Using global isotopic data to constrain the role of shale gas production in recent increases in atmospheric methane

Abstract

The accelerated increase in global methane (CH₄) in the atmosphere, accompanied by a decrease in its ¹³C/¹²C isotopic ratio (δ¹³C_CH4) from −47.1‰ to −47.3‰ observed since 2008, has been attributed to increased emissions from wetlands and cattle, as well as from shale gas and shale oil developments. To date both explanations have relied on poorly constrained δ¹³C_CH4 source signatures. We use a dataset of δ¹³C_CH4 from >1600 produced shale gas samples from regions that account for >97% of global shale gas production to constrain the contribution of shale gas emissions to observed atmospheric increases in the global methane burden. We find that US shale gas extracted since 2008 has volume-weighted-average δ¹³C_CH4 of −39.6‰. The average δ¹³C_CH4 weighted by US basin-level measured emissions in 2015 was −41.8‰. Therefore, emission increases from shale gas would contribute to an opposite atmospheric δ¹³C_CH4 signal in the observed decrease since 2008 (while noting that the global isotopic trend is the net of all dynamic source and sink processes). This observation strongly suggests that changing emissions of other (isotopically-lighter) CH₄ source terms is dominating the increase in global CH₄ emissions. Although production of shale gas has increased rapidly since 2008, and CH₄ emissions associated with this increased production are expected to have increased overall in that timeframe, the simultaneously-observed increase in global atmospheric CH₄ is not dominated by emissions from shale gas and shale oil developments.

Introduction

Methane (CH₄) is the third-most important greenhouse gas (after water vapor and carbon dioxide) and a significant contributor to global climate change^1–8. Its globally averaged marine surface annual mean mole fraction in the atmosphere steadily increased from ~1600 parts per billion (ppb) to ~1775 ppb in the 1980–1990s, stabilized around ~1775 ppb during the period 1999–2006, and then returned to the earlier pattern of increases leading to ~1860 ppb in 2018^1,3,9,10. There are anthropogenic (e.g., agriculture, wastes, fossil fuels, biomass burning) and natural (e.g., wetlands, freshwaters, geological seepage, wild fires) sources of CH₄ to the atmosphere. The carbon-isotopic composition of CH₄ (the ratio of stable isotopes ¹²C and ¹³C expressed as δ¹³C (‰) relative to the Vienna Pee Dee Belemnite standard), together with estimates of flux from each source-type, can be used to infer the relative contributions of various CH₄ emitters to the global budget by matching co-constrained global observations of CH₄ and its δ¹³C^3,4. Furthermore, temporal variations in δ¹³C of atmospheric CH₄ are a highly useful indicator of changes in the trends of various emitters and/or CH₄ sinks. The global mean atmospheric δ¹³C_CH4 trended upward between 1980 (approximately −47.7‰) and 1997 (approximately −47.1‰), and remained relatively stable until 2008, before decreasing towards the most recent values of approximately −47.3‰^1,6.

This recent increase in the atmospheric CH₄ burden, coincident with the depletion of ¹³C, has been attributed to the increasing contribution of biogenic CH₄ from wetlands and from agricultural activities such as cattle husbandry that produce CH₄ with δ¹³C usually more negative than −55‰^1,4,9. While atmospheric CH₄ sinks are less extensively studied, changes in the sink strength may at least partially explain some of the long-term observed trends in δ¹³C_CH4¹¹. Fossil fuel production also contributes to increasing atmospheric CH₄^5,12,13. However, CH₄ in fossil fuels is, on average, enriched in ¹³C (δ¹³C = −44‰^4,14) relative to globally-averaged atmospheric CH₄. Decreasing δ¹³C of atmospheric CH₄ since 2008 implies that emissions from biogenic sources are therefore increasing at a greater rate relative to emissions from fossil fuels. However, recent studies have suggested that emissions from conventional petroleum developments⁵ and from shale gas/oil developments in particular² (see Text S1 in Supplementary Information) have been the greatest single cause of the recent global increase of atmospheric CH₄. Here, we use a large global dataset of δ¹³C_CH4 from produced shale formations, which leads us to conclude that emissions from shale gas and oil production have not played a dominant role in the increase in atmospheric CH₄ since 2008.

Materials and Methods

Global isotopic dataset

We analyzed δ¹³C_CH4 data for 1619 samples of produced natural gas from 38 shale formations around the world originally presented in 73 studies (Table S1). This shale gas dataset is a subset of a larger global inventory of gas samples from conventional reservoirs, shales, coals, seeps and other geological settings originally published by Sherwood et al.¹⁴ and further expanded and discussed by Milkov and Etiope¹⁵ and Milkov et al.¹⁶. Although most gas samples come from formations dominated by true shale lithology (e.g., the Marcellus Formation, USA), we also include samples collected from unconventional low-permeability (tight) reservoirs dominated by very fine-grained sandstone or siltstone (e.g., the Montney Formation, Canada) or mixed clastic/carbonate lithologies (e.g., the Niobrara Formation, USA) developed through hydraulic fracturing and commonly included in the inventories of produced shale gas¹⁷. The produced gas may be free gas associated with relatively little condensate liquids (e.g., in the Haynesville Formation) or oil-dissolved gas (e.g., in the Eagle Ford Formation). Most samples come from the USA (n = 1238), followed by China (n = 252), Canada (n = 124), United Kingdom (n = 2), Sweden (n = 2) and Australia (n = 1).

Calculation of weighted δ¹³C_CH4 values

Values of production volume-weighted δ¹³C_CH4 for shale gases were derived by first calculating the proportion of gas production from each shale formation in the total production, then multiplying that value by the average δ¹³C_CH4 for the corresponding shale formation, and then summing up the results. Emission volume-weighted δ¹³C-CH₄ values were derived by first calculating the proportion of CH₄ emissions from each shale formation in total emissions, multiplying that value by average δ¹³C_CH4 for corresponding shale formation, and then summing up the results.

Results

The arithmetic mean δ¹³C_CH4 for all shale gas samples is −41.3 ± 0.2‰ (n = 1619, range from −70‰ to −23.3‰, median −41.4‰) (Fig. 1). The mean value is slightly more positive than −42.5 ± 0.3‰ reported by Sherwood et al.¹⁴ based on a smaller dataset of 647 samples. Methane from produced shales is, on average, more enriched in ¹³C than CH₄ produced from conventional oil and gas reservoirs (mean δ¹³C_CH4 = −44.0 ± 0.1‰, n = 6079 in the study of Sherwood et al.¹⁴; mean δ¹³C_CH4 = −42.8 ± 0.1‰, n = 12,697 in the study of Milkov et al.¹⁶) and significantly more enriched in ¹³C than the modern atmospheric δ¹³C_CH4 (−47.3‰^1,6). We note that shale gas is even more enriched in ¹³C relative to the global average δ¹³C_CH4 (about −54‰⁴) of all atmospheric sources prior to isotopic fractionation of atmospheric CH₄ by all sinks resulting in the modern atmospheric value above.

Figure 1.

δ¹³C_CH4 values from a global dataset of 1619 samples of produced shale gases from around the world. The data are displayed using a box plot, which shows distribution of values as histogram, average (mean) value (−41.3‰) as black star, median value (−41.4‰) as dotted line, first quartile (Q1), third quartile (Q3), lower adjacent value, upper adjacent value, and outliers. The first quartile (Q1) is the median of the lower half of the data set. This means that about 25% of the values in the data set lie below Q1 and about 75% lie above Q1. The third quartile (Q3) is the median of the upper half of the data set. This means that about 75% of the values in the data set lie below Q3 and about 25% lie above Q3. The lower adjacent value is the smallest observation that is greater than or equal to the lower inner fence, which is the first quartile minus 1.5 × IQR, where IQR stands for the interquartile range. The upper adjacent value is the largest observation that is less than or equal to the upper inner fence, which is the third quartile plus 1.5 × IQR. Outliers are all values that fall outside of either of the fences. Original data are in Table S1.

Global shale gas production increased from about 31 billion cubic meters (bcm) in 2005 to about 434 bcm in 2015¹⁸. In the USA, the cumulative production of shale gas from 2000 to mid-2019 reached approximately 4.5 trillion cubic meters (tcm), including about 4.1 tcm produced since 2008 (Fig. 2, based on dry gas production). Half of the cumulative shale gas was produced from the Marcellus, Barnett and Haynesville formations. Figure 3 summarizes δ¹³C_CH4 data on gases produced from these and other principal shale formations in the USA.

Figure 2.

Cumulative production (in billion cubic meters or bcm) of dry gas from shale plays in the USA. Data are from Energy Information Administration¹⁷.

Figure 3.

Main statistics on δ¹³C_CH4 for gases produced from main shale formations in the USA, Canada and China. The data are displayed using box plots (see Fig. 1 for legend). Original data are in Table S1.

In this study, we use the global δ¹³C_CH4 dataset to derive δ¹³C_CH4 representative of both produced gas (volume-weighted average) and the δ¹³C_CH4 signature when weighted for measured emissions across plays (emission-weighted average). Table 1 presents average δ¹³C_CH4 for the main producing shale plays in the USA. The 1002 available gas samples with δ¹³C_CH4 data are from plays that account for 94% of cumulative US shale gas production. The average δ¹³C_CH4, when weighted by the amount of cumulative production from each shale play during 2008–2019, is −39.6‰. A large proportion (28%) of cumulative shale gas production comes from the Marcellus Formation where CH₄ is significantly enriched in ¹³C (mean δ¹³C_CH4 is −32.0‰, n = 98). This latter source significantly influences the average volume-weighted isotope signature of CH₄ produced from shales in the USA.

Table 1.

Data used to calculate the average δ¹³C_CH4 in shale gases produced in the USA since 2008.

Shale formation	Total dry shale gas production from 2008 to mid-2019 (bcm)	Portion (%) of total dry shale gas production from 2008 to mid-2019	Average δ13CCH4 in produced gas (‰)	N of gas samples with δ13CCH4
Marcellus	1170	28	−32.0	98
Haynesville	520	13	−39.5	17
Barnett	460	11	−42.7	450
Permian basin (Wolfcamp, Avalon and others)	360	9	−49.5	13
Rest of US shales	236	6	na	na
Eagle Ford	319	8	−42.9	50
Utica	245	6	−31.8	4
Fayetteville	240	6	−38.2	101
Woodford (Anadarko and Arkoma basins)	210	5	−49.1	54
Mississippian (Anadarko basin)	137	3	−50.3	7
Niobrara-Codell (Denver basin)	133	3	−47.6	190
Bakken	85	2	−47.3	19
Total	4114	100		1003
Total with gas data	3879	94
Production volume-weighted			−39.6

Production data are from Energy Information Administration¹⁷. See Text S2 in Supplementary Information for specific details on Permian, Utica and Niobrara-Codell samples. na – not available.

The average shale δ¹³C_CH4 weighted by the amount of emissions measured in 2015 from the main USA shale plays¹⁹ is −41.8‰ (Table 2). Sensitivity analysis suggests that this value changes little when emission measurements from other years are considered (see Table S2, Text S3). We also calculated how the average δ¹³C_CH4 signature of shale-emitted gas changed over time. When weighted by production or emissions, the US average signature becomes heavier (thus, opposite to the direction of the atmospheric trend) by about 4–7‰ from 2000 to mid-2019 (Fig. 4). This is because the relative contribution of shales with relatively more positive δ¹³C_CH4 (e.g., Marcellus and Haynesville formations) to both production and emissions increased in that period.

Table 2.

Data used to calculate the emission-weighted average δ¹³C of CH₄ emitted from, mostly, shale gas production in selected plays and areas in the USA in 2015.

Shale formation and basin/area	Gas production (m3/d)	Emitted gas (% of production)	CH4 emissions (tones/hr)	Average δ13C (‰) of emitted CH4
Eagle Ford	1.54E + 08	2.5	83	−42.9
Haynesville	1.50E + 08	1	42	−39.5
Marcellus (N.E. PA)	1.40E + 08	0.4	18	−32.0
Barnett	1.20E + 08	1.5	46	−42.7
Fayetteville	6.80E + 07	1.5	31	−38.2
Niobrara-Codell (Denver)	3.80E + 07	2.1	18	−47.6
Bakken	3.70E + 07	5.4	29	−47.3
Total	7.07E + 08		267
Volume-weighted average		1.6
Emission-weighted average				−41.8

Gas production, percentage of emitted gas, and CH₄ emissions are from Peischl et al.¹⁹ and references therein. These results account for ~60% of total USA shale gas production in 2015.

Figure 4.

Increasing δ¹³C_CH4 signature of produced and emitted gas from shale developments in the USA from 2000 to mid-2019. Only formations with sufficient isotopic data (Marcellus, Barnett, Haynesville, Permian, Eagle Ford, Fayetteville, Woodford, Niobrara-Codell and Bakken) and emission data (Marcellus, Barnett, Haynesville, Eagle Ford, Fayetteville, Niobrara-Codell and Bakken) are used to construct this figure.

Gas samples from three other countries currently producing shale gas commercially (Canada, China, Argentina) indicate somewhat more positive δ¹³C_CH4 values than the USA (Fig. 3), resulting in a global volume-weighted δ¹³C_CH4 signature of −38.8‰ (Table S3, Text S4). The volume- and emission-weighted δ¹³C_CH4 values calculated here do not account for shale plays for which production has become negligible after the 1990s (see Text S4).

The US shale δ¹³C_CH4 weighted by the amount of cumulative production from 2000 to mid-2019 for each shale play is −40.0‰ (Table S4). The mean δ¹³C_CH4 of produced shale gas in the USA since 2008 is −39.6‰ (Table 1). The slight (by 0.4‰) enrichment in ¹³C during 2008–2019, relative to 2000–2019, is due to a relatively larger contribution of production from the Marcellus and Haynesville formations in 2008–2019, and a smaller contribution from the Barnett Formation during that period. The mean δ¹³C_CH4 of globally produced shale gas in 2018 is −38.‰ (Table S3). We also estimated an emission-weighted δ¹³C_CH4 of −41.8‰ for the principal US shale plays in 2015 (Table 2). These values are appropriate for utilization in models that constrain CH₄ emissions from shale developments to the atmosphere based on matching modelled global δ¹³C to observed δ¹³C. Methane from produced shales is, on average, significantly enriched in ¹³C relative to atmospheric CH₄ (δ¹³C_CH4 ~−47‰).

Discussion

Recently, atmospheric CH₄ became more abundant but also depleted in ¹³C, as δ¹³C decreased from about −47.1‰ in 2007 to −47.3‰ in 2017. If shale gas (with δ¹³C_CH4 around −40‰ as documented in this study) and conventional oil and gas (with δ¹³C_CH4 around −43‰¹⁶) were conceived to collectively dominate recent emissions of CH₄ to the atmosphere, then atmospheric CH₄ would very simply become more enriched in ¹³C relative to the current global mean δ¹³C, which is not consistent with global observations. While we agree that shale developments (and fossil fuel in general) represent an important CH₄ source, and that emissions from those sources have been likely increasing due to growing production, we conclude that the increases in global atmospheric CH₄ concentrations since 2008 are not as strongly attributable to shale gas and conventional oil and gas emissions as some studies claim^2,5, based on our global observations of isotopic fractionation.

Additionally, we must emphasize that the measured atmospheric δ¹³C_CH4 signal is the sum-total of all CH₄ source and sink terms. For example, a decrease in biomass burning emissions (significantly enriched in ¹³C (δ¹³C_CH4 −22.3 ± 1.9‰⁴), and an increase in fossil fuel emissions (including shale gas), could in principle result in the same global average atmospheric δ¹³C_CH4 signal over time as if both sources had no trend^4,5. The biomass burning category includes fires and solid biofuels (e.g., for use in cook stoves). Data on global CH₄ emissions from fires is not entirely conclusive. Remote sensing data of CH₄ and CO (and assuming (i) biomass burning CH₄/CO emission ratios and (ii) a partitioning of CO emissions across sectors) suggests decreased fire CH₄ emissions of ~3.7 Tg/yr from the 2001–2007 to the 2008–2014 periods⁵. In contrast, remote sensing of burned fire area suggests no such trend²⁰ (no trend over this period apart from inter-annual variation; Fig. S1). Furthermore, CH₄ emissions from solid biofuels are reported to have increased from 12.2 to 13.6 Tg/yr from 2000–2012²¹ (latest time series available). While this data does not indicate an immediately apparent decrease in global biomass burning CH₄ emissions, more research is needed. Potential trends in the various CH₄ sink processes such as the soil sink²² and the tropospheric OH sink¹¹ can further complicate the diagnosis of source trends. As a result, it is important to account for these processes, as well as other existing evidence such as latitudinal and seasonal CH₄ trends, when attributing the global signal^1,9.

From the above, it follows that attributing ~1/3 of the global CH₄ increase to North American shale gas production and another ~1/3 to conventional gas and oil with a simple mass balance approach² is not supported by observations because of unconstrained uncertainties. Based on long-term airborne CH₄ measurements over the US, previous analysis concludes that oil and gas industry CH₄ emissions (shale and conventional) over the past decade have increased at about the same rate as natural gas production volume⁷. The existence of unaccounted and poorly characterized emission sources within the oil and gas industry has also been demonstrated through intensive field studies in the USA²³, and additional international studies paint a similar picture^24,25, although little independent measurement data exist for many world regions including the Middle East, the Former Soviet Union, and Africa. Further research targeted for these areas, in addition to changing biogenic sources and sinks, will serve to further constrain the conclusions made in this work.

Based on existing knowledge of CH₄ source and sink terms and isotopic signatures, additional CH₄ emissions associated with increased shale gas development in the USA cannot account for a large fraction of the recent increase in atmospheric CH₄. Yet, oil and gas industry expansion remains a significant factor in the complex patterns of global atmospheric CH₄ emissions and concentrations^4,23–25. And, of equal importance, fossil fuel CH₄ sources may be mitigated with policy and best (or better) industrial practice that can effectively reduce emissions. We suggest that the rise in global CH₄ concentrations is most effectively seen not through a lens of what is the most important or dominant source of emissions, but rather understanding all sources and how they can collectively explain the observed patterns of atmospheric increases. Indeed, a reduction in emissions from any major source (such as fossil fuels or cattle husbandry) would be expected to lead to a reduction in the global CH₄ concentration¹. Therefore, although our analysis indicates that shale gas and conventional gas and oil production has not played a dominant role in the increase in atmospheric CH₄ since 2008, we should not lose sight of the powerful impact of interventions to reduce emissions from sources we have.

Conclusions

CH₄ recently increased in the atmosphere and simultaneously became more depleted in ¹³C. In this study, we compiled a large global dataset of isotopic composition of CH₄ produced from shale formations that account for most global shale gas production. Developments of shale gas and oil on average emit CH₄ significantly more enriched in ¹³C than the atmospheric CH₄ signal. Given current knowledge of global isotopic data and processes, the increase in US shale oil and gas apparently does not dominate the recent increased emissions of global CH₄ to the atmosphere. It is important to understand all sources of CH₄ that collectively contribute to recent atmospheric increases, and isotopic data provide key constraints for this.

Supporting Information

Supporting Information. ^{(133.9KB, docx)}

Supplementary Dataset. ^{(65.3KB, xlsx)}

Author contributions

A.V.M. conceived the study, compiled the shale gas dataset and calculated reported statistical values. S.S. compiled and evaluated the shale emissions dataset. A.V.M., S.S., G.A., O.A.S. and G.E. discussed the data, interpreted the results and wrote the paper with input from all authors.

Data availability

The dataset used in this study is available as Supplementary information.

Competing interests

A.V.M. worked in petroleum industry (BP, Sasol and Murphy Oil) for 13 years as geoscientists and manager. He is Director of Potential Gas Agency (PGA) at Colorado School of Mines, and PGA receives financial support from oil&gas companies, gas pipeline companies and distributors, and trade associations. He also regularly teaches technical courses at oil&gas companies and consults them on the issues of petroleum geochemistry and petroleum exploration. The other authors do not have any competing interest.

Supplementary information

is available for this paper at 10.1038/s41598-020-61035-w.