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October 14, 2020

In south Texas, a wide band of rocks stretches from the Mexican border all the way to western Louisiana, forming the highly productive Eagle Ford Group. These formations, made up primarily of shale and mudstone, are some of the most prolific oil and gas-producing rocks in the United States.

The Eagle Ford Group was also one of the first formations to be unconventionally drilled as part of the U.S. Shale Revolution, and since then, tens of thousands of wells have been drilled.

The USGS has assessed recoverable oil and gas resources in parts of the Eagle Ford Group several times, with the most recent assessment in 2018. In 2019, the USGS built upon the 2018 petroleum assessment with an assessment of water and proppant  requirements and water production that would potentially be associated with producing the undiscovered oil and gas resource.

An image of USGS research drilling rig on side of U.S. Route 90 in Kinney County, Texas.  The sky is streaked with clouds.
This 2018 image shows a coring and geophysical well-logging operation into the Eagle Ford Group adjacent to U.S. Route 90, Kinney County, Texas. (Credit: Dr. Stanley T. Paxton, US Geological Survey. Public domain.)

The Oil and Gas Assessment

In 2018, the USGS assessed potential quantities of oil and gas resources that could be produced in the Eagle Ford Group, estimating a mean of 8.5 billion barrels of oil, 66 trillion cubic feet of natural gas, and 1.9 billion barrels of natural gas liquids. This assessment was unique, because it ranks in the top five of assessments in both the oil and gas categories. 

USGS oil and gas assessments provide a probabilistic estimate, meaning the USGS estimate contains a range of potential amounts of oil and gas. For the Eagle Ford Group assessment, the USGS estimates a 95% chance of there being at least 5.27 billion barrels of oil and a 5% chance of there being at least 12.85 billion barrels of oil.

USGS petroleum assessments estimate undiscovered, technically recoverable resources. Undiscovered resources are those that are estimated to exist based on geologic knowledge and statistical analysis of known resources, while technically recoverable resources are those that can be produced using currently available technology and industry practices. Whether or not it is profitable to produce these resources is not evaluated.

In the past, this is where USGS oil and gas assessments have stopped. However, new assessment approaches have been developed in recent years, expanding what a USGS energy assessment can provide.

Image: Withdrawing Water for Hydraulic Fracturing
Equipment set up to pump water from a lake to an impoundment for hydraulic fracturing in the Fayetteville Shale of Arkansas.

Like Oil and Water

Water plays a significant role in the development of oil and gas. It is necessary to the production of oil and gas, so knowing how much water is needed is important for decision makers and resource managers. Oil and gas production often occurs in areas with limited water supplies, as well as areas with many competing users for available water resources, such as residential neighborhoods, agriculture, or other industry.

The resources in the Eagle Ford Group are mostly continuous resources, which are dispersed throughout a geologic formation rather than existing as discrete, localized occurrences, such as those in conventional accumulations. Because of that, continuous resources commonly require special technical drilling and recovery methods, such as hydraulic fracturing, which require substantial water volumes.

Hydraulic fracturing involves pumping large volumes of fluid containing water and proppant (primarily sand) into the petroleum reservoir to hold open the newly created fractures and improve fluid-flow characteristics. The USGS estimates that, to produce the full amount of the continuous oil and gas resources of the Eagle Ford Group, a mean of about 672 billion gallons of water will be needed for the hydraulic fracturing process.

Water is also a key component of drilling mud, which is a mixture of water and other materials that helps with the drilling of wellbores. Drilling mud brings the rock cuttings to the surface, helps to keep the wellbore stable, and stabilizes the drilling assembly. The USGS estimates that a mean of about 16 billion gallons of water would be required to complete the drilling and cement process to produce the oil and gas in the Eagle Ford Group.

Image: Natural Gas Development Water Impoundment
A water impoundment at a drill pad in the Fayetteville Shale gas play of Arkansas. (Credit: Bill Cunningham, USGS. Public domain.)

High Production Values

Water is not only sent down the wellbore during the drilling and hydraulic fracturing of the well. It also returns to the surface, along with the oil and gas, as a mix of flowback and formation waters. Formation waters are those volumes of water that exist in the rock formation with the oil and gas and are often produced alongside the oil and gas.

Produced waters often must be specially stored and disposed of due to the substances that are dissolved in them. Most produced waters have high levels of salts, and many have high concentrations of heavy metals or even radioactive materials. Knowing how much water could be produced alongside the oil and gas will help resource managers and decisionmakers know what storage capacity they will have to prepare for.

The USGS estimates that, during production of the oil and gas in the Eagle Ford Group, a mean of about 177 billion gallons of produced waters could be brought up along with the oil and gas.

Image: Hydraulic Fracturing Sand
Fine-grained silica sand is mixed with chemicals and water before being pumped into rock formations to prevent the newly created artificial fractures from closing after hydraulic fracturing is completed.​​​​​​​ (Credit: Bill Cunningham, USGS. Public domain.)

The Nitty Gritty

Hydraulic fracturing requires not only water to fracture the rock, but also something to hold the fractures open so the oil and gas can flow to the wellbore to be produced. The material that keeps the fractures open is called proppant and is usually a high-quality silica sand.

The USGS issued a report describing the types of sand most commonly used for proppant and where they can be found. The majority, around 70%, of sand used for proppant comes from the Great Lakes Region.

In the Eagle Ford Group, silica sands are the primary type of proppant needed. As part of its assessment for water resources and production in the Eagle Ford Group, the USGS also included an estimate for the amount of proppant that would be required: a mean of about 420 million tons.

Start with Science

The USGS assesses oil and gas resources as part of informing decision makers and resource managers about the amount of energy resources available and the other materials required to produce that energy. This update to the 2018 USGS Eagle Ford Group Assessment provides that valuable context and is a model for future USGS oil and gas assessments to provide a more holistic picture of the country’s energy mix, future constraints and potential.

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