Controls on Methane Occurrences in Aquifers Overlying the Eagle Ford Shale Play, South Texas

Abstract

Assessing natural vs. anthropogenic sources of methane in drinking water aquifers is a critical issue in areas of shale oil and gas production. The objective of this study was to determine controls on methane occurrences in aquifers in the Eagle Ford Shale play footprint. A total of 110 water wells were tested for dissolved light alkanes, isotopes of methane, and major ions, mostly in the eastern section of the play. Multiple aquifers were sampled with approximately 47 samples from the Carrizo-Wilcox Aquifer (250-1200 m depth range) and Queen City-Sparta Aquifer (150-900 m depth range) and 63 samples from other shallow aquifers but mostly from the Catahoula Formation (depth <150 m). Besides three shallow wells with unambiguously microbial methane, only deeper wells show significant dissolved methane (22 samples >1 mg/L, 10 samples >10 mg/L). No dissolved methane samples exhibit thermogenic characteristics that would link them unequivocally to oil and gas sourced from the Eagle Ford Shale. In particular, the well water samples contain very little or no ethane and propane (C1/C2+C3 molar ratio >453), unlike what would be expected in an oil province, but they also display relatively heavier δ C (>-55‰) and δD (>-180‰). Samples from the deeper Carrizo and Queen City aquifers are consistent with microbial methane sourced from syndepositional organic matter mixed with thermogenic methane input, most likely originating from deeper oil reservoirs and migrating through fault zones. Active oxidation of methane pushes δ C and δD toward heavier values, whereas the thermogenic gas component is enriched with methane owing to a long migration path resulting in a higher C1/C2+C3 ratio than in the local reservoirs.

Methane Occurrences in Aquifers Overlying the Barnett Shale Play with a Focus on Parker County, Texas

“…Methodology is similar to that used in Nicot et al (2017aNicot et al ( , 2017b. It is described in Appendix S2 and summarized below.…”

“…Several research papers documented the initial state of knowledge on dissolved methane distribution in groundwater (Osborn et al 2011;Jackson et al 2013a;Molofsky et al 2013). These initial studies were quickly followed by others in Pennsylvania (Heisig and Scott 2013;Baldassare et al 2014;Darrah et al 2014;Wilson 2014;Siegel et al 2015Siegel et al , 2016Christian et al 2016) and elsewhere: Colorado (Li and Carlson 2014;Sherwood et al 2016), Fayetteville in Arkansas (Warner et al 2013), Bakken in North Dakota (McMahon et al 2015), Texas (Nicot et al 2015(Nicot et al , 2017a(Nicot et al , 2017bWen et al 2016), and Canada (Cheung et al 2010;Moritz et al 2015;Humez et al 2016). The source of the dissolved methane is sometimes clear, in particular when it is microbial (Warner et al 2013;McMahon et al 2015) but still the object of conflicting studies when mostly thermogenic, such as in Pennsylvania (Jackson et al 2013a;Molofsky et al 2013;Darrah et al 2014) or Texas (Darrah et al 2014;Wen et al 2016).…”

Hydrocarbon‐Rich Groundwater above Shale‐Gas Formations: A Karoo Basin Case Study

“…In order to evaluate the environmental effects of hydrocarbon extraction in petroleum basins targeted for drilling, baseline groundwater quality has been evaluated using a multiple geochemical tracer approach (e.g., Molofsky et al ; Darrah et al , ; McMahon et al ; Nicot et al , , ). These studies have identified elevated levels of methane at concentrations up to its saturation point in shallow aquifers and revealed an association between methane and NaCl‐type waters, valley bottoms, faulting, and highly reducing aquifers (Warner et al ; Jackson et al ; Darrah et al , ; Harkness et al , ).…”

“…To this point, although it has been nearly seven years since the initial Osborn et al () paper, there is still a dearth of published genuinely predrill, baseline datasets. Although the baseline studies that have been published are of great value, the majority of these studies did not start until after unconventional shale‐gas drilling had commenced in these areas (Molofsky et al ; Baldassare et al ; Siegel et al ; Darrah et al ; Humez et al ; Nicot et al ; Harkness et al ; Nicot et al , ). Thus, it remains uncertain if observations from those datasets actually represent the natural background.…”

Attitudes, Perceptions, and Geospatial Analysis of Water Quality and Individual Health Status in a High-Fracking Region

“…The oil and gas industry exercises a number of precautionary measures to ensure that groundwater quality is not impacted by nature. Multiple reconnaissance efforts have recently evaluated groundwater quality throughout the Western Gulf Basin revealing elevated levels of biogenic and thermogenic natural gas [25,26]; benzene, toluene, ethylbenzene and xylene (BTEX) compounds [27]; and total organic carbon, and various organic solvents [24] in private and public water supply wells. However, these data are the first to evaluate the prevalence of organic and inorganic groundwater constituents within the context of community members' perceptions, providing unique insight into the relationship between residents and the UD industry operations.…”