This dataset is based on U.S. Geological Survey (USGS) resource assessments for “undiscovered” natural gas liquid resources, which are resources that have not yet been extensively proven by drilling (USGS 2014). Individual resource assessments describe the amount of petroleum resources in units with similar geologic features. We quantified the density of natural gas liquid resources by adding together the amounts in spatially overlapping assessment units and dividing these totals by polygon areas. Since assessments for geologic areas used in this analysis were completed at various times, the certainty related to these values is likely to vary according to geologic unit. USGS [U.S. Geological Survey]. 2014. Energy Resources Program. Oil and gas: assessments data. Department of the Interior, Reston, Virginia, USA.