Identification of long-chain 1,2-di-n-alkylbenzenes in Amposta crude oil from the Taragona Basin, Spanish Mediterranean : implications for the origin and fate of alkylbenzenes

Abstract

Homologous series of C₁₅-C₄₀ 1, 2-di-n-alkylbenzenes (with alkyl side chains containing 2 or more carbon atoms) were identified in the Amposta crude oil (Tarragona Basin, Spain). Structural assignments were confirmed by synthesis of the C₁₈ members of this series. These 1, 2-dialkylbenzenes in combination with monoalkylbenzenes and 2-alkyltoluenes dominate the alkylbenzene distribution in this "immature" oil. This phenomenon lends support to the hypothesis that alkylbenzenes are formed in the subsurface by cyclisation and aromatisation reactions of linear, functionalised precursors. In other, more mature, oils other alkyltoluenes isomers are present as well and 1, 2-dialkylbenzenes have not been reported. Isomerisation reactions of initially formed 2-alkyltoluenes during catagenesis may lead to the formation of thermodynamically more stable isomers (i. e. , ortho and para alkyltoluenes) encountered in these more mature crude oils.