NPR-A looking better and better for natural gas

Kay Cashman

Petroleum News Publisher & Managing Editor

When David Houseknecht says he expects USGS’s estimate for northern Alaska’s undiscovered, technically recoverable, natural gas to jump from 150 trillion cubic feet to 211 tcf, he also points out that the increase could be much larger because of public information gleaned from recent discoveries in the National Petroleum Reserve-Alaska. That information could lead the U.S. Geological Survey to increase its NPR-A gas estimate, which currently sits at 61.4 tcf.

Houseknecht, a top geologist with the USGS, told a committee of Alaska legislators July 28 that the jump from 150 tcf to 211 tcf would likely occur later this year when the federal agency releases the results of its assessment of undiscovered gas resources for state onshore and Native lands in northern Alaska. (The 150 tcf includes only onshore federal, offshore federal and state offshore.)

“The upside potential could be much greater,” he said, pointing to recent NPR-A discoveries that “certainly indicate there may be more gas present in NPR-A than we estimated just two years ago.” (See map on page 23.)

There have been more than 400 exploration wells drilled on the North Slope, but those wells “are clustered along the coastline … along the Barrow Arch” where a drill bit is more likely to encounter crude oil than natural gas.

“That’s where industry found oil so that’s where exploration has been focused,” Houseknecht said.

In the northern half of the North Slope “there is a greater probability of finding oil than gas even though gas is present. In the southern half of the North Slope, in the (Brooks Range) Foothills … there is a greater probability of encountering gas than oil,” he said.

“Worldwide the largest gas resources and reserves typically occur in geologic provinces” where there is little or no oil, Houseknecht said.

“The bottom line here is that most of the known accumulations of associated gas are up on the coastal plain near the Barrow Arch; most known accumulations of the non-associated gas are in the Foothills, farther south” where “significant gas shows are pervasive in the wells that have been drilled in the Foothills.”

But little is known about those wells because when oil explorers “encountered gas there they said, ‘oh shucks, this isn’t what we’re looking for’ and moved away,” most often without further testing or delineation of the gas accumulations, Houseknecht said.

“And for that reason there is a lack of drilling data in the Alaska North Slope Foothills, which increases the uncertainty of the estimates we have made.” That’s why “we have such a wide range between our 95 percent and 5 percent probabilities in the National Petroleum Reserve,” he said, referring to the USGS’s 40 tcf to 85 tcf estimate, which means the agency thinks there is a 95 percent chance that there is 40 tcf of undiscovered, technically recoverable gas in the reserve and a 5 percent chance that the number could be as high as 85 tcf. The average estimate the USGS uses is 61.4 tcf.

Because the four major gas plays in NPR-A extend across the Colville River into Native and state lands, all the way eastward to the pipeline corridor, Houseknecht said he expects the average gas resource estimate for those lands to be the same as NPR-A, 61.4 tcf.

The four gas plays — the Brookian Topset structural play, the Torok structural play, the Brookian Clinoform Central play and the Brookian Clinoform South-Deep play — are in the central or southern part of the reserve outside of the oil-rich Barrow Arch along the coast to the north.

NPR-A: Oil or gas province?

But there is a lot the USGS does not know about the North Slope’s geology. And what the agency is finding in its latest assessment has its scientists “puzzled,” Houseknecht said. The Alpine play “represents … the type of geology that exists in the Alpine fairway,” which stretches across the upper half of the reserve from the Alpine field on the east to within a few miles of NPR-A’s western boundary.

Bidding at NPR-A lease sales over the last five years indicates “industry believes there is significant potential extending westward across NPR-A,” he said.

Although industry has been interested in the Alpine play area’s oil potential, which is thought to contain more than 2 billion barrels of recoverable crude in NPR-A’s northeast corner, recent NPR-A discoveries indicate the play might also have significant gas potential, Houseknecht said.

Starting at the Alpine field and moving a few miles west to the Spark and then the Rendezvous oil discoveries, “there is an astoundingly rapid increase in the gravity of oil and the GOR over a very short lateral distance.” Houseknecht said USGS scientists are “struggling to understand this” because of the limited amount of data that is publicly available.

Field characteristics

That oil is 40 degree API gravity, which is the American Petroleum Institute’s measure for the lightness or heaviness of oil. Alpine oil is very light, or watery, as opposed to thick, molasses-like oil.

The GOR, or gas to oil ratio, at Alpine is 840. The GOR of a well, or field, is the number of cubic feet of gas it produces per barrel of oil, and Houseknecht said 840 is a very low value.

“If we step westward to the discovery at Spark the test indicates 55 degree oil, a much lighter oil than at Alpine, and probably a condensate, a petroleum compound that is a gas in the reservoir and precipitates to a liquid at the surface,” he said. (Condensate is almost pure gasoline and is generally 45 to 62 degrees API. Refiners pay almost as much for condensate as crude oil.)

Spark GOR is 10,200.

“If we step to the Rendezvous discovery the reported hydrocarbons discovered are 60 degrees gravity, even lighter, and a gas to oil ratio of almost 17,000.”

These rapid increases, Houseknecht said, lead to the question, “Is the big play, or plays, in NPR-A really going to be predominantly oil or will there be a very substantial gas resource that has perhaps already been discovered, or is waiting to be discovered, along the Alpine play fairway that industry has been treating primarily as an oil play?”

With only two of the recent discoveries in NPR-A in the public domain, he said, “there are lots of unknowns.”