Plenty of oil remains
USGS estimates 3.6 billion barrels undiscovered in central North Slope Alan Bailey for Petroleum News
The U.S. Geological Survey has published a new assessment of undiscovered, technically recoverable oil and gas resources under the central North Slope of Alaska. The agency estimates mean resources of 3.6 billion barrels of oil and 8.9 trillion cubic feet of natural gas in the region. The region is bounded on the west by the National Petroleum Reserve-Alaska and on the east by the Arctic National Wildlife Refuge. It runs south from the nearshore state waters of the Beaufort Sea to the northern border of the Gates of the Arctic National Park.
Scope of assessment The assessment does not include potential further oil and gas discoveries in the Torok and Nanushuk formations of the central North Slope that have been the focus of recent major oil finds - these exploration plays were fully considered in the USGS 2017 assessment for the NPR-A. And the assessment only includes conventional resources: It does not consider potential shale oil and gas resources in the region, nor does it consider methane hydrate resources. The USGS published an assessment of North Slope shale oil and gas resources in 2012, and an assessment of hydrate resources in 2019. Also, the assessment does not consider the economic feasibility of recovering the resources.
As in any undiscovered oil and gas assessment, there is a significant range of uncertainty in the results. The USGS reports a 95% probability of at least 950 million barrels of undiscovered oil in the region, with a 5% possibility of as much as 8.6 billion barrels. The corresponding range for potential natural gas is 1.6 tcf to 25.4 tcf.
Regional variations But these regional numbers mask significant differences between different oil and gas plays in different parts of the central North Slope. The USGS has considered plays or “assessment units” within the three major petroleum bearing rock sequences of the region: the Ellesmerian, Beaufortian and Brookian sequences. The older and deeper Ellesmerian and Beaufortian sequences host many of the producing oil fields in the central North Slope. As a consequence, the relatively mature exploration and development of these sequences in the northern part of the region result in a relatively high level of understanding of continuing resource potential. On the other hand, there are huge areas elsewhere, where there has been very little exploration - undiscovered resource potential in these areas remains subject to significant uncertainty.
The Brookian sequence is widely found at relatively shallow depths throughout the region. However, east of the area where current Brookian exploration and development has been focused, the Brookian rocks are younger than the Torok and Nanushuk formations that host recent discoveries. And, east of the trans-Alaska pipeline, there is a huge area of Brookian exploration interest where very little previous exploration has been conducted. The Brookian sequence was formed from the discharge of huge amounts of sediment into an evolving marine basin. To evaluate the Brookian oil and gas potential, the USGS scientists have separately considered Brookian topset formations, rocks formed from sediment deposition at the top of the slope at the edge of the basin, and foreset and bottomset formations, rocks from sediments deposited on the sides and bottom of the basin.
Another very important variable relates to the thermal maturity of the oil and gas source rocks in the region, in other words the extent to which the rocks have been heated in the subsurface to produce different types of hydrocarbon. In general, the thermal maturity tends to increase north to south from the Beaufort Sea coast. As a consequence, there is a higher potential for the generation of oil in the more northerly part of the region, with natural gas likely to be more prevalent to the south. This trend is particularly pronounced in the relatively deep Ellesmerian and Beaufortian rocks. For relatively shallow source rocks in the Brookian, however, the oil window extends farther south, into the foothills of the Brooks Range.
Six assessment units The upshot of all of this is that the USGS has used six assessment units to evaluate oil and gas resources: Ellesmerian targets in the north, Ellesmerian targets in the south; Beaufortian targets in the north; Beaufortian targets in the south; Brookian topsets; and Brookian foreset and bottomset. The relative ranges of estimated undiscovered resources in each of these units provide an indication of the relative uncertainties of the estimates in the different units. Uncertainty can be expressed as a range of possible resource quantities, from a 95% probability of volumes at the low end of the range, to a 5% probability at the high end.
An examination of the trends in this data reveals, as expected, that some significant oil potential remains in the Ellesmerian and Beaufortian in the north but that, in the south, given the thermal maturity distribution, the oil potential appears very low. On the other hand, while there is gas potential in the north, the gas potential is higher, but also more uncertain in the south.
In the north, the Ellesmerian has a range of 91 million to 869 million barrels of oil, with a mean of 356 million barrels. The corresponding figures for natural gas are 103 billion to 2.0 trillion cubic feet, with a mean of 642 billion. The range for the Beaufortian in the north is 182 million to 1.4 billion barrels of oil, with a mean of 577 million barrels. Natural gas figures have a range of 182 bcf to 2.5 tcf, with a mean of 852 bcf.
In rocks of similar age to the south, natural gas becomes the expected dominant resource. The Ellesmerian has a range of just 16 million to 216 million barrels of oil, with a mean of 65 million barrel. The range for natural gas is 765 bcf to 6.4 tcf, with a mean of 2.7 tcf. The Beaufortian in the south has a range of zero to 174 million barrels of oil, with a mean of 45 million barrels. Beaufortian natural gas in the south is estimated at a range of zero to 5.7 tcf, with a mean of 1.9 tcf.
Brookian potential A particular wildcard in the exploration outlook is the Brookian sequence, where the lack of past exploration leads to high levels of uncertainty for future exploration results. However, the assessment indicates that there is strong potential for significant oil and gas discoveries in this sequence.
The Brookian topset has a range of 443 million to 3.5 billion barrels of oil, with a mean of 1.6 billion barrels. Natural gas in this assessment unit has a range of 163 bcf to 3.7 tcf, with a mean of 1.1 tcf. The Brookian foreset-bottomset has oil in the range 239 million barrels to 2.4 billion barrels, with a mean of 975 million barrels. Natural gas estimates for this assessment unit are in the range 423 bcf to 5 tcf, with a mean of 1.6 tcf.
Differences from last assessment The total mean undiscovered oil volumes have dropped a bit from the 3.9 billion barrels that the USGS determined in its last central North Slope assessment, conducted in 2005. However, the new assessment does not include potential oil in the Nanushuk and Torok formations. And 1 billion barrels of oil have been discovered in the central North Slope since 2005, USGS scientists pointed out in a presentation accompanying the new assessment. The range of uncertainty in the oil estimates has also increased, because of a recognition of the complexity of stratigraphic traps thought likely to hold a significant amount of the oil, the scientists commented. A stratigraphic trap results from the manner in which sediments forming the rocks were deposited, as distinct from structures formed from the subsequent folding and faulting of the strata.
The assessment has also increased the level of uncertainty for potential gas resources, in part because of a lack of delineation of gas discoveries and in part because of the complexities of the structural traps involved. In addition, relatively few wells have been drilled since 2005, with generally modest results for gas discovery, the scientists commented.
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