The Promise of the Best of Both Worlds

GOM Deep Water + Subsalt Plays

Deep water and subsalt plays are the two hottest exploration frontiers in the Gulf of Mexico - and when the two are combined they equal the most prolific discoveries in a producing province littered with giant oil and gas fields.

The deep water Gulf has been a focus for major oil companies for over 10 years now, but today the most exciting play is searching for structures below the salt in waters thousands of feet deep.

The potential reward is definitely worth the risk. At least one subsalt deep water discovery has reserves in the one billion barrel range - and several others are not far behind.

In a presentation at last April's AAPG annual meeting in New Orleans, John H. Shinol, senior advising geophysicist for deep water exploration with Unocal's Spirit Energy unit, discussed the deep water subsalt potential. His paper ("Subsalt Exploration in the Gulf of Mexico Deep Water," co-authored with H. Scott Sumner and J. Tim Ford) outlined four of the major provinces in the deep water subsalt play based on geological, geophysical and associated petroleum system attributes.

"We are a large independent company playing this ultra-deep water subsalt play, going head-to-head with the major oil companies," Shinol said, "but, we think there is a great deal of opportunity in this play for company makers."

The four deep water subsalt provinces discussed are:

  • The Isolated Salt Tablets.
  • The Primary Basins.
  • The Eastern Sigsbee Salt Canopy.
  • The Central Sigsbee Salt Canopy.

Much of the deep water portion of the northwest Gulf of Mexico, Shinol said, is covered by shallow allochthonous salt with deep-rooted feeders. There are extensional Oligocene-Pleistocene faults and detachments updip with an arc of salt-cored compressional folds downdip that help accommodate the updip extension.

In addition, there are several major transform faults running northwest to southeast that accommodated the original opening of the Gulf of Mexico in Late Triassic and Early Jurassic, he added. Rifts on the abyssal plain are related to these transform faults, which may have had controls on salt thicknesses when it was originally deposited.

Shinol said there are basically four different types of salt prospects in these provinces:

  • Low relief salt-cored pillow folds.
  • Thrusted folds.
  • Higher relief folds.
  • Inverted sediment thicks called turtle structures, found predominately updip to the other three.

Isolated Salt Tablets

Deep water subsalt activity started in the Isolated Salt Tablets province, which is located in the eastern Mississippi Canyon area in water depths ranging from 3,000 to 6,000 feet. Productive reservoirs are Pliocene to Miocene unconfined turbidites found in the 10,000- to 15,000-foot depth range. These are usually found in combination structural/stratigraphic traps.

Non-subsalt drilling has proven there is good oil and gas charge in the area, Shinol said, mainly from late Jurassic to Cretaceous carbonates and clastics.

Exxon made the first subsalt discovery anywhere in the Gulf of Mexico here about 10 years ago at its then-called Mickey prospect. Reserve estimates range between 50 to 150 million barrels of oil equivalent below the tabular salt, but the discovery was deemed sub-economic and has yet to be put on production.

That well, however, got people's attention and focused efforts on the subsalt, both on the shelf and in deep water.

Then in the mid-1990s Texaco and partners drilled the Gemini prospect on the province's west side and hit a commercial gas field. That field was developed as a sub-sea completion and is currently producing from one reservoir.

Gemini was the first deep water subsalt production - and the wells can be prolific. In June 1999 one of the wells was tested at about 76 million cubic feet of gas and 1,370 barrels of condensate a day.

"The companies were drilling for hydrocarbon indicating amplitudes below salt, much like discoveries in the area that were not subsalt," Shinol said. "This area is characterized by very good seismic imaging, and hydrocarbon indicating amplitudes have been associated with each discovery both under the salt and outside the salt play.

"Gemini, for example, showed up as a very strong event on the seismic," he continued. "Texaco was so confident of its prospect that the firm stopped drilling and set casing just before entering the pay zones.

"In fact, this hydrocarbon indicating anomaly could be seen on non-reprocessed, mid-1980s vintage 2-D seismic," he said. "Also, the seismic is so good in this area that most of the prospects were seen very early, and the non-subsalt and the subsalt plays have been relatively drilled up."

Image Caption

Figure 1: Gulf of Mexico subsalt plays. Graphics courtesy of John H. Shinol

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Deep water and subsalt plays are the two hottest exploration frontiers in the Gulf of Mexico - and when the two are combined they equal the most prolific discoveries in a producing province littered with giant oil and gas fields.

The deep water Gulf has been a focus for major oil companies for over 10 years now, but today the most exciting play is searching for structures below the salt in waters thousands of feet deep.

The potential reward is definitely worth the risk. At least one subsalt deep water discovery has reserves in the one billion barrel range - and several others are not far behind.

In a presentation at last April's AAPG annual meeting in New Orleans, John H. Shinol, senior advising geophysicist for deep water exploration with Unocal's Spirit Energy unit, discussed the deep water subsalt potential. His paper ("Subsalt Exploration in the Gulf of Mexico Deep Water," co-authored with H. Scott Sumner and J. Tim Ford) outlined four of the major provinces in the deep water subsalt play based on geological, geophysical and associated petroleum system attributes.

"We are a large independent company playing this ultra-deep water subsalt play, going head-to-head with the major oil companies," Shinol said, "but, we think there is a great deal of opportunity in this play for company makers."

The four deep water subsalt provinces discussed are:

  • The Isolated Salt Tablets.
  • The Primary Basins.
  • The Eastern Sigsbee Salt Canopy.
  • The Central Sigsbee Salt Canopy.

Much of the deep water portion of the northwest Gulf of Mexico, Shinol said, is covered by shallow allochthonous salt with deep-rooted feeders. There are extensional Oligocene-Pleistocene faults and detachments updip with an arc of salt-cored compressional folds downdip that help accommodate the updip extension.

In addition, there are several major transform faults running northwest to southeast that accommodated the original opening of the Gulf of Mexico in Late Triassic and Early Jurassic, he added. Rifts on the abyssal plain are related to these transform faults, which may have had controls on salt thicknesses when it was originally deposited.

Shinol said there are basically four different types of salt prospects in these provinces:

  • Low relief salt-cored pillow folds.
  • Thrusted folds.
  • Higher relief folds.
  • Inverted sediment thicks called turtle structures, found predominately updip to the other three.

Isolated Salt Tablets

Deep water subsalt activity started in the Isolated Salt Tablets province, which is located in the eastern Mississippi Canyon area in water depths ranging from 3,000 to 6,000 feet. Productive reservoirs are Pliocene to Miocene unconfined turbidites found in the 10,000- to 15,000-foot depth range. These are usually found in combination structural/stratigraphic traps.

Non-subsalt drilling has proven there is good oil and gas charge in the area, Shinol said, mainly from late Jurassic to Cretaceous carbonates and clastics.

Exxon made the first subsalt discovery anywhere in the Gulf of Mexico here about 10 years ago at its then-called Mickey prospect. Reserve estimates range between 50 to 150 million barrels of oil equivalent below the tabular salt, but the discovery was deemed sub-economic and has yet to be put on production.

That well, however, got people's attention and focused efforts on the subsalt, both on the shelf and in deep water.

Then in the mid-1990s Texaco and partners drilled the Gemini prospect on the province's west side and hit a commercial gas field. That field was developed as a sub-sea completion and is currently producing from one reservoir.

Gemini was the first deep water subsalt production - and the wells can be prolific. In June 1999 one of the wells was tested at about 76 million cubic feet of gas and 1,370 barrels of condensate a day.

"The companies were drilling for hydrocarbon indicating amplitudes below salt, much like discoveries in the area that were not subsalt," Shinol said. "This area is characterized by very good seismic imaging, and hydrocarbon indicating amplitudes have been associated with each discovery both under the salt and outside the salt play.

"Gemini, for example, showed up as a very strong event on the seismic," he continued. "Texaco was so confident of its prospect that the firm stopped drilling and set casing just before entering the pay zones.

"In fact, this hydrocarbon indicating anomaly could be seen on non-reprocessed, mid-1980s vintage 2-D seismic," he said. "Also, the seismic is so good in this area that most of the prospects were seen very early, and the non-subsalt and the subsalt plays have been relatively drilled up."

The drawbacks to this Isolated Salt Tablets province?

The structures are not very large - in the 50 to 150 million barrels of oil equivalent range - and thus the economics can be suspect.

Turtles and Primary Basins

The second deep water subsalt province is the Primary Basins, the region where most of the non-subsalt deep water discoveries like Shell's Auger, Mars and Ursa were made early on in deep water exploration.

This province covers the eastern East Breaks, Garden Banks, Green Canyon and Mississippi Canyon areas, and there are two subsalt plays in this region.

♦   The first -  is searching for fault/salt closures beneath small salt tablets and overhangs, he said.

The first deep water subsalt discovery in the province was made in the mid-1990s at a fault/salt closure prospect called Pluto, a gas-condensate field with estimated reserves of less than 50 million barrels of oil equivalent. It is currently producing.

"There is potential for these targets under the salt overhangs, but the problem is they can be relatively difficult to image," Shinol said. "There have been several dry holes drilled in this play - however, if you find one (target), the sands are very thick, very clean and have great production parameters.

"It is difficult to see through the salt because the rate of deposition moved the salt relatively quickly in some areas. The further east you go the better behaved the salt is, but in the Garden Banks and Green Canyon regions it's difficult to image the targets."

Companies are looking for younger Miocene through Pleistocene confined turbidite sections; estimated potential from these structures is between 50 and 150 million barrels of oil equivalent.

♦   The Primary Basin's second - and more exciting - subsalt play is centered on the province's eastern side and is targeting deeper, larger four-way closure turtle structures.

These structures are expected to be productive from the early and middle Miocene section at about 23,000-28,000 feet, with reserve potential greater than 300 million barrels of oil equivalent.

"BP-Amoco's Crazy Horse field, which is the largest deep water discovery made to date with estimated reserves of greater than a billion barrels of oil equivalent, is interpreted to be a turtle structure that thins away from the crest," Shinol said.

This play is still emerging. Crazy Horse and one dry hole called Metallica are the only tests so far - but Chevron, Texaco and BP-Amoco currently are drilling an exploratory well 15 miles west-northwest of Crazy Horse at the Poseidon prospect, which appears to be another turtle structure at about 30,000 feet.

Leasing activity in this deep water subsalt province has been quite heavy, and major oil companies now hold most of the documented deep turtles.

Deeper turtle structures are difficult to image on 3-D seismic data. There are no diagnostic hydrocarbon indicating amplitudes related with this play, so companies are targeting structures.

Geologically, turtle structures can have some complications.

"These structures were synclines at one time and were receiving sediments," he said. "Then the structure inverted due to salt withdrawal - so what was a low is now a high. That in itself implies there may be a complex relationship between timing of trap formation, hydrocarbon charge and migration.

"That's one concern industry in general has with turtles," he continued. "Was there trap formation during the hydrocarbon generation phase?

"Also, there can be crestal faulting on these structures that can degrade the top seal of the traps," he said, "(but) the size of the potential discoveries makes the risk worthwhile."

Crazy Horse is by far the largest discovered oil field in the deep water northwest Gulf of Mexico - in fact, it's one-and-one-half to double the size of the next biggest field.

Eastern Sigsbee Salt Canopy

The Gulf's third deep water subsalt province is the Eastern Sigsbee Salt Canopy in north Atwater Valley and southeast Green Canyon.

The petroleum systems here feature high relief salt cored folds to the west and thrusted folds to the east. Lower and middle Miocene turbidite reservoirs extend across the crest of the folds, which formed during the mid-Miocene to Early Pliocene. Target reservoirs are found in the 17,000- to 26,000-foot range, and the structures can be prolific with greater than 300 million barrels of oil equivalent.

There have been five discoveries to date here, although four have been centered in the western part, in a small area of southeast Green Canyon within 30 miles of each other in water depths ranging from 4,000 feet to 6,500 feet out near the edge of the Sigsbee Escarpment.

BP-Amoco's discovery Neptune kicked off this trend, and soon thereafter BP discovered Atlantis and Mad Dog. K2/Timon was found just north of the earlier discoveries, and most recently Texaco discovered the Champlain Field in the middle of Atwater Valley, extending the play further east.

Mad Dog is currently the most prolific of these discoveries, with reserve ranging from 400 to 800 million barrels of oil equivalent from about 300 feet of net hydrocarbon pay. Spirit Energy holds a 15.6 percent interest in the field, which is a large, high relief fold under the edge of the Sigsbee Escarpment.

Atlantis, however, is vying with Mad Dog in terms of size. The first appraisal well at Atlantis added significant additional section, bringing the total net feet of pay to 500.

The province's eastern side, dominated by thrusted folds, has not enjoyed that same kind of success. Shell drilled a dry hole called West Blackjack in the late 1980s. Chevron drilled a dry hole at Windriver. Recently Phoenix in southeast Mississippi Canyon and Showboat in eastern Atwater Valley have been drilled with no fanfare.

These dry holes may be attributed to a regional problem with access to mature charge.

"The question is, how far east can this play go?" Shinol asked.

However, the successful wells on the province's western side have a proven oil and gas charge, and target Middle to Lower Miocene unconfined turbidites in large high relief folds.

"The seismic quality in this province is good to fair, but companies are playing very high relief features, which are difficult to miss," Shinol said. "For example, Mad Dog is a 10,000-foot feature - about as tall as Pike's Peak above regional elevation. Even though imaging is not the best quality, it's hard to overlook a 10,000-foot tall mountain in the subsurface. This part of the play is not a search for the subtle trap."

The province's western part is relatively drilled up now, so the play's future lies in taking the charge risk and moving east to determine how far the play will go.

Central Sigsbee Salt Canopy

The success in the western Eastern Sigsbee Salt Canopy has sparked interest in the adjacent Central Sigsbee Salt Canopy province in Walker Ridge and Keathley Canyon, which has yet to be tested by the drillbit.

"There is a good chance of a continuation of the Mississippi fan foldbelt trend into this province," Shinol said. "We can see folds in southeast Green Canyon that appear to extend into Walker Ridge.

"The structures are not quite as tall here," he continued. "Instead of 10,000 feet they are 1,500 to 4,000 feet tall.

"In general we believe the features in this province are salt-cored pillow folds as well as large turtle structures, similar to Crazy Horse, generally updip of the pillow folds," he said.

The Central Sigsbee Salt Canopy structures are large with potential for 300 million barrels of oil equivalent, or better, from Middle to Lower Miocene unconfined turbidites downdip and confined turbidites updip. Based on surface piston cores and geochemical modeling, the charge appears to be predominately oil.

"There's only been one shallow well drilled in this area - it was drilled outboard of the salt canopy and it is interpreted to have TD'd above the primary objectives for the foldbelt," he said. "We perceive this region will be much like the five discoveries in the western Eastern Sigsbee Salt Canopy."

Spirit Energy plans to spud its first test as operator in the ultra-deep water at its Dana Point prospect in September, targeting a subsalt pillow fold in Walker Ridge.

"We will be drilling in 7,000 feet of water, and plan to drill through 8,000 feet of salt as we drill down to about 26,000 feet," he said.

"We also have another prospect called Cedar Top that's a turtle structure to the northwest of Dana Point."

Dana Point will be the first test of a pillow fold in Walker Ridge. Also, Texaco and Chevron are currently drilling a turtle structure in Walker Ridge, and BHP plans to test a pillow fold out in front of the salt at its Chinook prospect in the next few months.

"These are two different play types," Shinol said, "and in the next six months the industry will achieve a lot of new well control in Walker Ridge that should help us determine the overall prospectivity of the Central Sigsbee Salt Canopy.

"Are our preconceptions right? Are the folds better than the turtles? Are the turtles better than the folds?

More to Come

While Walker Ridge is currently being tested, the Keathley Canyon portion of the Central Sigsbee Salt Canopy will get its turn next year.

"Keathley Canyon is an area where we feel Spirit Energy has made a break out play and has taken a competitive advantage with a strong lease position over what we believe is a trend of large, low-relief salt-cored pillow folds," Shinol said.

"This portion of the subsalt foldbelt had not been previously recognized by industry," he added. "We were able to identify the trend by regional interpretation of an extensive data set of depth imaged seismic.

"In the Keathley Canyon portion … the folds are oriented in a different direction than Walker Ridge, from the northwest to the southeast. We were a little worried about that issue and wondered if it was a geophysical problem or real geology. So we have gone through a great deal of rigor to make sure we were not causing false structures by the way we were depth converting our seismic.

"We think the northwest to southeast folds are related to one of the large transform fault zones in the area that probably had heavy control on the original salt thickness," he added. "So, we have a northwest-southeast running transform fault with thick salt to the northeast of it and thinner salt to the southwest. That thicker salt deformed more easily and the transform fault acted as a buttress to keep things pinned.

"In addition, we think this area relates well to the Miocene depocenters to the north that would have actually caused the deformation in different directions in this area."

Shinol said Spirit Energy expects to find a continuation of Lower-to-Middle Miocene objectives with some upside potential in the Paleogene and Mesozoic.

"We are now at the point where we need to drill in our Keathley Canyon acreage, which we call the Spirit foldbelt, to see if our concepts are correct," he said.

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