On the origin of molecular compositions in the Proterozoic extractable organic matter from the Jixian section, Northern China

Mesoproterozoic shales of low metamorphic grade from the Jixian section, northern China, were geochemically analyzed for bulk organic contents, molecular and isotopic compositions. One organic-rich sample from the Hongshuizhuang Formation is characterized by high total organic carbon and extractable organic material contents, high hydrocarbon generation potential, while four other organic-lean samples have almost no hydrocarbon generation potentials. Molecular compositions differ greatly between the organic-rich and lean samples. n-Alkanes in the organic-rich sample consist mainly of low molecular weight components with a maximum at n-C-12,C- while the organic-lean samples are enriched in medium molecular weight n-alkanes with the maximum around n-C-16. The monomethylalkanes and cyclic alkanes in the organic-rich sample are much more abundant than in the organic-lean counterparts, while acyclic isoprenoid alkanes are enriched in the organic-lean samples. Almost no steranes can be detected in the organic-rich sample, while a reasonable amount of steranes, dominated by pregnanes, occurs in the organic-lean samples. Bicyclic sesquiterpanes and pentacyclic terpanes are relatively concentrated in the organic-rich sample, while tricyclic terpanes are the major terpane compounds identified from the organic-lean samples. Alkylphenanthrenes are the major compound class in the aromatic hydrocarbon fractions. The ratios of parent to methylated aromatic homologues are higher in the organic-rich sample than in the organic-lean samples. All these provide convincing evidence for the existence of cyanobacteria organisms in the organic-rich Hongshuizhuang Formation. While both prokaryotes and eukaryotic organisms are likely contributors in the organic-lean samples, contamination from the Phanerozoic biomass cannot be ruled out.