The Geology of ANWR

Surface Geology

Nearly all of the surface of the 1002 area is covered by a thin layer, less than 30 m thick, of unconsolidated, frozen silt- to gravel-sized sediments of nonmarine origin. They originated from the erosion of the Brooks Range during the late Cenozoic.

The surface exposures of rock formations inside the 1002 area are mostly restricted to deposits of the Sagavanirktok, Jago River, and Canning Formations of Tertiary age. There are some smaller outcrops of Hue Shale, pebble shale unit, and Kingak Shale from the Cretaceous and the Jurassic. To the east of the Sadlerochit Mountains, in the southern border of the 1002 area, some of the oldest exposures, of limestones from the Mississippian Lisburne Group, are found.

Subsurface Structure

Tectonics

During the rifting that separated northern Alaska from the Canadian Arctic Island, the Barrow arch, a structural high, was formed. The northern side of the Barrow arch formed the continental margin, while the southern flank received the thrust of the formation of the Brooks Range orogen.

The part of the 1002 area located to the south and east of the Marsh Creek anticline and north of 69-degrees is the Brooks Range orogen. The Brooks Range is more than 1000 km long and up to 300 km wide, and the distribution and character of its major structures is not symmetrical. Deformations in this region occurred during the Cenozoic era, forming both east-northeastward and eastward structures. Parts of Jurassic to Cretaceous shales, Mississippian shales, and of a horizon in the pre-Mississippian basement rocks were separated from their source due to extreme folding and compression.

The structure of the Brooks Range is formed by a series of broad anticlines with a core of pre-Mississippian rocks and younger rocks deformed on the borders. To the north of the Brooks Range, the structure, called thin-skinned deformation, is composed of numerous folds and faults developed in rocks of Cenozoic and Mesozoic age. Several broad domes are present in pre-Mississippian rocks in that area.

Stratigraphy

The stratigraphic record of the North Slope is divided into three sequences: Franklinian, Ellesmerian and Brookian.

The Franklinian sequence comprises a thick succession of mainly sedimentary rocks with a minor amount of igneous rocks of Cambrian to Devonian age that lie beneath the pre-Mississippian unconformity, where a gap in the geologic record exists.

Two separate layers of rock are present in northern Alaska. The shallow marine carbonates are composed of rocks of Proterozoic age and rocks of Lower Devonian age, separated by unconformities where Silurian strata are missing. They are composed of quartzite (metamorphic rock consisting of quartz grains, formed by recrystalization of sandstone), argillite compact rock derived from mudstone or shale, product of weak metamorphism), and basalt (dark-colored igneous rock), with intrusions of sandstone (the consolidate equivalent of sand, with 85-90% quartz), and shale (fine-grained sedimentary rock, formed by clay, silt or mud). The deeper marine layers include quartzite, conglomerate, phyllite (metamorphic rock, finer than schist), argilite, limestone (sedimentary rock formed by calcium carbonates), and granite (light-colored, coarse-grained igneous rock). The compression, uplift and erosion of this sequence during the Ellesmerian orogeny formed the pre-Mississippian unconformity.

The Ellesmerian sequence is hundreds of meters thick, and is composed of layers of marine and nonmarine sedimentary rocks of Middle Devonian to Triassic age, that rests on top of the pre-Mississippian unconformity. The lower unit of this sequence is the Endicott Group, which, in the 1002 area, consists of Mississippian coal-bearing sandstone, conglomerate, and shale of the Kekiktuk Conglomerate and Kayak Shale, of Devonian and Mississippian age. The Kekiktuk Conglomerate is a proven oil-bearing reservoir. Large amounts of limestone and dolomine of the Lisburne Group were deposited in the North Slope up to a thickness of 500 to 1000 meters during the Mississippian and Pennsylvanian. It is separated from the overlying Sadlerochit Group by an unconformity formed during the Middle Pennsylvanian to Early Permian in when sea level fell.

Water resources

Ice: The North Slope is covered in snow from October to May of every year, and while snow might fall during any month, the precipitation during the summer is mainly rain. In spring, melting snow is the main source of water for lakes and streams in that region, and water stored in snowdrifts is released slowly and provides water for them during the summer. In the 1002 area, the depth of the snow increases considerably away from the coast.

Ice: Around mid-September, water bodies freeze across the North Slope, with an average maximum thickness of 1.80 m at the end of winter. By the middle of January, the ice is thick enough to support a loaded C-130 Hercules or a D-8 tractor.

Ground water: There is very little liquid ground water in the coastal plain, but there is ground ice underlying the surface, which can't be used as a water supply.

Thermal Maturity

Thermal maturity is the level of alteration of the organic matter in a given sedimentary rock due to high temperatures. It gives the absolute maximum temperature to which the rock has been exposed. When there is no igneous source of heat, the maximum temperature measured is just a product of burial heating. Therefore, the thermal maturity of a rock can be used to determine the uplift it has suffered, and this can show broad patterns of geologic structure in a studied region.

The tectonic makeup of Alaska is very complex and there is little stratigraphic correlation between areas. There, thermal maturity is used to determine the structure of basins and the deformations they have.

Thermal maturity indicators:

The most common available data for Alaska comes from vitrinite reflectance and conodon color alteration.

Vitrinite is organic matter composed of remains of woody plants that is found in classic sedimentary rocks. It originated in terrestrial environments. During burial heating, vitrinite loses volatile compounds and the carbon turns into graphite, increasing its reflectively. This reflectively is proportional to the temperature reached, is not influenced by pressure or common chemical reactions, and doesn't return to the original state after it has reached a given level. For heating periods greater than 1,000 years, the reflectance of vitrinite depends only on the maximum temperature reached and not in the time.

Conodons are teeth-like fossils of primitive fish that existed from the Cambrian to the Triassic. They were deposited in marine environments, and are found in carbonates and loose classic sedimentary rocks. Conodons are composed of layers of apatite (calcium fluoride phosphate Ca5F(PO4)3) and organic matter. When conodons are buried and the temperature rises, the organic matter is carbonized and its color changes from light yellow to black, then to white and finally to clear. There are established standards between the color of conodons and the temperature they were subject to.

Regional thermal maturity patters:

Tertiary basins of interior Alaska: The low thermal maturity of surface rocks show that the basins are at their maximum burial depth and they have suffered no uplift.

Aleutian Arc: The rocks mature with depth, being under mature at the surface. This shows that the basins are near their maximum burial depth.

Colville basin (North Slope): At the surface, the thermal maturity of rocks increases from the coast at the north to the Brooks Range origin at the south.

Yukon-Koyukuk basin, Kuskokwin flysh (sandstone+schist) belt: There is a nearly concentric thermal maturity pattern, showing greater uplift at the basin margins. The basin as a whole suffered uplifting and deformation.

Kandik basin (east-central Alaska): The basin shows uplifts and a fold and thrust belt, placing younger mature rocks over older immature rocks.

Petroleum Systems

The petroleum systems- meaning where the areas in which petroleum is likely to be found coupled with information on what type of hydrocarbons are present and with which geologic features- found in the North Slope include:

Ellesmerian:

Source rocks: type II kerogen
- Shublik Formation (gas)
- Kingak Shale (gas)
- Pebble shale unit

Reservoir rocks:
- Endicott Group
- Sedlerochit Group (quartz-rich conglomerate sandstone)
- Kuparuk River Formation of Jamison
- Lisburne Group (carbonate)

Overburden (to mature source rocks):
- Brookian sequence (deltaic deposits)

Type of hydrocarbons:
- Low gravity (med to heavy hydrocarbons)
- High sulfur content

Torok-Nanushuk (North East North Slope):

Source rocks: type III kerogen
- Torok Formation
- Pebble shale unit

Reservoir rocks:
-Nanushuk Formation

(sandstones, derived from formation of Brooks Range)

Overburden (Brookine sequence):
- Colville Group (shales)
- Sagavanirktok Formation

Types of hydrocarbon:
- High gravity (med to light hydrocarbons)
- Low sulfur content

Hue-Sagavanirktok (Point Thomson to Canadian border):

Source rocks: type II kerogen
- Hue Shale

Other rocks (either overmature or with type III kerogen -gas prone-)
- Shublik Formation
- Kingak Shale
- Pebble shale unit

Reservoir rock:
- Sagavaniktok Formation
- Canning Formation

That concludes the brief overview of the geologic history of ANWR.

Sources

Johnson, M.J. & Howell D.G. (1999). Thermal Maturity of Sedimentary Basins in Alaska - An Overview. Thermal Maturity of Sedimentary Rocks in Alaska: Digital Resources. U.S. Geological Survey Bulletin 2142.

Chapter GG  Geographic and Geologic Setting
by Kenneth J. Bird
in the USGS Open File Report 98-34