ABSTRACT

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-C₁₂, while the organic-lean samples are enriched in medium molecular weight n-alkanes with the maximum around n-C₁₆. 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.