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October 23, 2017

Although often associated with helping fuel the Nation’s growth during the Industrial Revolution, coal is very much part of our space-age present. In 2016, coal-fired power plants provided 30.4 percent of the country’s electricity, and it is an important source of employment in many states.

Image shows a sample of anthracite coal on a rock backdrop
This is anthracite, the highest rank of coal. It is a hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter. Anthracite is not as commonly mined as other ranks of coal. It played a significant role in Pennsylvania coal during the Industrial Revolution in the United States. Learn more about our coal research here: Donna Pizzarelli, USGS. Public domain.)

Here at the USGS, we have studied coal for more than 100 years, and worked with others, predominantly state geological surveys, to provide the basic geological research and  information to assess the Nation’s coal resources. We also coordinate with other federal agencies, including the Bureau of Land Management and the Energy Information Administration (EIA), who also have responsibilities for managing coal resources or reporting coal information. The largest and most well-known areas of coal the USGS has assessed are the Appalachian Basin and Illinois Basin in the eastern U.S. and the Williston Basin, Colorado Plateau,  and the Powder River Basin in the western United States.

But just as the role for coal has evolved over the years years, so have our scientific methodologies and procedures used in assessing this important resource.

Image shows coal fields of the conterminous united states color-coded by the type of coal
A map of the various coal fields of the conterminous United States.(Public domain.)

It’s Not How Much There Is, But How Much You Can Get

Traditionally, USGS coal assessments have focused on the remaining coal resources in a basin or region, based on minimum thickness and maximum depth of cover parameters.  

However, beginning with our 2015 assessment of the Powder River Basin in Wyoming and Montana, we began to calculate recoverable coal resources and coal reserves from the remaining coal resources.  Recoverable coal resources are calculated by subtracting coal resources that are lost due to environmental, societal, or legal restrictions, as well as those that are lost due to geological constraints and mining technology limitations.  Coal reserves, meanwhile, are the portion of recoverable coal resources that can be extracted profitably.  

The amount of recoverable coal resources is usually much smaller than the amount of remaining coal resources.   Likewise, coal reserves are usually just a small fraction of the recoverable coal resources.  

Image shows a flowchart of how coal reserves fit into coal resources
A chart showing the various stages of certainty in coal resources.(Public domain.)

For instance, in 2015, we estimated that the Powder River Basin contained a mean of about 1.07 trillion short tons of remaining coal resources. That’s how much coal we believe to be in the entire Powder River Basin. Of that 1.07 trillion short tons, we estimated that about 162 billion short tons were recoverable coal resources, after environmental, societal, and legal restrictions, geological constraints, and technological limitations were determined. Finally, out of that 162 billion short tons of recoverable coal resources, we estimated that only 25 billion short tons could be classified as coal reserves, based on the average price of Powder River Basin coal when we conducted the assessment.

So, as a result, the 1.07 trillion short tons of remaining coal resources in the Powder River Basin yields an estimated 25 billion short tons of coal reserves, or 2.3 percent of the remaining coal resources, based on the price of coal in 2013. This is important because it gives resource managers and companies a more complete picture of the amount of coal in the Powder River Basin.

The USGS is taking this approach forward in our current and future assessments of coal resources. Currently, we are assessing coal resources and reserves in the Green River Basin of Colorado and Wyoming. The Green River Basin contains large amounts of coal resources that have not been assessed in detail in previous assessment studies and much of the coal resources in this area lie under Federal lands, which we are mandated by Congress to assess.  Additionally, we have acquired a large amount of previously proprietary and unavailable geological data for this area.  

Image shows a map of Colorado, Utah and Wyoming with polygons of the Greater Green River Basin and the USGS coal assessment area
A map of the Greater Green River Basin in Colorado, Utah and Wyoming showing the coal assessment areas. (Public Domain)

Mapping the Cost Curves

One of the difficulties in determining coal reserves is that they fluctuate with the price of coal.  What may be profitable to mine one day may not be the next, as prices for commodities like coal can vary significantly. An assessment of coal reserves needs to be able to account for those changes.

In fact, current coal reserve estimates are not static: reserves grow and shrink as coal prices change. If reserve volumes shrink, it does not always mean that the amount of coal in the ground has changed-it can reflect  that less of the coal resources can be mined profitably.  If the price of coal increases, then the amount of recoverable coal resources classified as coal reserves may increase.

Coal mine in the Powder River Basin of Wyoming and Montana(Credit: USGS. Public domain.)

In our 2015 Powder River Basin assessment, we met the challenges of changing prices by including a cost curve that would allow readers to estimate what portions of the recoverable coal resources could be considered reserves at various price points.

The current and upcoming assessments in the Green River Basin will include similar cost curves.  We are working to make them even more sophisticated and accurate through price analysis and economic modeling based on analogs of mining operations in the region.  

Image shows a sample of lignite on a rock background
A sample of lignite, the lowest rank of coal. It is primarily mined for burning in steam-generation power plants. Read more about our coal research here: Donna Pizzarelli, USGS. Public domain.)

Refining the Models

As technology and modeling have advanced in recent years, what they can tell us about what lies beneath the Earth’s surface has also greatly expanded. Because some of these formations are extremely large and located hundreds, even thousands of feet below ground, models are essential to allowing us to accurately estimate coal resources and reserves.

To make sure our models are providing us the best information, we have started including stochastic geostatistical analysis modeling to determine the optimum spacing for test drill holes. The current standard, as prescribed in USGS Circular 891, is to drill test holes at an arbitrary spacing (every quarter mile) to collect geologic data.  The geologic data is modeled using mathematical algorithms to extrapolate out from the test drill holes to the entire formation. By utilizing geostatistical modeling, we can define the optimum spacing for drill holes needed to best define the geology.

In addition, we are looking at the other end of our assessments-how resource managers can use the assessments to help make the best decisions on how to address the resource potential. One of the ways we are doing this is by utilizing the assessments to determine which areas possess the optimum combination of mineable coal thickness, cover or overburden, and favorable geologic conditions to warrant further exploration and possible economic development.

Image shows four men in visibility gear and hard hats at a coal mine
USGS coal project personnel visiting Trapper Mine in northwest Colorado in June, 2016.(Credit: Brian Shaffer, USGS. Public domain.)

Start with Science

The United States is richly endowed with coal resources.  Coal has been and will remain, into the foreseeable future, an integral part of the fuel mix for the Nation’s electrical supply.  It is also critical to the revitalization of the Nation’s infrastructure, as some coal, known as metallurgical coal, provides the carbon used in the making of steel.  Even though the Powder River Basin is the largest deposit of low-sulfur, sub-bituminous coal in the world, it is just one of many large coal basins in the United States capable of providing sufficient coal resources for both electrical generation and steel making processes.  The USGS will continue to provide critical information for the Government, as well as for private industry, to define the location, extent, thickness, and quality of the Nation’s coal resources and reserves.  

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