Organic sulphur in macromolecular sedimentary organic matter: I. Structure and origin of sulphur-containing moieties in kerogen, asphaltenes and coal as revealed by flash pyrolysis

The distributions of sulphur-containing compounds generated by flash pyrolysis of macromolecular sedimentary organic matter (kerogen, coal, asphaltenes) were studied by gas chromatography in combination with Sselective flame photometric detection or mass spectrometry. The abundance of S-containing pyrolysis products in the pyrolysates relative to other products was highly variable depending on the sample but the types of products were generally similar, being mainly composed of “gaseous” compounds (e.g., hydrogen sulphide) and low molecular weight alkylthiophenes and alkylbenzothiophenes. The distribution patterns of the alkylated thiophenes were dominated by a limited number of all theoretically possible isomers. The alkyl substitution patterns of the dominant isomers bear a strong similarity to those of the organic S compounds present in the GC-amenable fractions of bitumens and immature oils. Therefore, it is suggested that these S-containing pyrolysis products are formed by pyrolysis of related thiophenic and benzothiophenic moieties present in the macromolecular sedimentary substances. Specific examples include those with linear alkyl, iso and anteiso alkyl, isoprenoid alkyl and steroidal carbon skeletons. The presence of higher molecular weight alkylthiophenes and alkylbenzothiophenes with these same carbon skeletons in pyrolysates of S-rich kerogens provided further evidence for the presence of these S-containing moieties. It is likely that these moieties have been formed by abiogenic S incorporation into sedimentary organic matter during early diagenesis.