Reading the Rocks: The Search for Oil in 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

At the turn of the twentieth century, early explorers found oil seeps and oil-stained sands in Alaska's North Slope, the area north of the Brooks Range. From the crest of the mountain range to the coastal areas of the Beaufort and Chuckchi seas, this region of rolling foothills, wild rivers, and coastal plain wetlands provides habitat for millions of waterfowl, caribou, Arctic peregrine falcons, and other wildlife.

Oil exploration began in 1923 in the federally owned, 23 million-acre area now known as the National Petroleum Reserve-Alaska (NPRA). Since then over 13 billion barrels of oil have flowed from more than a dozen fields through the Trans-Alaska Pipeline to the ice-free port of Valdez on Alaska's southern coast.

Wilderness, or Not? Murky Legislation Ignites a Turf War

In 1960, concern over preserving natural resources led Congress to designate 8.9 million acres of coastal plain and mountains of Northeast Alaska as the Arctic National Wildlife Range. Twenty years later, Congress passed the Alaska Lands Act, which doubled the size of the range to nearly 20 million acres, including 8 million acres as "wilderness," and renamed it the Arctic National Wildlife Refuge (ANWR). The entire refuge lies north of the Arctic Circle and 1,300 miles south of the North Pole.

Acknowledging the possible presence of valuable hydrocarbon reserves, Section 1002 of the Act set aside a 1.5 million-acre section of coastal plain at the northeastern tip of the refuge and authorized the evaluation of the oil and gas potential of the area by means other than drilling.

The dual status of ANWR's 1002 area set the stage for a turf war that has simmered ever since, as oil interests, environmentalists, politicians, and even scientists wrangle over whether to open up ANWR's coastal plain to energy development or preserve its integrity as part of the last near-pristine wilderness left in the United States.

Premium Habitat for Unique Wildlife


"It is a whole place, as true a wilderness as there is anywhere on this continent and unlike any other that I know of."

—Morris Udall, former U.S. Congressman

The coastal plain of the Arctic National Wildlife Refuge is a unique tundra ecosystem stretching just 110 miles along the Beaufort Sea and measuring only 14 to 50 miles across. Dotted with thousands of small ponds, the tundra turns in the south into gently rolling hills that become the foothills of the northern Brooks Range, which dominates the refuge with glacier-clad peaks up to 9,000 feet tall.

This unique compression of habitats concentrates an extraordinary variety of species, including caribou, three kinds of bears, wolves, wolverines, musk oxen, Arctic and red foxes, Dall sheep, trout and grayling, snow geese and tundra swans, and millions of birds that pass through during the brief Arctic summer. For tens of thousands of years, the Porcupine caribou herd has migrated north each spring to calve on the coastal plain and fatten up on its nutritious vegetation. The coastal waters support a variety of marine mammals, including the endangered bowhead whale.

Despite the intense winter cold, short summers, and waterlogged soil, the refuge is also home to many low shrubs, mosses, lichens, and grasses. Arctic vegetation grows slowly and is exceedingly susceptible to trampling. In some places, Native American trails thousands of years old are still visible.

Though only a small fraction of ANWR is under consideration for oil exploration, the coastal-plain area houses far greater biodiversity than anywhere else on Alaska's North Slope. A plethora of native plants and animals depend for their survival on a strip of land whose unforgiving climate and limited resources allow only a small window for survival and regeneration.

Oil to the Left of ANWR, Oil to the Right

The oil industry has made its mark on Alaska's North Slope, changing it from pristine arctic tundra to the site of a sprawling industrial complex. Major petroleum discoveries have been made to the east of ANWR's coastal plain, in Canada, and to the west lie the Prudhoe Bay, Lisburne, Endicott, Milne Point, and Kuparuk oil fields. Producing approximately 1.5 million barrels of oil a day, these fields are the source of approximately 25 percent of U.S. oil production.

The basis of all we know about the oil- and gas-bearing potential of the coastal plain is a U.S. Geological Survey (USGS) analysis of a single set of 2D seismic data acquired along a coarse three-mile by six-mile grid during the winters of 1984 and 1985, as well as projection of geological information from adjacent wells. In the late 1990's, as interest in ANWR's energy potential increased, scientists spent three years on a new USGS assessment of the 1.5 million--acre region. They ran the seismic data acquired in the mid-'80s through new computer models, conducted new field studies, and incorporated new information from 41 wells drilled over the years near the borders of the refuge.

Released in 1998, the report concluded that the ANWR 1002 area contains between 4.3 and 11.8 billion barrels (bbo) of technically recoverable oil, with a mean value of 7.7 bbo. About half as much profitable petroleum as Prudhoe Bay was estimated to hold in 1977, this is more than was previously estimated. Most of the oil is thought to lie in the western part of the reserve, which is closest to existing roads and pipelines. The report also concluded that most of the oil is likely to occur in a number of smaller pockets rather than in a single large accumulation, which makes recovery more expensive, with a greater tax on the environment. ""If 'the pie' has been cut into many small pieces, and/or late-stage faulting has allowed oil to leak out of the trapping structure, it will affect the economics of the prospect," explains Charlie Mandeville, research scientist at the American Museum of Natural History and former exploration geophysicist.

Plenty of Uncertainties Remain

Wildlife on Alaska's North Slope has managed to coexist with oil exploration, but the impact remains unclear. In March 2002, a USGS report based on 12 years of biological research concluded that wildlife in the 1002 region is especially vulnerable to the kinds of disturbances that development may bring. The report singled out snow geese as at risk of displacement because of increased activity, including air traffic, and said that the geese would not necessarily find adequate feeding grounds elsewhere. Denning polar bears, another fixture on the coastal plain, might also be adversely affected, but the report added that "aggressive and proactive management" could minimize the problem.

Other sensitive ecological issues include:

• The caribou question: Proponents of drilling point out that the Central Arctic herd, a separate group whose summer habitat encompasses the Prudhoe Bay and Kuparuk oil fields to the west, has grown in size since exploration began. However, the 1002 coastal plain provides calving habitat for a herd nearly five times as large as the Central Arctic herd in an area one-fifth the size. The USGS report said that "oil development will most likely restrict the location of concentrated calving areas." Since the Porcupine herd has little other high-quality habitat, and calf survival has been linked to the animals' ability to move freely, higher mortality is likely. The caribou also congregate with their newborn calves in the most rapidly greening areas, and there are some indications that they're being displaced from these favorite summer forage areas around existing oil fields to the west.


• The musk oxen question: Formerly found across arctic Alaska, musk oxen were wiped out in the mid nineteenth century. A small herd was reintroduced to the coastal plain in 1969. Numbers there have stabilized, and the population continues to expand to the east and west. Musk oxen do not migrate, but survive the brutal winters by hunkering down and conserving energy by limiting movement as much as possible. This renders them particularly "vulnerable to disturbances" from oil and gas exploration, according to the USGS report, because drilling activity is most intense during the winter.


• The fresh-water question: Roads made of water and crushed ice have now largely replaced their gravel counterparts during wintertime oil exploration. It takes about a million gallons of water to construct a mile-long stretch of ice road. While liquid water is plentiful around the Prudhoe oil fields, even at minus 20 degrees Fahrenheit, a 1989 survey estimated the total wintertime freshwater supplies in the ANWR coastal plain at only 9 million gallons. Shallow tundra lakes freeze solid, and the largest pockets of unfrozen water lie along the major interior rivers. Wildlife biologists are concerned that if these few wet lakes are depleted, migrating waterfowl may find less to eat, and fish that overwinter in the spring-fed Canning River may suffer.

High-tech Tools to the Rescue?

Interior Secretary Gale Norton has repeatedly maintained that modern technologies allow oil to be extracted from ANWR without harming the environment. New procedures—winter-only exploration, ice roads and airstrips, eco-sensitive seismic surveys, better waste disposal—have indeed greatly reduced the impact of oil exploration. Future drill pads could be served by short airstrips and placed far from nutritious cottongrass patches. Many musk oxen wear radio collars, allowing their movements to be tracked so contact can be minimized. Oil companies have learned where to build ramps or elevate pipelines so they don't interfere with caribou calving and migration. Supporters maintain that oil drilling in the coastal plain will require a total footprint—the amount of space taken up by infrastructure—no larger than that of the average airport.

Environmentalists argue, however, that no matter how sophisticated our tools, the infrastructure necessary to extract oil will always take a toll on the environment. Prudhoe Bay, too, was once pristine. And since the oil in the 1002 area is believed to be scattered in many small pockets, development is likely to crisscross the coastal plain in a tight web, leaving little refuge indeed.

What's in Store for ANWR?

In the more than two decades since a 2D seismic survey was conducted inside ANWR's 1002 area, technology has made huge advances, enabling scientists to generate detailed 3D portraits of the Earth's interior. A new seismic survey would certainly increase the resolution of structures and permit a tighter assessment of reservoirs in the area, but the hard to face fact of the matter (and what makes the debate so slippery) is that until we go up there and drill some prospects, we are dealing with no more than that — the prospect of one day producing oil from ANWR.

On April 18, 2002, the Senate rejected a provision to open the Arctic National Wildlife Refuge to oil exploration. However, at the end of Autumn 2002, Congress must reconcile two versions of an energy bill, and drilling in ANWR remains a top priority for the Bush administration.

Getting oil out of the Earth is a complicated and risky business. Success depends on evaluating many different kinds of information, some obtained from chipping a piece of rock off a mountain ridge, some from peering through an electron microscope or sifting through the slurry coming up from a well, and some obtained by geophysical exploration techniques.

Over the last few decades, new technologies have transformed the field of oil exploration, enabling geologists to:

• choose an optimal location and drill fewer wells. This results in fewer dry holes, cuts exploration costs, and reduces environmental impact.
• identify as-yet-untapped oil reserves in frontier areas.
• learn more about the reservoir rock as drilling proceeds (e.g., how readily the rock conducts fluid) and tailor extraction methods accordingly.
• track the status of a hydrocarbon reservoir across all stages of its life in order to extract the maximum amount. 

Lab Work Traces the Geologic History of the Rocks and the Region

Some of the most valuable information for scientists interested in "reading the rocks" comes from the microfossils of tiny living organisms left inside shales and sandstones. These help determine the age of the rock, and when geologists compare samples from different regions, they can infer a history of the area. Index fossils--organisms that lived on Earth only briefly--are especially useful as time markers that can be tracked from basin to basin.

Some microfossils help geologists track the thermal history of the surrounding rock: whether it's been heated enough to create hydrocarbons. Vitrinite represents the remains of woody plant material and is abundant in many shales. It becomes more reflective as it is buried, compacted, and heated. Vitrinite reflectivity is directly proportional to temperature, so it's widely used to measure whether sedimentary rocks that contain organic carbon reached temperatures high enough to produce hydrocarbons. Conodonts are small tooth-shaped microfossils related to early jawless fishes. They grew by acquiring thin layers of a transparent mineral called apatite interspersed with thin layers of organic matter. When sediments containing conodonts are converted to rock, the organic matter within the microfossil changes from yellow to brown to black to gray to crystal clear, depending on how hot the sediments got during the process. Analyzing this color spectrum is another way to measure whether sedimentary rocks have been heated enough to create hydrocarbons.

 Other information--such as the absolute age of rock layers, established through radiometric age-dating (measuring the rate at which certain elements naturally decay), and the relative age of rocks, established by examining position and the fossil content--also contributes to a detailed geologic portrait that is invaluable to geologists and petroleum engineers. Evaluating both the relative and absolute ages of various features in the rock allows geologists to assess when structural traps formed relative to the formation and migration of hydrocarbons in the region. This is extremely important in evaluating a potential oil reserve, because a trapping mechanism that developed relatively late is far less likely to be as effective as one that developed before a significant amount of hydrocarbons accumulated.

Exploration Begins with Sound Waves

Just as ultrasound uses acoustic waves to generate a picture of a fetus inside the womb, seismic exploration tracks the echoes made by acoustic waves as they travel into the subsurface and reflect back to the surface. Conducting a 3D seismic survey involves laying down straight lines of sensitive microphones across the ground in a tight grid, typically 100 feet apart. Specially designed vibrating trucks, which have replaced the dynamite charges used until about 20 years ago, generate sound waves of different frequencies. The microphones pick up the reflected signals, which are recorded on magnetic tape and sent to data processors. These in turn generate a near-continuous cube of dataa three-dimensional portrait of the inner Earthrendering maps and cross sections obsolete. As former exploration geophysicist and current research scientist at the American Museum of Natural History, Charlie Mandeville, points out, "This is an immense asset in an area of complex structural geology."

Drills on the Cutting Edge

Computer-generated simulations of Earth's crust are full of clues about where to drill-but they're only clues. As Jennifer Burton, site geologist with Anadarko Petroleum puts it, "The well bore is the ground truth."

Not long ago, the drill consisted of fairly rigid 4 1/2-inch steel pipes, connected to a bit, that were rotated from the surface. Coiled-tube drilling has changed all that. The tube itself is made of flexible steel stored on huge spools and capable of bending in a big arc. Once in the subsurface, the pipe stays stationary as "mud" (once actual mud, and now a synthetic mixture) is pumped down the drill hole at high pressure. This mud powers a turbine that rotates a bit at the end of the tubing.

The mud is dense enough to float small rock fragments, called cuttings, to the surface, which technicians analyze for yet more information about the rock's oil-bearing potential. Sensors studding the drill surface also transmit a steady stream of data about rock type, thickness, temperature, porosity, water saturation (indicating what percentage of oil remains in the rock), and other characteristics. Not only is far more information available, it's available in real time, which is crucial during horizontal drilling because it enables geoscientists to follow thin reservoir layers that contain oil and/or gas.

Horizontal drilling is the other major breakthrough. Unlike the near-vertical paths of earlier conventional drilling, coiled-tube technology makes it possible to drill virtually horizontally, and to travel four or fives miles out from the borehole. That means that a drill pad doesn't have to be placed in an environmentally sensitive area like a wetland or migration route in order for the oil beneath the area to be extracted. Several boreholes can extend in different directions from a single drill pad, and old wells can provide access for horizontal drills. The payoff is big. According to the Department of Energy, "Because of both economic and environmental restrictions, development of the North Slope would have halted several years ago without the advent of horizontal wells."

An Evolving Picture of the Rock Below

Oil exploration really gets fine-tuned when seismic and drilling information are combined over the life of a reservoir. At the cutting edge is 4D seismic (the fourth dimension is time), in which a time-lapse picture is built out of data re-recorded at intervals and plotted by computer onto a 3D model. Combined with direct samples acquired during drilling, this data shows petrochemical engineers and production geologists how characteristics such as fluid saturation, temperature, and fluid movements change over time. This in turn guides secondary and tertiary recovery efforts, because, as Mandeville explains, "What you flood the reservoir with, to force the oil and gas out, depends on what type of cements and framework grains are present." Further, 4D seismic data allows exploration geologists and geophysicists to better recognize the characteristic seismic "signatures" of hydrocarbon-bearing rock formations versus those that hold water, an insight they can apply to future explorations. 

The Limits of High-Tech Tools: a Limited Oil Supply

According to figures from the U.S. Department of Energy, at the current rate of consumption--approximately 77 million barrels a day, from a global reserve of approximately 1,000 billion barrels--only 36 years' worth of crude oil remain within the Earth's crust.

Some energy experts believe that advances in exploration and drilling technologies will simply empty existing reserves faster. They foresee a peak in oil production around 2010, followed by a steep decline by mid-century. 

Others argue that these advances, paired with higher prices, will greatly increase the amount of oil that can be extracted economically from reserves, as has indeed happened in recent decades. They tend to assume that production rates will stay virtually steady until the supply runs out. This optimistic position is exemplified in a March 2002 report from the Department of Energy titled "Oil Resources in the Twenty-First Century: What Shortage?" The report anticipates that non-conventional resources--defined as those that "cannot be produced economically at today's prices and technology"--"will act as a buffer against prolonged periods of high oil prices well into the middle of this century, and perhaps well beyond." No one disputes that oil reserves are finite and only a few decades separate the two sides of the debate.

As Mark Myers, director of the Alaska Division of Oil and Gas, puts it, "Development in ANWR would have significant effects and advantages for the United States. But it's not going to solve the problem. We need multiple tiers to improve our energy situation. We need better energy efficiency. We need alternate fuels technology."