Formation and abundance of doubly-substituted methane isotopologues (13CH3D) in natural gas systems

Abstract

Formation of the Carbon-13 (13C) and deuterium (D) doubly-substituted methane isotopologues (13CH3D) in natural gases is studied utilizing both first-principle quantum mechanism molecular calculation and direct FTIR laboratorial measurements of specifically synthesized high isotope concentration methane gas. For 13CH3D, the symmetrically breathing mode A0 emerges as IR-detectable attributed to the molecular symmetry lowering to C3v from Td of the non-isotopic methane (CH4), along with a large vibrational frequency shift from ~3000 to ~2250 cm^−1. Our studies also indicate that the concentration of 13CH3D is dependent on the environmental temperature through isotope exchanges among methane isotopologues; and the Gibbs’ Free Energy difference due to Quantum Mechanics Zero-Point vibrational motions has the major contribution to this temperature dependency. Potential geologic applications of the 13CH3D measurement to natural gas exploration and assessments are also discussed. In order to detect the 13CH3D concentration change of each 50 °C in the natural gas system, a 10^−9 resolution is desirable. Such a measurement could provide important add-on information to distinguish natural gas origin and distribution.