Some biogenic-related compounds occluded inside asphaltene aggregates
Résumé
The occluded compounds inside asphaltene aggregates were released from 16 asphaltenes through mild degradation processes oxidized by the H 2O2/CH3COOH system. Prior to the oxidation procedure, all of the n-C7 asphaltenes (n-heptane-insoluble asphaltenes) were subjected to Soxhlet extraction by acetone continuously for 240 h to rule out interferences from the adsorbed/co-precipitated compounds, which were likely introduced during the asphaltene preparation processes. Among these occluded compounds, a series of even-carbonnumbered n-alk-(1)-enes were detected from 13 asphaltenes, 22,29,30-trisnorhop-17(21)-ene and 17α-hop- 20(21)-ene were simultaneously detected from 3 asphaltenes, and C23 tricyclic terp-12(13)-ene and C29 18α-30-norneohop-13(18)-ene were found co-existent in another 3 asphaltenes. From all 16 asphaltenes, a series of n-alkanoic acid ethyl esters were detected from the occluded compounds, which were generally dominated by the even-carbon-numbered n-alkanoic acids in the esters. The above-reported compounds occluded inside asphaltene aggregates are interpreted as hydrocarbon representatives of materials generated from kerogen at an early stage. These unusual compounds were not detected from the crude oil maltenes or from the acetone extracts of asphaltenes, except for some n-alkanoic acid ethyl esters that were observed in the acetone extracts of a few asphaltenes. The occurrence of a series of even-carbon-numbered n-alk-(1)-enes and some terpenes inside asphaltene aggregates indicated that the macromolecular structures of asphaltenes have protected these compounds from being influenced by the alteration processes in the oil reservoirs and enable them to survive over geological time. In consequence, the geochemical study of these occluded compounds should be very meaningful, especially for heavily altered oils, such as biodegraded oils, from which it is difficult to obtain useful geochemical information directly from the crude oil maltenes because the hydrocarbons have been heavily depleted.