The U.S. Geological Survey has released its first estimate of conventional natural gas resources west of the National Petroleum Reserve-Alaska.
Rock formations in the region - west of NPR-A, east of the Chukchi Sea and north of the Brooks Range - “are believed to contain no recoverable oil deposits,” so the study does not include an assessment of those resources, the survey said Feb. 5.
USGS estimates a mean of 1.4 trillion cubic feet of undiscovered, technically recoverable natural gas lies in the area in conventional accumulations in Upper Devonian and Lower Cretaceous strata.
The western North Slope was included in a 2017 assessment of conventional resources in the Cretaceous Nanushuk and Torok formations in NPR-A and adjacent areas and in assessments of continuous resources of the entire North Slope in 2012.
The 2017 assessment said the Nanushuk South and Torok South assessment units show “evidence that the rocks have been heated to temperatures at which oil is converted to natural gas.” The 2017 assessment looked at northeast and northwest areas for both Nanushuk and Torok and said the northwest assessment areas “are inferred to contain oil-charged traps and a larger proportion of gas-charged traps because of source-rock character and evidence for a northward gas flush during the late Cretaceous and early Cenozoic uplift in the northwestern NPR-A.”
2021 assessment
The assessment released Feb. 5 said the area west of NPR-A includes a mix of federal, state and Alaska Native lands, including adjacent state waters.
The assessment estimates a mean of 1.4 tcf of natural gas, but “also found that geologic conditions in the area are too hot to support oil, so no assessment was made of oil resources in this study.”
“There has been speculation for decades that this area west of the NPR-A might be rich in oil. However, the limited geologic data we have indicate the rocks assessed contain modest natural gas, but likely no oil,” USGS Associate Director for Energy and Minerals Sarah Ryker said in a statement on the new assessment.
“This study suggests that oil deposits were formed in the area and were transformed into gas when the area was heated to high temperatures,” said USGS scientist Dave Houseknecht, who led the assessment.
USGS said the assessment looked at “undiscovered, technically recoverable conventional resources … those that have collected into discrete accumulations that can be extracted using traditional techniques.”
Conventional resources, the survey said, “are in contrast to continuous resources, which are those spread throughout a rock layer and typically require enhanced recovery techniques, such as hydraulic fracturing.”
Undiscovered resources are estimated to exist based on geology and other data, but not proved by drilling or other means, and technically recoverable resources are those that can be extracted with today’s standard industry practices and technology, as opposed to reserves, which are quantities that are currently profitable to recover.
Older than Torok formation
USGS said the 2021 assessment looked at strata older than the Torok formation and younger than the acoustic basement.
The Torok and younger Nanushuk are Brookian formations, which were considered in the 2017 assessment.
Older formations include the Beaufortian Kingak Shale; and the Ellesmerian Sag River and Shublik formations, and Sadlercohit, Lisburne and Endicott groups.
USGS said there is sparce subsurface data in the assessment area: three exploration wells (Akulik, Eagle Creek and Tungak Creek) drilled between 1978 and 1982 and some 500 miles of vintage 2D seismic from 1970-71.
Those data were supplemented with geologic maps of the western North Slope and adjacent areas and additional well and 2D seismic from the NPR-A and Chukchi Sea.
USGS said the geologic history of this western area “resulted in deeper burial and higher thermal maturity of Upper Devonian to Lower Cretaceous strata in the assessment area compared to coeval strata across much of the North Slope. Consequently, all strata considered in this assessment have been exposed to temperatures generally unfavorable for the preservation of oil.”
While “originally oil-prone source rocks occur in the Ellesmerian, Beaufortian, and Brookian … sequences in the assessment area,” only gas was encountered in wells drilled in or near the assessment area.
Resources summary
Three assessment units were considered: the Western North Slope-North AU, the Western North Slope-South AU and the Western North Slope-Lisburne Hills AU.
The Lisburne Hills AU was not quantitively assessed as the area’s geology does not seem favorable.
Most of the mean total undiscovered resources are in the North AU, a mean of 1.274 tcf of natural gas, with a mean of 133 bcf of natural gas in the South AU, for the total mean of 1.407 tcf of total undiscovered resources. The range for the North AU of fully risked, estimated mean gas resources was from 0 to 4.865 tcf; for the South AU the range was from 0 to 767 bcf.
USGS said it estimated undiscovered, technically recoverable resources in conventional accumulations of a minimum size of 30 bcf, similar to 5 million barrels of oil.
