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October 6, 2017

No, this EarthWord isn’t a trait of adult petroleum basins, but it is related to how old they are...

EarthWords is an on-going series in which we shed some light on the complicated, often difficult-to-pronounce language of science. Think of us as your terminology tour-guides, and meet us back here every week for a new word!

Image shows an excavator in front of a coal seam
A coal seam in a Wyoming mine. Credit: Bureau of Land Management(Public domain.)

The EarthWord: Thermal Maturity


  • Thermal maturity refers to the amount of changes that have occurred in organic matter in rock layers due to heat.


  • Thermal comes from the Old Greek therme, which referred to heat.

  • Maturity comes from the Latin maturare, meaning “to ripen.”

Use/Significance in the Earth Science Community:

  • Thermal maturity is one of the factors that indicates whether or not coal or petroleum have formed from their source rocks.

  • As sedimentary rock layers are laid down, the organic material undergoes heat and pressure. After enough time has passed, and if other conditions are right, the organic material can become coal or petroleum.


  • USGS incorporates thermal maturity into its coal and petroleum resource assessments. Determining the thermal maturity of various layers is a key component to estimating how much of the organic material has made the transition into coal or petroleum.

  • In addition, the thermal maturity helps USGS scientists determine the quality of the coal or the type of petroleum (oil, oil shale, or natural gas).

  • By using the same, publicly available methodologies for all of its assessments, USGS ensures that each basin can be compared to the others and provide a trusted, apples-to-apples estimate for the Nation’s energy resource endowment.

Next EarthWord: Eureka often accompanies this EarthWord...

Hungry for some science, but you don’t have time for a full-course research plate? Then check out USGS Science Snippets, our snack-sized science series that focuses on the fun, weird, and fascinating stories of USGS science.

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