Alaska is well-known for containing some of the world's richest oil and natural gas deposits, also called hydrocarbon formations. Hydrocarbon formations exist where they do as a direct result of regional geologic activity that has spanned millions of years. The National Petroleum Reserve - Alaska (NPRA) is one of these places, and scientists estimate that 896 million barrels of oil and 53 trillion cubic feet of natural gas remain to be discovered.
Hydrocarbon Formations
Hydrocarbon formations are found in ancient sedimentary basins, where cycles in sea-levels deposit multiple layers of sedimentary material, burying and re-burying organic material. In order for the organic material to become an oil and gas reservoir, two types of sedimentary rock must exist: 1.) a hydrocarbon trap rock that is porous, such as sandstone, and 2.) a hydrocarbon cap rock that is impermeable, such as shale. As sea-levels rise and fall over periods of millions of years, layers upon layers of organic material are buried. As these layers of organic material sink farther down in the stratigraphic column, the pressure from the overlying sediment compresses the organic material and it eventually turns into coal, oil or natural gas.
In Alaska, a similar process took place along the North Slope, but it wasn't just sea-levels changing that buried the organic matter. This region of the Earth's crust has always been very tectonically active. Today, the subduction of the Pacific Plate under the North American Plate has created the very active Aleutian Volcanic Arc, and Alaska is vulnerable to earthquake hazards. The tectonic evolution of Alaska has also contributed to the presence of hydrocarbon formations, since the folding and stretching of crust is a process that leads to basin formation.
The Formation of Fossil Fuels in the North Slope
The tectonic evolution of Alaska has been complex and includes crustal deformation that led to mountain building events (also called orogeny), as well as rifting events. Scientists have pieced together the mutliple events that took place over the last 355 million years to come up with a sequence of events that led to the accumulation of gas and oil in NPRA. These events have been categorized into three Major Sequences, which are roughly summerized below:
- The Ellesmerian Sequence - the hydrocarbon formations within this sequence were deposited 355 to 215 million years ago. This basin was created as a result of the proto-Pacific Plate, an ancient plate that is now under Siberia, an ancient plate that is now part of Europe and the proto-North American plate all crashing into each other. The forces caused the crust along the edges to fold at two locations, called the Ellesmerian orogenic belts. A basin was created in the middle of these two orogenic belts, and this is where the Ellesmerian sequence was deposited.
- The Beaufortian Sequence - the hydrocarbon formations within this sequence were deposited 215 to 113 million years ago. This basin was created as a result of rifting that created what is now the Western Canada Basin and the Beaufort Sea.
- The Brookian Sequence - the hydrocarbon formations within this sequence were deposited 145 to 15 million years ago. Tectonic activity during this period was marked by crustal compression that caused the crust to uplift in some places (what is now Brooks Mountain Range) and be buried in others (present day location of the coastal plains).
During the evolution of all these sequences, the sea-level changed multiple times. The tectonic activity produced giant basins and sedimentary material filled in those basins burying organic material. Today, these basins are stacked one on top of the other and multiple exploration and drilling projects are focused on capturing the valuable resources within these sequences.
Sources:
Kumar et al., 2002, "A Digital Atlas of Hydrocarbons Within and Adjacent to The National Petroleum Reserve - Alaska", usgs.gov
The National Petroluem Reserve Fact Sheet 2010, usgs.gov
Hubbard et al., 1987, "Geoogic Evolution and hydrocarbon habitat of the 'Arctic Alaska Microplate'". Journal of Marine and Petroleum Geology, Volume 4.
Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
