Alaska North Slope Basin

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The Alaska North Slope Basin is located on the northern side of the Brooks Range located in Alaska, United States. The basin is bordered by Russia in the west, Canada in the north and east, and the United States in the south. This basin is considered a foreland basin due to its proximity to the Brooks mountain range. In total, the basin covers 240,000 km^2, 75,000 km^2 located offshore.

Contents History of the Alaska North Slope (ANS) Future Petroleum Potential Structural geology Tectonic history Depositional history Stratigraphy Petroleum geology Source rocks Traps and seals Hydrocarbon production Further readings Referefences

History of the Alaska North Slope (ANS)

The history of the ANS goes as far back as the 1920s when then President Harding created the National Petroleum Reserve - Alaska in 1923 when the conversion from coal to oil of the U.S. Navy started. Afterwards, the basin was nearly abandoned up until the events of WWII in the 1940s when exploration and drilling commenced in the region once more.


Map of North Alaska. The blue dotted line represents the south border of the ANS. NPRA to the west, Arctic Wildlife Range to the east, and Prudhoe Bay in the middle.

The Navy started drilling in 194 and by 1953 it had already tested out 3 oilfields: Umiat, Fish Creek, and Simpson and six gas fields: Barrow, Gubik, Titaluk, Wolf Creek, Square Lake, and Meade. Most of these wells were typically shallow drilled (~3000ft) which was not uncommon for its times and did not give the production output that was hoped for.

As the 1960s started Congress created the Arctic Wildlife Range (AWR) which grouped approximately nine million acres of the East ANS and removed them from future exploration projects. The Department of Interior and the Bureau of Land Management started to offer incentives and leases to oil companies to further explore the areas permitted west of the AWR in order to continue exploration in the area. The oil companies involved at the time were Richfield (present day ARCO), Humble (present day Exxon), and British Petroleum (BP).

By the late 1960s exploration and drilling operations from the major companies involved had slowed dramatically as the Alaska Native Claims Settlement Act (ANCSA) created a moratorium on federal lands. On March 12, 1968, after many resultless years, there was a discovery made in Prudhoe Bay with a joint effort from ARCO and Exxon’s State #1 Well that oil had struck. As soon as the discovery was made plans for new exploration wells started, the creation of the Trans-Alaska Pipeline System (TAPS) was announced and the state of Alaska 23rd lease sale earned almost one billion dollars net in bids. The increase in drilling after the events of Prudhoe Bay led to further discoveries of other fields such as Kuparuk thanks to a joint effort between Sinclair and BP.

“Oil and Gas Development on Alaska's North Slope: Past Results and Future Prospects ” Arthur C. Banet, Jr. U.S. Department of the Interior, Bureau of Land Management. BLM - Alaska Open File Report 34. March 1991.

Future Petroleum Potential

On January 23, 2020, the United States Geological Survey published an assessment of undiscovered oil and gas resources in the central ANS, where the majority of its oil production is located. The assessment divides up the central area in two tables and each table shows the estimated undiscovered resources divided into strata. The first table informs us about the number of oil and gas fields, their size, max, mins, medians, and calculated means while the latter table indicates the confidence levels of each strata for the resource amount assessed.

Engineering Operations

Since 2000, BP has been the sole operator of the Prudhoe Bay Reservoir, located within the Alaska North Slope Basin. Once retrieved from one of the thousands of wells pumping the reservoir, oil is sent through pipelines to pumping stations, processing centers, then on to further consumption.

Structural geology

The basin is shaped as an elongated oval from east to west and, as a foreland basin, was formed by lithospheric flexure, the bending and folding of the lithosphere which can be considered a trapping mechanism. The folds along the Alaska North Slope are 50 to 100 km long and occur at intervals of 20 to 25 km. Moving from south to north, the proximity of the folds gets closer and tighter. The geologic structure of the basin was developed between the late Cretaceous period and the early Tertiary period.

Tectonic history

The Brooks Range was created by a fold belt through Triassic period rocks that extend through Northern Alaska all the way north to Canada. There are deflections at both ends of the range that contribute to a larger sigmoidal deflection. Early rifting is present as the Siberian-Chukchi shelf has shifted southeastward. This rifting developed the deformed basin with folding and faulting present.

Depositional history

Late Cretaceous and Tertiary nonmarine sandstones make up the majority of the basin. The cretaceous rocks display a trend of being thicker in the south and are developed thicker in the north. Volcanic rocks and ash beds are present during the Jurassic period and Turonian period, predominating in the Seabee formation. Deep marine deposits are present and are covered by delta deposits, creating a rich depositional environment. At the surface, there is heavy ice that does not completely melt as well as braided streams.

Stratigraphy

Three major sequences compose the Alaska North Slope. These sequences include the Ellesmerian, the Beaufortian, and the Brookian.

The Ellesmerian Sequence was deposited from the Mississipian to the Triassic. The sequence thickens out into the Chucki Sea south into the Colville Trough and west into the Hannah Trough. It is composed of carbonates and marine deposits near the passive margin. The Ellesmerian Sequence is fundamental to hydrocarbon production in the Alaska North Slope, as it contains the sandstone reservoir found in Sadlerochit formation, where the Prudhoe Bay obtains its hydrocarbons from. The carbonate dominated Lisburne Group also has reservoir potential, as well as serves as great source rock.

The Beaufortian Sequence was deposited from the Jurrassic to the Lower Crustaceous. This sequence is primarily composed of shale and possesses syn-rift deposits caused by local erosion. The Lower Crustacean Unconformity, found within the sequence, functions as a pathway hydrocarbons which have been trapped by Crustaceous mudstones. One important Custaceous mudstone is the Pebble Shale Unit. The Pebble Shale Unit and Kingak Shale serve as two fundamental source rocks in the Beaufortian Sequence.

The Brookian Sequence was deposited from the Crustaceous to the Tertiary. This sequence is the thickest of the three and coexists with the Brooks Range. It is primarily composed of shallow marine and terrestrial siliciclastic deposits. Key formations found in the Brookian Sequence include the Hue Shale, Torok, Seabee, Schrader Bluff, and Nanushuk.


Petroleum geology

Source rocks

There are four key source rocks identified in the Alaska North Slope. These include the Middle-Upper Triassic Shublik Formation found in the Ellesmerian Sequence, the Kingak Shale and Pebble Shale Unit, both of which are found in the Beauforitian Sequence deposited during the Jurassic-Lower Crustaceous, and lastly the Crustaceous-Tertiary Cretaceous Hue Shale located in the Brookian Sequence.

Traps and seals

The Pebble Shale Unit located within the Beauforitian Sequences caps off the hydrocarbons in the Lower Cretaceous Unconformity. The northern regions of the Nanushuk and Torok formations mainly contain stratigraphic traps, and few stratagraphic-structural traps. The northeastern regions of these two formations primarily contain oil-charged traps with a few gas-charged traps closer to the southern border. The northwestern region also contains oil-charged traps, and more gas-charged traps. Within the southern regions of the Nanushuk and Torok formations, there are mainly structural traps and few stratagraphic-structural traps. Additionally, the Barrow Arch, a rock belt that runs from Utqiagvik to just west of the Arctic National Wildlife Refuge, also acts as an oil trap.

Hydrocarbon production

The Alaska North Slope is one of North America’s primary prolific oil-producing basins. Namely, the Prudhoe Bay Oil Field is one of the top producers of the ANS responsible for 12 billion of the almost 15 billion barrels produced from the basin. The source rock near the Prudhoe Bay reserves holds an estimated 2 billion barrels of recoverable gas which can be divided between tight oil and shale gas. The remaining 2.5 billion barrels of oil produced in the ANS is attributed to Kuparuk River with 2 billion barrels, and Duck Island with 446 million barrels. The Trans-Alaska Pipeline System is used to transport oil from the Alaska North Slope to Valdez in the Pacific Ocean.


References

Tailleur, Irvin L, and W. P. Brosge. “Tectonic History of Northern Alaska: ABSTRACT.” AAPG Bulletin, vol. 54, 1970, doi:10.1306/5d25cb61-16c1-11d7-8645000102c1865d. Wiley, Thomas J. “Sedimentary Basins of Offshore Alaska and Adjacent Regions.” Open-File Report, 1986, doi:10.3133/ofr8635. Houseknecht, D.W., Whidden, K.J., et al. 2020, Assessment of undiscovered oil and gas resources in the central North Slope of Alaska, 2020: U.S. Geological Survey Fact Sheet 2020–3001, 4 p., https://doi.org/10.3133/fs20203001. “Oil and Gas Development on Alaska's North Slope: Past Results and Future Prospects ” Arthur C. Banet, Jr. U.S. Department of the Interior, Bureau of Land Management. BLM - Alaska Open File Report 34. March 1991. Houseknecht, David W.; Bird, Kenneth J. "Studies by the U.S. Geological Survey in Alaska, 2005" (PDF). Professional Paper 1732–A Oil and Gas Resources of the Arctic Alaska Petroleum Province Kenneth J. Bird, Cornelius M. Molenaar (1992). "The North Slope Foreland Basin, Alaska: Chapter 13". 136. Houseknecht, David W.; Bird, Kenneth J. (2006). "Oil and gas resources of the Arctic Alaska Petroleum Province". Professional Paper. doi:10.3133/pp1732a. ISSN 2330-7102. Margaret Kriz Hobson (April 3, 2013). "SHALE OIL: Geologist's Alaska gamble could turn into America's next big shale play". EnergyWire, E & E Publishing. Retrieved April 4, 2013.

https://pubs.er.usgs.gov/publication/70030896

https://pubs.usgs.gov/fs/2017/3088/fs20173088.pdf

Schenk, C.J., and Houseknecht, D.W., 2008, Geologic Model for Oil and Gas Assessment of the Kemik-Thomson Play, Central North Slope, Alaska: U.S. Geological Survey Scientific Investigations Report 2008–5146, p. 28.

Houseknecht, D.W., Whidden, K.J., et al. 2020, Assessment of undiscovered oil and gas resources in the central North Slope of Alaska, 2020: U.S. Geological Survey Fact Sheet 2020–3001, 4 p., https://doi.org/10.3133/fs20203001.

External links

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/foreland-basin

https://en.wikipedia.org/wiki/Brooks_Range