Identification and distribution of chrysene, methylchrysenes and their isomers in crude oils and rock extracts

Chrysene, its methylated homologues and their structural isomers are important polyaromatic compounds in crudes oils, coals and sediment extracts. Chrysene, benzo[a]anthracene, methylchrysenes and methylbenzo[a]anthracenes were unequivocally identified in mass chromatograms by comparison with authentic standards and by retention indices reported in the literature. The response factors relating those of chrysene and methylchrysenes to internal standards [perdeuterated chrysene (chrysene-d12) and pyrene (pyrene-d10)] were obtained by GC–MS analyses of mixture solutions with different concentration ratios. The oil families of Ordovician reservoirs in the cratonic region of the Tarim Basin (NW China) are distinguished by the distribution patterns of chrysene, methylchrysenes and their isomers. The majority of the oil accumulations and sediment extracts from the Middle–Upper Ordovician source rocks are characterized by low concentrations of benzo[a]anthracene and methylbenzo[a]anthracenes relative to chrysene and methylchrysenes, respectively. In contrast, some oils with relatively high abundances of benzo[a]anthracene and methylbenzo[a]anthracenes were derived from Cambrian and Lower Ordovician source rocks. The effect of maturity on the distribution patterns of methylchrysenes in Tertiary lacustrine shales from the Liaohe Basin (East China) was investigated. The 2-/1-methylchrysene ratio keeps a nearly constant value with increasing maturity at the low maturity stages and then it abruptly increases, corresponding approximately to the onset of intense C 15 + hydrocarbon generation. This indicates that chrysene and its methylated homologues are useful molecular indicators in petroleum geochemistry.