Ethane C-C clumping in natural gas : a proxy for cracking processes ?

Abstract:

Ethane (C₂H₆) is the second-most abundant alkane in most natural gas reservoirs, and is used to produce ethylene for petrochemical industries.

It is arguably the simplest molecule that can manifest multiple ¹³C substitutions. There are several plausible controls on ∆¹³C₂H₆in natural gas: thermodynamically controlled homogeneous isotope exchange reactions analogous to those behind carbonate clumped isotope thermometry; inheritance from larger biomolecules that undergo thermal degradation to produce natural gas; mixing of natural gases that differ markedly in bulk isotopic composition; diffusive fractionation; or combinations of these and/or other, less expected fractionations. There is little basis for predicting which of these will control isotopic variations among natural ethanes, but we think it likely that addition of this new isotopic proxy will reveal new insights into the natural chemistry of ethane.

We have developed a method to measure the abundance of ¹³C₂H₆ in natural samples, using high-resolution mass spectrometry. We define ∆¹³C₂H₆ as 1000 . ((¹³C₂H₆/¹²C₂H₆)_measured/(¹³C₂H₆/¹²C₂H₆)_stochastic -1). We studied several suites of natural gas samples and experimentally produced or modified ethane. Natural ethanes, including closely related samples from a single natural gas field, exhibit surprisingly large ranges in ∆¹³C₂H₆ (4 ‰ overall; up to 3 ‰ in one gas field). Such ranges cannot be explained by thermodynamic equilibrium at a range of different temperatures, or by diffusive fractionation. Kinetic isotope effects associated with ‘cracking’ reactions, and/or inheritance of non-equilibrium carbon isotope structures from source organics are more likely causes. We observe a correlation between ∆¹³C₂H₆ and the concentration of alkanes other than methane in several suites of natural gases, suggesting the causes of clumped isotope variations are tied to the controls on gas wetness. An experiment examining ethane residual to high-temperature pyrolysis confirms this trend could be an isotopic fingerprint for ethane destruction.