Position-specific carbon isotopic composition of thermogenic propane: Insights from pyrolysis experiments

Abstract

Position-specific isotope analysis (PSIA) of propane provides novel insights into formation mechanisms and thermal maturity of natural gas. The evolution of thermogenic propane in natural gas was simulated with pyrolysis experiments using types I, IIA, III kerogen and an oil. PSIA of propane involved on-line pyrolysis with gas chromatography–isotope ratio mass spectrometry to explore evolution characteristics and fractionation mechanisms of propane position-specific δ13C values. Results indicate the formation of propane can be considered in three stages. Stage I (<1.3% EasyRo) involves propane production mainly from primary kerogen cracking, with its precursors having homogeneous isotopic distributions and with position-specific 13C variation patterns depending on the kerogen molecular structure. In Stage II (1.3% EasyRo to maturity of peak propane yield), propane is derived mainly from secondary cracking of bitumen/oil, resulting in 13C enrichment of terminal (δ13Ca) and central (δ13Cb) C atoms with maturity and a strong linear relationship between δ13Ca and δ13Cb. A suggested new pattern is explained by a Rayleigh model for the evolution of propane position-specific δ13C values in this stage. In Stage III (maturity greater than the peak propane yield), propane destruction mainly controls its amount and isotopic signature. Propane site-preference (SP) value (δ13Ca–δ13Cb) can be used to determine its formation mechanism and maturity, with positive and negative values respectively indicating primary kerogen cracking and secondary oil/bitumen cracking. Increasing maturity causes decreasing values. The application of propane PSIA to the Tarim Basin gas samples demonstrates its potential for determining the mechanism of formation and maturity of natural gas.