Aromatic compounds in lacustrine sediments from the Lower Cretaceous Jiufotang Formation, Chaoyang Basin (NE China)

Abstract

Twenty-three lacustrine sediment samples from the ZK1006 well in the third member of the Lower Cretaceous Jiufotang Formation in the Chaoyang Basin (NE China), were studied using gas chromatography-mass spectrometry to investigate their aromatic compounds. Aromatic fractions in these sediments are mainly composed of alkylbenzenes, biphenyl, naphthalene, phenanthrene, fluorene, dibenzofuran, dibenzothiophene and their alkyl derivatives, benzohopanes, aromatic triterpenoids, and mono- and tri-aromatic steroids. Environment-related aromatic parameters (e.g. ternary plot of fluorene, dibenzofuran and dibenzothiophene, and dibenzothiophene/phenanthrene) suggest the sediments were deposited in a brackish, lacustrine sedimentary environment with low sulphur content. Maturity-related aromatic parameters (e.g. methylphenanthrene index, methyldibenzothiophene ratio, triaromatic steroids ratio) show that all the samples have reached the early oil generation window. Source-related aromatic parameters (e.g. retene/(retene + cadalene), arborane/fernane-derived triterpenes) are controlled by vertical variation of primary productivity in the early Cretaceous lake, from a higher plant-dominated shallow lake to an aquatic organism-dominated semi-deep to deep lake. The large variation in the origins of the organic matter in the oil shales, siltstones and mudstones is interpreted to be due to the effects of volcanic ash and gravity-flow deposition, which directly contributed to the local biomass.