Methylation and demethylation of naphthalene homologs in highly thermal mature sediments

Abstract

Molecular compositions of naturally matured source rocks from the Carboniferous Keluke Formation in the Qaidam Basin and laboratory matured samples from the Cretaceous Qingshankou Formation in the Songliao Basin were analyzed to investigate the thermal evolution behaviors of methylation and demethylation of naphthalene homologs. Sediments in the Keluke Formation were variably mature with measured vitrinite reflectance (Ro) ranging from 0.74% to 1.88%. Concentrations of alkylated naphthalene homologs, including dimethylnaphthalenes (DMNs), trimethylnaphthalenes (TMNs), tetramethylnaphthalenes (TeMNs) and pentamethylnaphthalenes (PMNs), exhibit an increasing trend with thermal maturation below 1.1 %Ro, but present an abrupt shift with decreasing concentrations above 1.1 %Ro. The isomerization, methylation and demethylation processes play significant roles in the distribution of alkylnaphthalenes, but dominates at different thermal maturity levels. Isomerization and methylation play dominant roles at relatively low thermal maturity levels (Ro < 1.1%) whereas demethylation dominates the distribution of alkylnaphthalenes in highly mature sediments (Ro > 1.1%) when less stable isomers of alkylnaphthalenes, such as 1,2,5-TMN, 1,2,5,6-TeMN and 1,2,3,5-TeMN, are almost eliminated and more stable isomers dominate the remaining homologs. The transition from methylation to demethylation of alkylnaphthalenes occurs at ca. 1.1 %Ro, which is lower than that of alkylphenanthrenes (1.35 %Ro) due to thermally induced condensation of aromatic hydrocarbons. The demethylation of alkylnaphthalenes at higher thermal maturity levels (Ro > 1.1%) results in the failure of thermal maturity parameters derived from alkylnaphthalenes, such as TMNr and TeMNr.