breaks Blocks 171, 172, 215, and 216.

Phillips expects this year to spud its Snakewood wildcat on South Marsh island South Addition Block 194. In addition, Anadarko's Birch subsalt prospect is ready to drill.

Although the company has not made definite plans beyond 1994, Phillips in the next few years could gradually increase its subsalt activity in the gulf. "Our goal for this year and next is to explore two or three new subsalt prospects in addition to continuing to delineate or develop whatever subsalt discoveries we make," a company official said.

For its part, Amoco expects to participate in three to five subsalt wildcats in the next 2 years, including operated and nonoperated wells. That count might be in addition to Mahogany drilling.

"We're going to continue updating our subsalt drilling plans, especially in the next few weeks while information from Sale 147 still is coming in," an Amoco spokesman said.

Japex (U.S.) Corp., Houston, reportedly is preparing to spud a company operated subsalt wildcat late this month in about 180 ft of water on Ship Shoal Block 250, where its lease is near the end of a 5 year term.

Louisiana Land & Exploration Co. (LL&E), New Orleans, expects to spud at least one subsalt test in the gulf in 1995 but none this year. The operator of a near salt test under way on Eugene Island Blocks 110-111, LL&E might be the operator of its first subsalt test.

"We would like to operate," an LL&E official said, "but it will depend on what we pick as our first locations, who our partners are there, and what the acreage situation is like. It's possible another company could be the operator-but not probable at this point."

SUBSALT LEASING

Anadarko blitzed OCS Sale 147, winning 24 of 29 bids for subsalt tracts in the central gulf. Anadarko's winning offers included a $40 million bid for Ship Shoal South Addition Block 337, a northeast offset to the Mahogany discovery (OGJ, Apr. 11, p. 36). Anadarko believes Block 337 is 500 ft high to the Mahogany strike (OGJ, Apr. 25, Newsletter).

All of the nine tracts on which Phillips and Anadarko offered 50-50 al)parent wining bids in Sale 147 are near subsalt prospects, including:

• Shoal Block 361 and Eugene Island blocks 345-346 near Mahogany.

• South Timbalier Block 299 near Teak.

• Vermilion blocks 357, 375, and 376 near the Mesquite and Padauk prospects on Vermilion Block 411.

• South Marsh Island Block 121 near Mesquite.

• East Cameron Block 374 near Padauk.

Amoco, meanwhile, had three winning solo bids in Sale 147 for subsalt acreage, two joint bids with Chevron U.S.A. Inc., and one joint bid with Anadarko.

LL&E participated in four winning bids in Sale 147, including three with subsalt potential and one near salt.

Companies contacted for this story expressed satisfaction with results of their subsalt bids, but said it is too early to know how the new acreage will affect their exploration and development plans in the gulf.

Anadarko's Paul Taylor, vice-president of corporate communications, said drilling results will affect activity sooner than the new tracts under lease.

"Depending on how tests on South Timbalier 260 go, it could change our priorities somewhat," Taylor said. "The 2 Mahogany delineation well will tell us whether we need a second rig to begin drilling on the north end of the structure. So to definitely say we're going to drill five subsalt tests this year or seven, we don't know."

While some of Amoco's new subsalt acreage will affect its plans in the gulf, the timing of future subsalt tests depends more on internal and external logistical preparations. Amoco has about 90,000 acres under lease in the gulf with subsalt potential, including about 50,000 acres won with apparent high bids at Sale 147.

A TECHNOLOGICAL PLAY

LL&E's apparent winning bids in Sale 147 increased its total subsalt leasehold in the gulf to 68 tracts, 36 of which are held by production. All the tracts lie near existing infrastructure on the shelf in less than 600 ft of water.

While all its subsalt prospects could be placed on stream quickly if a discovery is made, the company is taking plenty of time to fully study prospects on acreage held by production.

"We consider the subsalt play to be highly technologically intensive, and there's a limit to how fast you speed that up," an LL&E official said. "As time goes on, we're going to firm up our plans, but right now we're collecting and processing a lot of seismic data."

As the gulf's subsalt play spreads to include more companies, the need for 3D seismic data is growing.

In mid-April, the Geco-Prakla unit of Schlumberger Oilfield Services disclosed plans to start a 48 tract, nonexclusive, 3D seismic survey focusing on subsalt prospects in the gulf's West Cameron South Addition federal planning area. Geco-Prakla will use triple 4,000 m streamers and dual source acquisition. Common depth point coverage is to be obtained under platforms in the area with the company's two vessel system.

The new data will be processed using dip moveout and steep dip, one pass, 3D time migration. Existing data from adjacent surveys are to be used during processing as a means to ensure a high quality data set.

Geco-Prakla has accumulated nonexclusive 3D seismic data on more than 800 tracts in the federal OCS of the Gulf of Mexico.