Methylated naphthalenes as indicators for evaluating the source and source rock lithology of degraded oils

Abstract

Methylated naphthalenes in oils/condensates are relatively resistant to bio- and thermal degradation and can be useful compounds for evaluating the source organic matter and source rock lithology. Assuming that cadinene, abietic acid, and cholesterol are major precursors of methylated naphthalenes in oils and condensates, laboratory hydrous pyrolysis experiments for these possible precursors were performed to understand the precursor/products relationship. The major dimethylnaphthalenes (DMNs) and trimethylnaphthalenes (TMNs) in pyrolyzates of cadinene and abietic acid were 1,2,5-TMN and 1,6- and 1,5-DMNs, whereas those in pyrolyzates of cholesterol were 2,3,6-TMN and 1,3- and 1,7-DMNs. The experimental results suggested that the dimethylnaphthalene ratio (DMR) of [1,5- + 1,6-DMNs]/[1,3- + 1,7-DMNs], and the trimethylnaphthalene ratio (TMR) of 1,2,5-TMN/2,3,6-TMN, can be potential indicators of source organic matter. The relative abundance of TMNs in the pyrolyzates was controlled largely by the maturity level and presence or absence of montmorillonite catalyst. The relative abundance of total TMNs to total MNs (ΣTMNs/ΣMNs ratio) can be a novel indicator for evaluating the clay-catalytic effect in source rocks. A lower ΣTMNs/ΣMNs ratio is related to clay-poor source rocks, such as coal, carbonates, and siliceous rocks, whereas a higher ΣTMNs/ΣMNs ratio suggests shaly (clay-rich) source rocks. The DMR, TMR, and ΣTMNs/ΣMNs ratios of various oils and condensates are very consistent with their source organic matter and source rock lithology, indicating that these indicators are practically useful.