Torok sandstones in Brooks Range show evidence of gas accumulations
It sometimes seems as if optimism over natural gas resource potential in Arctic Alaska rises at about the same rate as optimism falls over finding a means of bringing that gas to market. On May 9 at the annual meeting of the Pacific Section, American Association of Petroleum Geologists, a few months after the U.S. Geological Survey rebranded the National Petroleum Reserve-Alaska as a gas province, and a few days before ConocoPhillips and BP threw in the towel on their hopes of building a North Slope gas line, scientists from Suncor Energy described an exciting gas play that they have been investigating in the Brooks Range foothills. The play involves the Cretaceous-age Torok formation, a sequence of shales, siltstones and sandstones thousands of feet thick, deposited as detritus from the emerging Brooks Range along the northern side of the mountain range.
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In 2009 Calgary-based Suncor bought out Petro-Canada, the company that had been working with Anadarko Petroleum and the BG Group to explore for gas in the foothills.
World-class resourceSuncor’s Jeff Bever told the AAPG conference that the Torok formation in the foothills appears to be gas charged, with estimates of gas in place running to tens of trillions of cubic feet. To the north, near the Beaufort Sea coast, the formation hosts the reservoirs of the Tarn and Meltwater oil fields. Scientists have long understood that because of the distribution of subsurface temperatures, hydrocarbon source rocks have been cooked to form oil under the more northerly part of the North Slope, but that gas has preferentially formed to the south.
“We think there’s a lot of gas in the foothills in the Torok. It’s world class,” Suncor’s Robin Slotboom told the conference.
In the foothills region there are extensive sandstone units with good reservoir potential in the lower part of the Torok, in what geologists call “bottomsets,” sands dumped towards the base of the subsea slope on the side of the marine basin that once lay on the north side of the mountains, Bever said.
Less than 10 wells penetrate the lower part of the Torok in the foothills, but these wells all found strong gas shows, Bever said. Unfortunately, however, none of the wells extensively tested the “bottomset” units and no gas production tests have been conducted in the Torok, he said.
But evidence from 2-D seismic data and from well logs points to the excellent gas potential of these lower Torok sands. Analysis of potential source rocks and well mud logs supports the likely presence of extensive and contiguous gas-saturated reservoir rocks and a succession hundreds of feet thick of porous sandstone reservoirs. The seismic data indicate that the sandstones extend fairly continuously across the region. There are thick shale units to seal the gas into the sandstone, and geologic faults give rise to structural hydrocarbon traps tens of miles long. Subsurface pressures within the reservoir rocks are high.
In addition to the proximity of traditional North Slope hydrocarbon source rocks, geologists think that the Torok itself contains hydrocarbon sources.
Reservoir riskSlotboom said that the key geologic risk in the Torok play is the reservoir quality. There is considerable variation in the porosity (the ability to hold gas), while the permeability (the ability to flow the gas) is rather low. But there are systems of fractures in the rocks that would provide flow routes for the gas, with the use of long horizontal wells for penetrating clusters of fractures being a possible production technique, he said.
Reservoir modeling has indicated that gas could flow from the reservoirs at rates greater than 5 million cubic feet per day, with a potential cumulative gas production per well of 10 billion cubic feet, Slotboom said. Analogues from producing gas fields in similar geology in western Canada support these flow estimates, with even the lower permeability reservoir units providing acceptable flow rates in the Canadian fields, Slotboom said.
And, in addition to the potential for developing gas fields involving conventional gas reservoirs in the Brooks Range foothills, the more deeply buried Torok rocks have the potential for more economically challenging unconventional gas resources, in continuous gas-saturated sands.
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