Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases

Abstract

Natural gas is a key energy resource, and understanding how it forms is important for predicting where it forms in economically important volumes. However, the origin of dry thermogenic natural gas is one of the most controversial topics in petroleum geochemistry, with several differing hypotheses proposed, including kinetic processes (such as thermal cleavage, phase partitioning during migration, and demethylation of aromatic rings) and equilibrium processes (such as transition metal catalysis). The dominant paradigm is that it is a product of kinetically controlled cracking of long-chain hydrocarbons. Here we show that C2+n-alkane gases (ethane, propane, butane, and pentane) are initially produced by irreversible cracking chemistry, but, as thermal maturity increases, the isotopic distribution of these species approaches thermodynamic equilibrium, either at the conditions of gas formation or during reservoir storage, becoming indistinguishable from equilibrium in the most thermally mature gases. We also find that the pair of CO2 and C1 (methane) exhibit a separate pattern of mutual isotopic equilibrium (generally at reservoir conditions), suggesting that they form a second, quasi-equilibrated population, separate from the C2 to C5 compounds. This conclusion implies that new approaches should be taken to predicting the compositions of natural gases as functions of time, temperature, and source substrate. Additionally, an isotopically equilibrated state can serve as a reference frame for recognizing many secondary processes that may modify natural gases after their formation, such as biodegradation.

Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH4H2H2O from 3 to 200 °C

“…The carbon isotopic composition of thermogenic methane is generally thought to be controlled by kinetic isotope effects (e.g., Tang et al, 2000) However, Smith et al (1998) showed experimentally that the thermal decomposition of acetic acid yields CO 2 and methane with carbon isotopic compositions consistent with generation in carbon isotopic equilibrium from 290 to 650°C. Most recently, based on observed differences between the  13 C of CO 2 and thermogenic methane vs. measured methane clumped-isotope based temperatures, Thiagarajan et al (2020) proposed that thermogenic methane and CO 2 can achieve carbon isotopic equilibrium in the subsurface through reactions that promote methane oxidation and CO 2 reduction.…”

“…Additionally, proposed that thermogenic methane may form in (or achieve) hydrogen isotopic equilibrium with water based on the observed agreement between apparent methane clumpedisotope temperatures (from 118-204°C) and measured differences in the D of methane and water versus those expected for CH 4 -H 2 O hydrogen isotopic equilibrium. Most recently, Thiagarajan et al (2020) proposed that methane equilibrates hydrogen with other gaseous alkanes (e.g., ethane, propane, etc.) in thermogenic gas reservoirs.…”

“…We are aware of six published theoretical estimates of 1000ln 13  CH4(g)-CO2(g) as a function of temperature at isotopic equilibrium that could be used for such an exercise: Craig (1953), Bottinga (1969), Richet et al (1977), Chen et al (2019), Thiagarajan et al (2020), and Gropp et al (2021). For temperatures lower than those accessed by experiments (i.e.…”

Position-specific distribution of hydrogen isotopes in natural propane: Effects of thermal cracking, equilibration and biodegradation

“…This could reflect similarities between methane and propane in the mechanisms and kinetics of carbon-bound hydrogen equilibration. This could reflect the ability of both to undergo catalystmediated exchange or that they are both participants in radical chain reactions that allow interconversion of molecules and radicals and exchange of atoms (Xia and Gao, 2018;Thiagarajan et al, 2020b).…”

Clumped isotope effects of thermogenic methane formation: Insights from pyrolysis of hydrocarbons

“…Wang et al, 2015;Young et al, 2017;Douglas et al, 2017;Stolper et al, 2018;Giunta et al, 2019). This discrepancy could be taken as evidence for an alternative hypothesis that catagenetic reactions proceed through locally reversible metastable equilibria (Helgeson et al, 2009;Mango et al, 2009;Thiagarajan et al, 2020b). Alternatively, like interpretations of some natural microbial methane, thermogenic methane might be produced by irreversible, kinetically controlled reactions, but subsequently driven to equilibrium by some more reversible chemical process.…”