Essay: The (Long) Story of ANWR
They Don't Call It Fossil Fuel for Nothing
Oil and natural gas are made of hydrocarbons: chemical compounds that contain mostly carbon and hydrogen. Hydrocarbons are created when organic matter—everything from leaf litter to gastropods to marine algae and zooplankton—accumulates and is cooked in the interior of the Earth. Depending on the temperatures and pressures the matter is subjected to, the end product is either oil or gas. Denser than gas, oil contains more energy per cubic centimeter and is therefore more valuable.
Oil and gas, which usually occur in combination, typically accumulate in sedimentary rocks: rocks formed when broken fragments of pre-existing rocks are deposited somewhere by water or wind. As the sediment accumulates, the layers at the bottom of the sedimentary basin are subjected to increasing pressure and temperature (if you drill into the Earth, the temperature increases by about 30 degrees Centigrade per kilometer), forming trapped reservoirs of oil and gas. This process occurs over millions of years, which is why hydrocarbons are referred to as fossil fuels: like fossilized remains of plants and animals, they, too, formed in the geologic past.
Where in the World Are These Deposits Found?
“Oil and gas accumulations are relatively rare,” explains Mark Myers. “In many places some, but not all, of the elements are in place.”
Those elements include source rock, formed from organic-rich sediments cooked under high temperature and pressure; reservoir rock, a body of rock with open void space where hydrocarbons have accumulated the way a sponge holds water; and a trapping mechanism, such as cap rock, which keeps the oil in the reservoir.
Bringing all these elements together, oil deposits are typically found along continental shelves—at the present-day edges of continents, or where continents ended in the distant past. Thanks to the record preserved in rock, we can trace the key sequences of geologic events, going back tens to hundreds of millions of years, that make Alaska's North Slope an ideal target for oil exploration.
Alaska's North Slope: Oil and Gas Fields in the Making
From a geologist's standpoint, the best place to look for oil and gas is near where other oil and gas has been found. You're surrounded by that potential in the 1002 area.
-Mark D. Myers, Alaska Division of Oil and Gas
Alaska's North Slope, and the Brooks Range just to the south, constitute a portion of a terrane referred to as the Arctic Alaska Microplate, a continental fragment that now contains several large oil and gas fields, including Prudhoe Bay, Kuparuk, NPRA, Alpine, and Badami. Geological studies indicate that this microplate was once located nearer to the Canadian Arctic Islands. Several key events in this microplate's geologic past make it especially attractive for hydrocarbon exploration.
In the Beginning: A Slowly Subsiding Continental Shelf
Three hundred and seventy to 210 million years ago, a passive continental margin existed within the Arctic Alaska Microplate. Passive margins involve little tectonic activity, no collision or subduction between plates, and most importantly, delivery of sediments to the continental edge by agents of erosion such as wind and water, which leads to subsidence (gradual depression of the plate) due to the weight of accumulating sedimentary debris. (The Eastern seaboard of North America is a current example of a passive continental margin.) The passive margin allowed for the accumulation of thick piles of marine and shallow-marine sediments, including sands, silts, mud, and, when the amount of suspended sediments was low, for the formation of marine carbonates such as limestone and dolomite. Over millions of years of subsequent burial and heating, these sediments were transformed into sandstones (reservoir rocks), shales (source rocks and/or cap rocks), and carbonate rocks (reservoir rocks)—the perfect ingredients for an oil and gas system.
From Passive to Active: Creation of a New Ocean Basin
Approximately 200 to 130 million years ago the tectonics of the Arctic Alaska Microplate changed from a slowly subsiding passive margin to an active rift margin. An active rift margin is formed when plates move apart, allowing a new ocean basin to form (such as the modern Red Sea/Gulf of Aden region). A greater amount of opening at one end of the rift caused the microplate to rotate approximately 60 to 70 degrees counterclockwise to accommodate the new oceanic crust that was formed. Upwelling of the Earth's hot mantle during the rifting caused the microplate to heat up and rise, exposing uplifted regions to erosion by wind and water, leading to additional sediment accumulation. The higher temperatures also sped up and increased the maturation of organic matter to produce more hydrocarbons. And finally, the new orientation of the rock layers allowed for the creation of hydrocarbon traps.
Crash! Continents on a Collision Course
Creation of this new oceanic crust and basin eventually led to collision of the Arctic Alaska Microplate with terranes in Alaska's south and southwest (roughly 65 to 20 million years ago). This violent collision produced the uplifted Brooks Range mountains and a thick wedge of sediments shed during their creation. The lowermost stratographic unit of this sequence is the organic-carbon-rich Hue Shale, an excellent oil-producing source rock. And Sagavanirktok sandstone, which crops out at the surface of the 1002 area, has oil stains indicating that it was once an oil reservoir and may still be a reservoir at greater depths. Beginning roughly 65 million years ago, and continuing today, the creation of folds and thrust faults (similar to those found in the western portion of the Appalachian Mountains and eastern margin of the Canadian Rockies) resulted in numerous fold- and/or fault-related structural traps for hydrocarbons.
It is crucial to note that structural traps in the region were created before most of the hydrocarbon migration, which took place roughly 50 to 45 million years ago. “So you had an excellent reservoir rifted at exactly the right place for source rocks to be laid over the top. Given the vast area and the millions of years involved, the combination of location and timing worked out perfectly,” says Charles Mull, petroleum geologist, Alaska Division of Oil and Gas.
Of course, continued structural deformation to the present day leads to some breaching of early-formed traps, resulting in seeps or pools of oil right on the North Slope's surface. ANWR contains quite of few of these, which Jennifer Burton, site geologist with Anadarko Petroleum calls "a fantastic indicator that you've got an active petroleum system." Charles Mull makes the point, saying, “In many places, you can break off a chunk of sandstone and it smells like your car's crank case.”
The Bottom Line
Although the geology strongly suggests that there is oil under ANWR, our picture of the subsurface and how much oil we could find is still a bit blurred. The same surface geology can lead to widely varying amounts of petroleum reserves underground. The detailed configuration and rock characteristics greatly affect how much oil can be practically and profitably extracted from a given reserve. And while new tools enable geologists and petroleum engineers to “read the rocks” before and during exploration with ever greater accuracy, the window of opportunity closed in 1985, when ANWR's coastal plain was closed to further geophysical surveys because of environmental concerns. Despite everything we've learned about ANWR's past, the Earth still guards many of its closely-held secrets.
The most recent analysis by the USGS of seismic survey data acquired in the mid '80s concludes that ANWR's 1002 area contains between 4.3 and 11.8 billion barrels of technically recoverable oil. With such a wide range, it's no wonder the portrait of ANWR warps and wavers according to the agenda of those doing the talking.
The bottom line is the geology strongly suggests significant oil and gas potential in there, but we ultimately don't know until we go out there and explore.
-Mark D. Myers, Alaska Division of Oil and Gas
More About This Resource...
Our innovative Science Bulletins are an online and exhibition program that offers the public a window into the excitement of scientific discovery. This essay was published in December 2002 as part of the Reading the Rocks: The Search for Oil in ANWR Earth Feature.
- It begins with an overview about how oil and gas are formed, explaining why oil is the more valuable of the two.
- After explaining that oil and gas accumulations are relatively rare, it details the history of Alaska's North Slope and why it is attractive for hydrocarbon exploration.
- It concludes by stating that how much oil is under ANWR is still blurred, with estimates ranging from 4.3 to 11.8 billion barrels of recoverable oil.
Supplement a study of earth science with a classroom activity drawn from this Science Bulletin essay.
- Ask students what they know about oil and gas. How are they formed? Where are reserves located? How did scientists identify those locations?
- Have them read the essay (either online or a printed copy).
- Have them research oil consumption in the US and calculate the range of time 4.3 and 11.8 billion barrels would last for.
SubtopicMinerals and Resources