Research Article

Groundwater methane in relation to oil and gas development and shallow coal seams in the Denver-Julesburg Basin of Colorado

Owen A. Sherwood, Jessica D. Rogers, Greg Lackey, Troy L. Burke, Stephen G. Osborn, and Joseph N. Ryan
PNAS first published July 11, 2016; https://doi.org/10.1073/pnas.1523267113

  1. Edited by Peter H. Gleick, Pacific Institute for Studies in Development, Environment, and Security, Oakland, CA, and approved June 7, 2016 (received for review November 24, 2015)

Article Figures & SI

Figures

  • Fig. 1.
    Fig. 1.

    Study area in the DJ Basin of Northeastern Colorado. (A) Overview map showing distribution of dissolved methane and thermogenic gas occurrences in 924 groundwater wells in relation to aquifers and oil and gas wells. (B) Closeup of Wattenberg field, also showing the distribution of coal-bearing deposits and coal mining fields (23), and northeast-trending wrench faults (12). Location of coal samples discussed in text is shown in both panels. Aquifers of the Denver Basin aquifer system (Dawson through Laramie-Fox Hills) are listed from youngest (Top) to oldest; Dakota-Cheyenne and High Plains represent separate aquifer systems (44).

  • Fig. 2.
    Fig. 2.

    Genetic characterization plot (22) of C1/(C2 + C3) vs. δ13CC1 for groundwater aquifers compared with natural gases from producing formations in Wattenberg field. Repeat samples from water wells are included. Groundwaters fall within microbial and thermogenic domains and along a mixing line (bold curve) calculated for thermogenic [δ13CC1 = −46‰; C1/(C2 + C3) = 5] and apparent Laramie-Fox Hill (LFH) microbial end-member values [δ13CC1 = −72‰; C1/(C2 + C3) = 1,000]. Arrows show oxidation trends using indicated fractionation factors (αCH4-CO2), following calculations in ref. 22. There were no data for Dawson or High Plains aquifers.

  • Fig. 3.
    Fig. 3.

    Plot of dissolved methane concentrations vs. groundwater aquifer, colored by methane genetic origin. Underlying vertical lines and boxes represent median ± 95% confidence limits (SI Appendix). Numbers to right of each data series represent number of samples. Repeat water well samples included.

  • Fig. 4.
    Fig. 4.

    Natural gas isotope plot (45) of δ13C vs. reciprocal carbon number for groundwater aquifers compared with production gas formation averages. δ13CC2 and δ13CC3 more clearly distinguish between microbial and thermogenic gases compared with δ13CC1. Water samples with thermogenic isotope signatures overlap with production gases. Repeat water well samples included.

  • Fig. 5.
    Fig. 5.

    Example of confirmed wellbore barrier failure (COGCC Complaint 200097544; SI Appendix). (A) Wellbore diagrams of gas well and water well (lateral offset = 103 m). Mechanical integrity testing of gas well revealed leaks in uncemented production casing at 71–99 m depth, below the surface casing, which was set above the top of the Upper Arapahoe aquifer. Production gases from the J-Sand formation thereby migrated to domestic water well screened in the Upper Arapahoe aquifer. Horizontal scale is exaggerated by 50×. (B) Natural gas isotope plot (45) showing identical δ13CC1–3 signatures in the gas well and water well.

  • Fig. 6.
    Fig. 6.

    Time series plots. (A) Methane concentration vs. sample collection date, colored by genetic origin. Line shows number of samples measured for dissolved methane per year. Implementation dates of COGCC regulatory and Colorado Oil and Gas Association (COGA) voluntary baseline water quality monitoring shown in top margin (SI Appendix). (B) Cumulative number of cases of thermogenic stray gas. Numbers indicate the number of impacted water wells where n > 1.

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Groundwater impacts of unconventional oil and gas
Owen A. Sherwood, Jessica D. Rogers, Greg Lackey, Troy L. Burke, Stephen G. Osborn, Joseph N. Ryan
Proceedings of the National Academy of Sciences Jul 2016, 201523267; DOI: 10.1073/pnas.1523267113
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Proceedings of the National Academy of Sciences: 118 (8)
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