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Isotopic Evidence for Quasi-Equilibrium Chemistry in Thermally Mature Natural Gases

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Isotopic Evidence for Quasi-Equilibrium Chemistry in Thermally Mature Natural Gases Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases Nivedita Thiagarajana,1, Hao Xiea, Camilo Pontonb, Nami Kitchena, Brian Petersonc, Michael Lawsond, Michael Formoloe, Yitian Xiaoe, and John Eilera aDepartment of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125; bGeology Department, Western Washington University, Bellingham, WA 98225; cExxonMobil Corporate Strategic Research, Annandale, NJ 08801; dExxonMobil Upstream Business Development, Spring, TX 77389; and eExxonMobil Upstream Research Company, Spring, TX 77389 Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved January 17, 2020 (received for review April 25, 2019) Natural gas is a key energy resource, and understanding how it experiments only produce 10 to 65% methane (7–10). Additionally forms is important for predicting where it forms in economically hydrocarbons exist in geologic environments at temperatures that important volumes. However, the origin of dry thermogenic natural are much greater than their predicted stability based on experi- gas is one of the most controversial topics in petroleum geochem- mentally derived models of catagenesis (11, 12). istry, with several differing hypotheses proposed, including kinetic Most explanations for this discrepancy argue that natural gas processes (such as thermal cleavage, phase partitioning during is generated with a chemistry more similar to products of crack- migration, and demethylation of aromatic rings) and equilibrium ing experiments, but its molecular proportions are then modi- processes (such as transition metal catalysis). The dominant para- fied by secondary processes, such as methane enrichment by digm is that it is a product of kinetically controlled cracking of long- buoyancy-driven fractionation during the migration of gas from its n chain hydrocarbons. Here we show that C2+ -alkane gases (ethane, source reservoir (13, 14) or methane enrichment from exception- propane, butane, and pentane) are initially produced by irreversible ally high-temperature catagenesis at equivalent thermal maturities cracking chemistry, but, as thermal maturity increases, the isotopic far in excess of those explored by most experiments (12). Addi- distribution of these species approaches thermodynamic equilib- tionally, methane-rich thermogenic gases are often suspected of rium, either at the conditions of gas formation or during reservoir containing a component of biogenic gas (e.g., ref. 15). None of storage, becoming indistinguishable from equilibrium in the most these processes challenge the fundamental view of catagenesis as a thermally mature gases. We also find that the pair of CO2 and C1 family of irreversible reactions that break down larger hydrocarbon EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES (methane) exhibit a separate pattern of mutual isotopic equilibrium molecules to form smaller ones (Fig. 1A). (generally at reservoir conditions), suggesting that they form a sec- One alternative hypothesis holds that thermogenic gases are ond, quasi-equilibrated population, separate from the C2 to C5 com- generated through catalytic equilibrium processes. Previous work pounds. This conclusion implies that new approaches should be has suggested that hydrocarbons, water, and authigenic minerals taken to predicting the compositions of natural gases as functions in hydrocarbon reservoirs are in a metastable equilibrium (16–19), of time, temperature, and source substrate. Additionally, an isotopi- while other laboratory experiments have suggested that methane, cally equilibrated state can serve as a reference frame for recogniz- ethane, and propane are in a metathetic equilibrium (20). Support ing many secondary processes that may modify natural gases after their formation, such as biodegradation. for the metastable equilibrium hypothesis comes from theoretical calculations (16) as well as laboratory experiments that mix pre- stable isotopes | compound-specific isotope analysis | clumped isotopes | cursor hydrocarbons with catalysts, which, upon heating, produce n natural gas | methane low-molecular weight -alkane gases with molecular proportions Significance atural gas is primarily composed of volatile alkanes contain- Ning five or fewer carbon atoms and is widely distributed in the subsurface. It is a key resource (1), and is increasingly viewed as The mechanisms of natural gas formation are important to the carbon cycle and predicting where economical amounts of nat- a transition fuel that can reduce CO2 emissions relative to coal and oil as the world’s economies move toward carbon-free en- ural gas form. However, the formation mechanisms of natural ergy systems. However, the utilization of natural gas poses en- gas are not clear, with hypotheses including both irreversible vironmental risks, as leakage contributes significantly to total chemical processes such as thermal cracking of long-chain hy- atmospheric methane, a significant greenhouse gas. Ethane, drocarbons and thermodynamic equilibrium processes such as transition metal catalysis. Here we show that hydrocarbon spe- propane, and butane have short atmospheric lifespans, but they cies in natural gases are initially produced by irreversible cracking also increase the production of tropospheric ozone, a respiratory chemistry, but, as thermal maturity increases, the H and C iso- hazard and pollutant (2, 3). topic distributions within and among coexisting light n-alkanes Understanding the mechanisms of natural gas formation is approach thermodynamic equilibrium, either at the conditions of critical for predicting where it forms in economic volumes and gas formation or during reservoir storage. Our finding has sig- for recognizing its release to the environment. Hydrocarbons in nificant implications for natural gas exploration. natural gas are believed to come from two sources, one from “ ” biological processes ( biogenic gas ) (4) and the other from the Author contributions: N.T., H.X., C.P., and J.E. designed research; N.T., H.X., C.P., N.K., B.P., thermal cracking of kerogen and oil (“thermogenic gas”). There M.L., M.F., and Y.X. performed research; N.T., H.X., C.P., and J.E. analyzed data; and is disagreement over the origin of thermogenic gas. The pre- N.T., H.X., C.P., B.P., M.L., M.F., and J.E. wrote the paper. dominant view is that it is created by irreversible, thermally acti- The authors declare no competing interest. vated breakdown of hydrocarbon solids and liquids, with reactions This article is a PNAS Direct Submission. dominated by the cleavage of carbon−carbon bonds (or “crack- Published under the PNAS license. ing”) (5, 6) (Fig. 1A). However, laboratory experiments involving 1To whom correspondence may be addressed. Email: [email protected]. heating of hydrocarbons generally produce hydrocarbon gases that This article contains supporting information online at https://www.pnas.org/lookup/suppl/ differ in molecular composition from natural gases. Natural gas doi:10.1073/pnas.1906507117/-/DCSupplemental. reservoirs tend to have greater than 80% methane, while pyrolysis First published February 11, 2020. www.pnas.org/cgi/doi/10.1073/pnas.1906507117 PNAS | February 25, 2020 | vol. 117 | no. 8 | 3989–3995 Downloaded by guest on October 5, 2021 proportions and 13C/12C ratios that resemble thermodynamic A C H equilibrium and not thermal cleavage (21). Here we extend that argument in three ways: 1) We have measured or compiled C and H isotopic compositions of methane (C1), ethane (C2), propane (C3), butane (C4), pentane (C5), and CO2 for 119 gases from 20 conventional, unconventional, oil- CH C H associated, and nonassociated natural deposits around the world. H This extended database provides a far wider sampling and allows us to use stricter tests of the equilibrium hypothesis than have been H previously considered. 2) Additionally, we include methane clumped isotope compositions (Δ18 values) for a large subset of the gases 2CH C H we examine (24–26). The Δ18 value principally reflects enrich- 13 ment (or depletion) in CH3D relative to a stochastic distri- bution of 13C and D among all methane isotopologues, and, in isotopically equilibrated systems, the Δ18 value is a function of C H C H temperature. 3) We compare measured isotopic compositions to a comprehensive set of theoretically predicted equilibrium and nonequilibrium fractionations between CO2 and C1 to C5 spe- H cies. Our equilibrium models are based on new ab initio calcu- lations of molecular partition function ratios (see SI Appendix for details). Models of nonequilibrium carbon and hydrogen B x2 isotope fractionations are based on previous estimates of KIEs C H associated with homolytic cleavage and beta-scission of methyl, ethyl, propyl, butyl, and pentyl radicals from an n-alkane pre- cursor (27, 28) (SI Appendix). C H Results and Discussion C H We find that our most thermally immature samples (Ro < 0.8) are CH consistent with stable isotope distributions controlled by KIEs. However, natural gases of moderate or greater maturity (Ro > 0.8) CH have C2+ gases (ethane, propane, butane, and pentane) that are C H consistent with thermodynamic equilibrium for carbon (Fig. 2 and C H SI Appendix,Fig.S1) and hydrogen isotopes (SI Appendix,Fig.S2), considering the combined analytical and model uncertainties. This H* pattern is also seen in a global compilation (29) of natural gases SI Appendix x2 H ( ,Fig.S3). There are three important exceptions to CH this pattern which we discuss in the following
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