Taking another look
USGS conducting new NPR-A assessment before also reviewing ANWR potential
Following a directive from Interior Secretary Ryan Zinke to update the estimates of undiscovered oil and gas resources in Arctic Alaska, the U.S. Geological Survey is conducting a new assessment of the National Petroleum Reserve-Alaska and has started the data processing for a new assessment of the coastal plain of the Arctic National Wildlife Refuge, USGS geologist Dave Houseknecht told the Resource Development Council on Nov. 2.
These revised assessments will be followed by a re-assessment of the central North Slope, and of other North Slope lands, Houseknecht said.
The Nanushuk and TorokThe NPR-A assessment is particularly focusing on the resource potential of the Nanushuk and Torok formations, the rock units at the center of recent major oil discoveries in the Pikka unit, at Willow and in Smith Bay. These discoveries have been made since the USGS last assessed the NPR-A in 2010 and have triggered a rethink of the potential for finding oil and gas in the Brookian sequence, the youngest of the major petroleum bearing rock sequences in Arctic Alaska. In fact, the new oil discoveries had in themselves caused USGS to plan an updated NPR-A assessment, Houseknecht said.
In part, progress towards a new assessment depends on access to new data: New independent oil companies operating in Alaska are proving particularly helpful in giving the USGS scientists ready access to data and rock samples, Houseknecht said. The USGS has also been reviewing 3-D seismic data from NPR-A and is waiting for the release by the Alaska Department of Natural Resources of new data that becomes available as a consequence of tax credit rules.
NPR-A firstCurrently the USGS scientists are in the process of gathering public input for their NPR-A assessment, before moving behind closed doors to conduct the assessment without external influence.
Houseknecht emphasized that, apart from an area along a geologic structure called the Barrow Arch, along the Beaufort Sea coast, Arctic Alaska remains essentially unexplored for oil and gas, with just a thin scattering of exploration wells. The offshore is also underexplored by global standards, he said.
“It’s really an exceptionally immature area in terms of exploration density,” Houseknecht said of the region as a whole.
The Barrow Arch, a structural high in the subsurface geology, has been a focus of exploration and development for good reason, given that it forms a kind of underground ridge, towards the crest of which oil and gas migrates upwards from prolific source rocks to the north and south. And the earlier oil fields discovered on the arch have traps with strong structural components, in which the folding and faulting of the rock strata have juxtaposed reservoir rocks against strata that seal hydrocarbons from escape to the surface.
Stratigraphic trapsBut the development of the Alpine oil field in the Colville River Delta region, together with the more recent Torok and Nanushuk discoveries, have demonstrated that stratigraphic traps in the region, traps formed from the geometry of the strata as they were deposited, also have huge hydrocarbon potential.
“This has really revolutionized exploration on the North Slope, in particular because everyone always thought the stratigraphic traps were too small to be economically viable,” Houseknecht said. “And what we know now, based on the discovery of Alpine and more recently some of the Nanushuk and Torok discoveries, is that these are not only economically viable, they represent the biggest three or four oil fields that have been discovered in Arctic Alaska.”
While smaller than the Prudhoe Bay field, new discoveries could approach the Kuparuk River field in scale, depending on how development progresses. And discoveries near to existing infrastructure appear economically viable. However, Houseknecht emphasized that, with little data yet released for the new discoveries, people do not truly know how big they are.
“We’re at the point of the blind man describing the elephant and we’re not sure whether we have a hold of the trunk or the tail yet,” he said.
An upside of the new Nanushuk and Torok play is that it extends across a huge area, with seismic data suggesting similar hydrocarbon traps all the way west to the Chukchi Sea coast, well offshore towards the Beaufort Sea shelf edge, and even out across parts of the Chukchi Sea. And a tantalizing issue is that the 3-D seismic data needed to delineate specific prospects only covers a very small part of this huge region.
Shelf marginGeologists have figured out that the rocks were formed by the filling of a deep marine basin with sediment, with the sediment pouring out into the basin from west to east. As the basin filled, the shelf that marked the western margin of the basin shifted west to east across what is now NPR-A before stopping just east of where the Colville River now flows.
At times when the sea level in the ancient basin was somewhat low, clean sand was deposited in a shore system, to later form the sand reservoir bodies found in the Pikka and Willow discoveries. Then, as the sea level dropped lower, much sand was eroded from the shelf edge and dumped at the base of the basin slope, to form the type of reservoir found in the Torok at Smith Bay. At Smith Bay it appears that multiple pulses of sand were cycled to the toe of the basin slope, with each pulse of sand representing a distinct shelf margin, Houseknecht said.
Estimation challengesInspection of a west to east seismic section across NPR-A reveals one of the challenges that the USGS geologists face in preparing a resource estimate. Under the more westerly part of the reserve the curved shapes that depict the erstwhile migrating basin margin are quite feint, while to the east the curves suddenly become more distinct. The USGS geologists attribute this change to a change in the rate at which the basin margin was migrating across the region: Radiometric dating of the sediments indicates a much faster rate of migration in the west than in the east. Presumably, with more time for the rising and falling ancient sea to wash away at the basin margin in the east than in the west, the margin became more distinctly delineated.
But the really prospective basin margins that have been discovered in the eastern NPR-A sit in the area with the more distinct basin margins. So, one uncertainty revolves around whether the more western part of the region contains hydrocarbon reservoirs of similar quality to those in the east.
Analysis of available 3-D seismic data has revealed intriguing results, with dozens of potential exploration targets that have yet to be tested. Houseknecht showed an illustration of one seismic anomaly that covers about 44 square miles: No one knows whether this feature contains oil but, if oil is present, there could be 300 million barrels or more of recoverable resource.
“This is the kind of data we are in the process of reviewing, in order to update our assessment of the Nanushuk and Torok,” Houseknecht said, cautioning that this type of detailed seismic data is only available for the northeastern part of the NPR-A.
One general concern with the Nanushuk and Torok play is the potential for poor reservoir quality. The impact of the reservoir quality on oil recoverability will not be understood until some field development takes place, Houseknecht said.
ANWR coastal plainTurning to the coastal plain of ANWR, Houseknecht commented that the region has not been re-assessed for about 20 years because no new data for the region has become available over that time. There are about 1,451 line miles of old 2-D seismic data of poor quality. As a precursor to its planned new assessment, the USGS is having this data reprocessed, to try to tease out more detail than is apparent in the processed data currently available. The USGS scientists anticipate conducting an interpretation of the reprocessed data soon, Houseknecht said.
However, given the wide spacing between the seismic lines, it would be possible to hide a stratigraphic trap on the scale of Willow or Pikka between the lines, he said.
One area badly in need of better insights from seismic is the eastern part of the coastal plain, where large subsurface structures attracted exploration interest in the past and where, as a consequence, Arctic Slope Regional Corp. acquired some subsurface land. Given that geologists know that the Ellesmerian sequence, including the prolific Shublik source rock and Ivishak reservoir formation, has in the past been eroded out of much of the ANWR coastal plain, there is a critical question regarding whether the Ellesmerian sequence exists under the eastern structures. Sunken fault blocks known to exist under the nearshore Beaufort Sea nearby ANWR have preserved the Ellesmerian strata. But, do similar fault blocks exist under the eastern part of the ANWR coastal plain?
Aside from the question of the presence or otherwise of the Ellesmerian, the Brookian sequence shows much hydrocarbon potential in the ANWR coastal plain, Houseknecht said. There are structural hydrocarbon traps in the east, some oozing oil, while in the west there are stratigraphic traps, analogous to those in the Nanushuk and Torok of the NPR-A.
A range of possibilitiesHouseknecht also cautioned that resource assessments can only estimate a range of possibilities for how much oil and gas may exist in a region. And that estimated range tends to change, as more information becomes available. For example, the estimate for NPR-A increased following the discovery of the neighboring Alpine oil field, decreased again when more gas than expected was discovered in subsequent NPR-A exploration drilling, and now seems set to increase again, following the Nanushuk and Torok discoveries.