Quantification of C10–C14 Adamantanes in High-Viscosity Naphthenic Oils

Abstract

The paraffin–naphthenic fractions (with boiling points below 310°C) prepared from three high-viscosity naphthenic crude oils, classified as types B1 and B2 (according to Petrov’s classification), were subjected to thiocarbamide complexation. The molecular composition of polycyclic hydrocarbon biomarkers and C11–C13 adamantanes in the oil samples suggested a predominantly marine genotype of the precursor organic matter (OM). The molecular composition also suggested source rocks of a clayey type. Nonetheless, the biomarkers detected in one sample indicated some contribution of terrigenous components to the precursor OM. All the oils were generated under the conditions of the main oil generation zone and, presumably, underwent microbial transformations in the deposits. The compositions of C10–C14 adamantanes in the initial paraffin–naphthenic fraction, in the thiocarbamide adduct, and in the filtrate that remained after the adduction were comparatively characterized for each oil sample. The test conditions allowed us to have adamantane more than 100-fold concentrated (in the adduct), to quantify it in the oils, and to evaluate the concentrations of C11–C14 alkyladamantanes in the oils using adamantane as an internal standard. C10–C14 adamantanes exhibited selective adduction ability, with the extraction ratios of individual components being different. Taking into account these extraction ratios, the component concentrations were evaluated on crude oil basis: 2.7 to 7.6×10–3 wt % for adamantane and 87 to 267×10–3 wt % for total C10–C14 adamantanes. The identification of adamantanes in the initial paraffin–naphthenic fractions, adducts, and filtrates revealed the presence of some other tricyclanes (probable precursors of alkyladamantanes) as well as decaline homologues. Like adamantanes, these compounds exhibited selective ability to complex with thiocarbamide.