A team of researchers from The University of Texas at Austin have indicated that high concentrations of methane found in drinking water wells at two counties located near Fort Worth are likely to originate from shallow naturally occurring gas deposits rather than gas leaks due to fracking operations in the deeper Barnett Shale.
In a report that was recently published in the scientific journal Groundwater, the scientists build on earlier studies related to the quality of well water in the Barnett Shale, using chemical as well as geographic evidence to link the elevated methane levels of certain wells to naturally occurring shallow deposits.
Natural gas consists primarily of methane gas, and is found in large deposits in shale rock formations deep underground. However, smaller deposits of methane can also be found much shallower, typically just a few hundred feet below the surface. These shallower deposits —such as the one found in this geological formation known as the Strawn Group— form as methane gas from deeper deposits moves upwards towards the surface.
"Over geologic time, methane has accumulated into these shallower reservoirs," explains Jean‐Philippe Nicot, a research scientist at the Bureau of Economic Geology, a unit of the UT Jackson School of Geosciences. "These fresh-water wells are very close to these shallower reservoirs and may be the source of the methane."
With around 20,000 gas wells or more, the Barnett Shale formation underlying Fort Worth is one of America's largest and highly productive gas fields. But the boom in natural gas production and fracking operations has also been linked to potentially hazardous methane levels in drinking water wells, particularly in Silverado, a neighborhood located in Parker County, just outside Fort Worth.
The scientists sampled over 450 water wells from 12 counties located in the Barnett Shale to determine the extent and source of methane contamination of wells. They found that the groundwater supplying 85% of the wells had very low levels of methane (10 milligrams per liter of water), levels which are considered potentially dangerous due to the flammable nature of methane gas.
In order to determine where the methane originated from, the scientists decided to examine the cluster a little closer, analyzing water samples from 58 wells, working from the center of the cluster (where Silverado residential area is located) outwards until methane was not detected in well water.
"What we wanted to do was understand how much methane there is and determine the size of the high methane hotspot," Nicot said.
Methane gas is produced in one of two ways: 1) thermogenically, when organic material is broken down in an environment that has a high temperature and high pressure, typically at great depths, although this methane can migrate upwards over time; or 2) biogenically, as a result of methane-forming microbial activity, which typically occurs at shallow depths.
To determine whether the methane was from biogenic or thermogenic sources, as well as the depth the contamination originated from, the scientists used carbon isotope analysis combined with additional analysis.
"Combining alkane, noble gas and nitrogen compositions and isotope ratios allowed us to distinguish natural gas sourced from the deep Barnett Shale from the shallow Strawn Group," explained Toti Larson, a researcher at the Jackson School's Department of Geological Sciences.
While these findings suggest methane found in water wells of Hood and Parker counties most likely originates from the Strawn Group, the scientists point out that they cannot completely rule out the possibility that some of this methane may have originated from gas leaks that occurred during fracking operations.
Jean‐Philippe Nicot, Patrick Mickler, Toti Larson, M. Clara Castro, Roxana Darvari, Kristine Uhlman, Ruth Costley. Methane Occurrences in Aquifers Overlying the Barnett Shale Play with a Focus on Parker County, Texas. Groundwater (March, 2017). DOI: 10.1111/gwat.12508