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Billions in leftovers Myers talks about untapped 20M to billion barrel fields north of Brooks Range Alan Bailey Petroleum News Staff Writer
There’s a prevailing view that, with production from major oil fields on Alaska’s North Slope declining, the North Slope has become a mature oil and gas province. But Mark Myers, director of Alaska’s Division of Oil and Gas, takes a different view.
The easier structural plays, particularly onshore and near shore, have been drilled but many stratigraphic plays have yet to be explored, Myers told Petroleum News in early February. Myers also sees opportunities to develop smaller structural plays close to the existing oil and gas infrastructure.
Myers thinks that success in the Tarn and Alpine fields has moved exploration attention away from the big Prudhoe Bay-style structures towards stratigraphic traps. At the same time the Northstar field has demonstrated great success with an offshore development.
Sustained high oil prices, the use of horizontal drilling and the ability to produce from low permeability reservoirs are making more oil fields economic, Myers said. Classic North Slope plays The classic North Slope oil and gas plays occur along a structural high known as the Barrow Arch under the Beaufort Sea coast of the North Slope. These plays originated from the discoveries of oil fields like Prudhoe Bay and Kuparuk River many years ago.
The Prudhoe Bay field consists of a huge combined stratigraphic and structural trap involving Triassic sandstone reservoirs in a Mississippian to lower Cretaceous sequence of sediments known as the Ellesmerian sequence.
Companies are still looking for opportunities in this Ellesmerian play, especially near the existing oil and gas infrastructure, Myers said. “You’ve got a ton of Ellesmerian structural plays,” he said.
The reservoirs for the Kuparuk River field involve sandstones in what’s called the Beaufortian or Rift sequence of Jurassic or lower Cretaceous age — the deposition of the sandstones is associated with rifting or pulling apart of the Earth’s crust that occurred during the opening of the Canada Basin of the Arctic Ocean.
Although some of the Beaufortian sands can be thin and discontinuous, other areas of more continuous sands give rise to large reservoirs.
“So you get a huge range of potential sizes in the same rift breakup sequence but there are a lot of plays in that 20 million to 70 million or 80 million barrels size,” Myers said. “There are still plays in that 300 million, 400 million or 500 million to plus a billion size — they’re still out there, but they’re almost all stratigraphic.”
Success with the Alpine field and its Beaufortian Jurassic sandstone reservoir has spurred interest in similar Jurassic plays. There is a series of upper Jurassic sands just below the Alpine sands, Myers said.
“There’s at least a billion barrels in place, we think, in that trend,” Myers said.
Because of the low permeability of the reservoirs in the Alpine play the gravity of the oil really impacts the ease of oil production. And the oil gravity depends on which of the multiple source rocks in the area generated the oil, Meyers said.
“The source rock’s critical and often you get multiple source rocks in a given area,” Myers said. “If you look at the Tarn play on the west side of Kuparuk you’ve got 38 to 37 API gravity in close proximity of 26 to 22 gravity in Kuparuk, because of changes in the sourcing.”
However, horizontal drilling techniques can help extract oil from the low permeability reservoirs, Meyers said. Brookian stratigraphic plays There is a major Cretaceous and Tertiary sequence of petroleum bearing sedimentary rocks above the Ellesmerian and Beaufortian sequences in northern Alaska. Known as the Brookian sequence, this younger rock sequence extends all the way from the northern edge of the Brooks Range out over the North Slope and across the continental shelves of the Beaufort and Chukchi seas.
Stratigraphic plays involving topset or turbidite strata in submarine fans typify this Brookian sequence. And Myers sees great potential for these plays.
“Some of the … submarine fans are very large,” Myers said: “If you had reservoir quality and if you had closure you could approach the billion-barrel mark in some these if you had structural fill.”
Then there are other situations where you may find smaller fans with as little as 20 million barrels of oil, Myers said, and where several smaller fans stack together the combined volume of oil could reach around 100 million barrels.
However, production problems in the Badami field have shown that the Brookian plays aren’t without risk, Myers said. And compaction of reservoir rocks at depth may prove problematic.
“But there are some good looking fans as well … so it’s a question of finding higher quality sands,” Myers said. Some of the younger Brookian sandstones contain particularly clean sand because they’ve reworked older sediments, he said.
Because the Brookian sequence tends to overlie Beaufortian or Ellesmerian rocks, there are opportunities to explore for situations where there is more than one play at the same location. A current exploration focus involves looking for classic Ellesmerian plays at locations where there is also Brookian potential, Myers said.
Brookian plays may dominate the coastal plain of the Alaska National Wildlife Refuge, at the eastern end of the North Slope — Myers concurs with a U.S. Geological Survey assessment that the Brookian sequence probably contains the preponderance of oil in that area. However, a couple of intriguing structural trends in the northeast of the ANWR 1002 area include potential Kuparuk-style plays. And Ellesmerian plays occur in thrust sheets along the front of the Brooks Range. The potential in NPR-A The National Petroleum Reserve-Alaska, to the west of the North Slope oil fields, offers some interesting exploration possibilities. Along the north of NPR-A there is keen interest in Kuparuk or Alpine style plays in Beaufortian sands.
“In NPR-A the government drilled a lot of the structures, so the plays that are left are Jurassic pure stratigraphic plays,” Myers said. “But some Kuparuk plays which are largely stratigraphic in nature could have a structural element to them.”
However, as people move further out from the infrastructure, field size becomes a critical issue.
Some of the individual Jurassic plays beyond the Alpine field are small, Myers said. “You’d have to find a pretty sizable incised valley or forced regression sand out there that would have 200, 300, 400 million barrels,” he said.
But Myers sees the potential for a large find in a Brookian stratigraphic play in NPR-A.
“So with the turbidite plays there are some very big submarine fans out there — if one of those works it could be in that hundreds of millions of barrels range,” Myers said.
And prolific source rocks have generated huge quantities of oil and gas in the area.
“Most of the sands when you find them are oil charged but it’s porosity preservation and permeability that are the issues,” Myers said. The Brooks Range Foothills Geologists have long considered the Brooks Range foothills, in the southern part of NPR-A and extending east to the Dalton Highway, to be a gas prone province. Myers thinks that reservoir quality will prove critical in locating gas fields in this area.
“When you look at it there’s a lot of oil-stained rock in outcrop, so the question is ‘can you find enough effective porosity and permeability to make good gas reservoirs?’” Myers said.
Rock units such as the Cretaceous Fortress Mountain formation and the Lisburne dolomites that outcrop in the foothills probably have enough porosity for gas. However, other rock formations that have been buried to relatively shallow depths exhibit better reservoir characteristics.
And in the foothills some rocks, probably from less deeply buried strata, provide evidence of reasonable thermal maturity for oil, Myers said.
Remote areas on the Beaufort Sea and Chukchi Sea outer continental shelves lie a long way from the existing oil and gas infrastructure but probably hold large quantities of oil and gas. For example there is a very large Eocene and younger basin on the U.S. side of the Mackenzie Basin and some intriguing, large fault blocks in Tertiary strata at the outer edge of the Beaufort Sea continental shelf.
Some spectacular-looking structures lie under the Chukchi Sea.
“If you can find a way to take the economic risk it has huge long-term potential,” Myers said. More drilling Myers thinks that the oil and gas industry in northern Alaska needs more companies drilling more wells.
“What’s fairly slowed down exploration has just been the lack of a critical mass of companies with exploration ideas,” Myers said.
However, Myers thinks that companies like Kerr-McGee and Pioneer are moving things in the right direction.
“Once they start developing and turning cash flow we’re incrementally going to see a lot more activity, particularly if they make money out of it,” Myers said.
And Myers thinks that a gas line from the North Slope will bring a huge boost to the economics of exploration and field development. However, it is essential that there is adequate provision for gas line expansion, so that companies can factor the future export of gas into the commerciality of exploration drilling.
“We think that it’s incredibly important that they create a structure that’s user friendly for expansion — the reserve base justifies that,” Myers said. Reducing costs Myers sees cost reduction as a critical factor in encouraging further oil and gas field development in northern Alaska. For example, the use of mobile, lightweight drilling rigs combined with a longer drilling season would cut the cost of exploration drilling and enable more wells to be drilled.
“I think we need to get some lightweight rigs up here — we need to get several of them working the North Slope,” Myers said.
The state is interested in helping reduce costs through road construction and by building exploration staging areas.
“One of the things that the state is thinking about is investing in staging areas in the foothills, where you could over-summer equipment,” Myers said. “… To do that we can rehabilitate some of the existing airstrips and old pads out there.”
These staging areas would reduce the need for lengthy ice roads and thus extend the effective drilling season. New permanent roads proposed by the state could also substantially reduce access costs.
Companies can themselves improve exploration economics by drilling in areas where several petroleum plays occur in close proximity.
“The people who have been most successful at exploration have drilled a number of different play concepts in a single year,” Myers said. “If you have a prospect that can stack up multiple objectives that gives you the best opportunity … and then being able to drill multiple wells in a winter season from a single rig becomes a critical element.”
However, developing modest sized fields depends on minimizing costs by sharing the use of production and export facilities with other fields.
“A lot of these areas, particularly onshore, where you may not find right off a 500 million barrel field … you might find a cluster of 50 million or 100 million barrel fields,” Myers said. “We have to find clever ways to produce these through a single producing facility.
The state recently commissioned a report on the status of oil and gas facility sharing on the North Slope. The state has also been looking at facility sharing practices in the North Sea.
“We would like to see more certainty in the commercial terms around access to the existing infrastructure,” Myers said.
The state would be happy to see clarity and certainty emerge from normal commercial negotiations. However, we have alternatives through codes of practice, arbitration or regulation, Myers said.
The state also wants to see lower tariffs on the trans-Alaska pipeline and has protested the recent tariff increase. Optimistic about the long term Myers feels optimistic about the long-term future of the oil industry in northern Alaska. We’ve seen the independent companies buying leases and now we’ve got to convert that to drilling activities, he said.
“So, I think we’ll see just a tremendous amount more oil produced, especially from the stratigraphic plays over time,” Myers said. “I think someone will stumble into that 500 million to a billion barrel field size.”
And where is that next big find?
“In the long term if I were to bet on a big prospect, the Brookian stratigraphic plays are where I’d put my money,” Myers said.
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