The Chevron Jack #2 well at Walker Ridge represents one of the recent high points in the Gulf of Mexico’s storied history. The well, in the deepwater Lower Tertiary Wilcox trend, was drilled to 25,000 feet subsea in 7,000 feet of water, and tested 6,000 bopd. Photo courtesy of Chevron
In the early 1990s Gulf of Mexico production had dropped below 20-year record low levels, and much of the industry was off looking for finds anywhere but the Gulf.
Then, in 1993, Anadarko announced its Mahogany subsalt discovery, igniting a whole new play in the Gulf’s extensive subsalt environs.
Add the deep gas play and the ever-increasing action in the deep water, and the Gulf became and remains a top producer for the energy-gorging United States, where most offshore regions have been declared off-limits for drilling.
One of the more recent high points in the Gulf’s long history was the successful production test of Chevron’s Jack #2 well in 2006 at Walker Ridge in the deepwater Lower Tertiary Wilcox trend. The well was drilled to 25,000 feet subsea in 7,000 feet of water and tested 6,000 bopd.
The test triggered a near-frenzy in the mainstream media once potential recovery for the entire play was publicized to be in the billions of barrels.
The highly touted Jack well actually followed a string of significant deepwater discoveries in the Lower Wilcox, dating back to 2001.
Only a few years prior to these discoveries, few geoscientists envisioned the presence of any significant Paleogene sands in the deepwater Gulf, let alone hundreds of feet of net sand spread across a vast area of the present-day lower continental slope and abyssal plain.
“It came as a surprise to many geologists that any significant sands would be encountered this far out in the basin,” said AAPG member Jon Blickwede, senior staff geologist with Statoil’s Global Exploration-Americas division in Houston. “The deepwater Wilcox play is a good example of how the Gulf province continues to surprise us in terms of discovering new resources in parts of the stratigraphic section that industry had not previously perceived as having any potential.
“It’s a myth that the Gulf is in its elder years and played out,” Blickwede added. “In the geographic sense, most of the Gulf of Mexico basin is under-explored, and some of the best global exploration opportunities are sitting under our noses.”
Challenges and Obstacles
But there are challenges – of various kinds.
Both federal and state political posturing has kept the geologically promising eastern one-third of the GoM off the Florida coast off limits to drilling for more than two decades, Blickwede noted.
To date, only about 70 wells have been drilled in this entire eastern Gulf area, even though a number of promising plays have been identified [Related Story].
A prospect in one of these plays – Destin Dome, offshore the Florida panhandle – was the site of a 2-3 tcf discovery about 30 years ago. Political intervention on both the federal and (Florida) state level put the skids on developing this significant resource.
The Yucatan platform of Mexico also looms as a promising locale for GoM exploration. It encompasses an area about three-fourths the size of the North Sea, yet only about 50 wells have been drilled here – most without the benefit of modern seismic.
The eastern margin of the intra-platform basin on the Mexican side is probably the best place to pursue this play, Blickwede said, as the westward tilt of the Yucatan platform would favor secondary migration toward the east.
This play has a pedigree of sorts.
“The intra-platform basin as it’s conceived has one of the best analogs imaginable,” Blickwede noted. “It has elements analogous to the super giant accumulations of the Arabian platform.”
Yucatan’s petroleum potential has been documented by one major discovery in northern Guatemala – the Xan Field – and two smaller discoveries in Belize.
There’s been considerable attention focused on drilling off the coast of Cuba, which has the smallest and least explored offshore area of the three countries having sovereignty over the GoM.
A well drilled in the offshore Cuban sector three years ago was rumored to have been a discovery, according to Blickwede.
A number of countries are cozying up to the Cuban government to get on board to drill in waters reportedly as close to Florida as 50 miles – an ironic twist given the moratorium covering most of the eastern Gulf.
Mexican Gulf Targets
Across the Gulf to the west from Cuba, the Mexican sector of the deepwater is perhaps the most promising under-explored Gulf area. Fewer than 10 wells have been drilled in water depths greater than 500 meters, Blickwede noted.
In contrast, about 1,800 wells have gone down in these same depths offshore Texas and Louisiana.
The more highly explored areas of the GoM also hold plenty of potential for new plays – and one of these entails the Gulf-wide Cretaceous-Tertiary boundary mass transport breccias.
This complex clastic, or “cocktail,” unit at the K-T boundary has been demonstrated to be the main reservoir in giant fields in Mexico’s Bay of Campeche. Outside of this locale and also western Cuba where it’s a productive reservoir as well, it would be a new play.
“Most people perceive the K-T boundary breccia to be restricted perhaps to just the immediate area around Chicxulub crater in Mexican waters,” Blickwede said.
“But I’m suggesting it may be much more extensive geographically,” he added, “and under certain conditions of diagenesis or deformation it may be a viable reservoir target in other parts of the basin where we may not have considered it before.”
Given the once-inconceivable find of great volumes of Lower Tertiary Wilcox sands in the deep basin, there’s some head-scratching as to just how deep in the stratigraphic section the operators might go.
Outside the eastern Gulf, the Mesozoic has not been actively pursued in the U.S. offshore GoM. An exception was Shell’s Baja #2 well drilled in 2000 in Alaminos Canyon Block 557, which penetrated a section at least as old as the Lower Cretaceous, according to Blickwede.
Effective porosity and permeability can be limiting factors as one goes deeper in the section, but there are examples of high porosity and permeability being preserved at great depths.
“In the northeastern Gulf in the Jurassic Norphlet play in those aeolian sands offshore from Mobile Bay, there are some significant gas accumulations below 25,000 feet subsea with porosities above 20 percent and associated good permeabilities,” Blickwede said.
“In that case, it’s been documented that it’s apparently due to very continuous clay coatings on quartz grains, which protect the sands from cementing up with quartz overgrowth cement – which at those depths and temperatures is what you’d most worry about in a highly quartzose sand like the Norphlet.
“There are parts of the Cretaceous in the Tuscaloosa where a similar phenomenon occurs.
“The porosity basement in terms of depth of burial is difficult to put a firm number on,” Blickwede noted. “It sort of depends on the composition and diagenetic conditions of the sand.”
Carbonates are a different breed of cat.
“Carbonates don’t necessarily have to be exposed to fresh water to develop good porosity and permeability post deposition,” Blickwede said. “Hydrothermal fluids from below can come into contact with carbonates, and the organic acids in those fluids can post-depositionally develop good porosity and permeability.
“So there are scenarios where probing deeper doesn’t necessarily mean you’ll run out of effective porosity.”
First, Some Questions
If you’re determined to head out to the deepwater Gulf with the goal of identifying new plays in under-explored parts of the stratigraphic column, Blickwede has a to-do list:
- Question paradigms – the deepwater Wilcox is a good example.
- Look for anomalies, exceptions to the rule.
- Use your understanding of the regional geology for clues, and evaluate whether the play concepts make sense in the regional context.
- Push the data, especially the seismic, to its limits.
But he, like so many others, believes the time is right to make good things happen.
“It’s a great time in the industry for explorationists to be creative and bring new ideas forward,” Blickwede said. “The state of the industry is very aggressive now, and there’s a heightened receptiveness to new ideas or exploration concepts.”