Rapid estimation of the organic sulphur content of kerogens, coals and asphaltenes by pyrolysis-gas chromatography

Abstract

A pyrolysis-gas Chromatographic (py-g.c.) method for estimation of the Sorg/C ratio in kerogens and other forms of sedimentary macromolecular organic matter is described. The method is based upon flash pyrolysis at 610 °C for 10s and areal integration of the FID peaks attributed to 2,3-dimethylthiophen, 1,2-dimethylbenzene and n-non-1-ene. Comparison of the ratio [2,3-dimethylthiophen]/[n-non-1-ene + 1,2-dimethylbenzene], denoted TR, with chemically determined atomic Sorg/C ratio revealed a distinct correlation (r2 = 0.89), suggesting that TR appears to broadly reflect the latter ratio for a variety of geological materials. Py-g.c. of untreated, whole rock samples indicated that whilst the atomic Stot/C ratio determined by elemental analysis was strongly influenced by the presence of inorganic sulphur, TR showed no such dependence. The independence of TR from matrix effects demonstrates that this method has an advantage in that samples may be directly analysed for organic sulphur content with little or no preparation. A trivariate plot was used to depict the relative abundances of each of the three individual components used to derive TR. On the basis of their position within this plot, samples derived from marine and hypersaline sedimentary environments could be clearly distinguished from those of freshwater lacustrine or predominantly higher plant origin. Maturity was also shown to significantly influence the relative abundance of thiophens, most notably in those sequences which contained kerogens with high initial organic sulphur contents. In these samples a preferential loss of 2,3-dimethylthiophen relative to alkenes and aromatic hydrocarbons was observed with increasing maturation. Quantitative pyrolysis data showed that the absolute amounts of thiophens generated from sulphur-rich kerogens is high and that there is a marked depletion in yields with increasing maturity. It is believed that the extension of py-g.c. to the characterization of macromolecularly bound organic sulphur will prove useful in fossil fuel research as a supplementary source of information on organically bound sulphur.