Some Biogenic-Related Compounds Occluded inside Asphaltene Aggregates

Abstract

The occluded compounds inside asphaltene aggregates were released from 16 asphaltenes through mild degradation processes oxidized by the H2O2/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-carbon-numbered 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.

Determination of the series of even carbon numbered n-alk-1-enes trapped inside geomacromolecules

“…Sixteen asphaltenes from the Tarim Basin, NW China, were analyzed for trapped hydrocarbons (Yang et al, 2009), among which the series of even carbon numbered n-alk-1-enes have been recovered from 13 samples, which generally ranged from C 16 eC 30 with the ratios of the paired -ene/-ane >1. Besides these alkenes, in the trapped fraction 22,29,30-trisnorhop-17(21)-ene and 17a-hop-20(21)-ene were detected from 3 asphaltenes, and C 23 tricyclic terp-12(13)-ene and C 29 18a-30-norneohop-13(18)-ene were found in another 3 asphaltenes (Yang et al, 2009).…”

“…However, recent studies have shown that a series of even carbon numbered n-alk-1-enes are present in the trapped fractions of asphaltenes (Yang et al, 2009;Zhao et al, 2010Zhao et al, , 2012Tian et al, 2012) and solid bitumens (Cheng et al, 2012(Cheng et al, , 2013. They may be stable in these geomacromolecules owing to the protection afforded by the matrix, which inhibits transformation of the trapped n-alk-1-enes during later geochemical processes.…”

Multiple-sourced features of marine oils in the Tarim Basin, NW China – Geochemical evidence from occluded hydrocarbons inside asphaltenes

“…Well protected from secondary alteration events, these occluded hydrocarbons represent the original oil derived from kerogen (Liao et al, 2006). A H 2 O 2 /CH 3 COOH oxidative system was used to release occluded hydrocarbons from the asphaltenes (Liao et al, 2005(Liao et al, , 2006Yang et al, 2009;Zhao et al, 2010). The procedure, details of which can be found in Liao et al (2006), included the preparation of asphaltenes by n-hexane precipitation, acetone extraction of n-hexane-asphaltenes to obtain the adsorbed fraction, and H 2 O 2 /CH 3 COOH oxidation of the acetone-extracted asphaltenes to release the occluded fraction.…”

“…The adsorbed fractions also gave evidence of biodegraded oil mixed with normal oils, with no obvious odd to even predominance, but enrichment with high carbon number hydrocarbons (C 30 +). However, the occluded fraction inside the corresponding asphaltenes shows different distribution characteristics, without evidence of biodegradation, but slight even carbon number predominance and the presence of even carbon number terminal n-alkenes (Liao et al, 2006;Yang et al, 2009;Zhao et al, 2010). Liao et al (2005Liao et al ( , 2006 proposed that these hydrocarbons, including n-alkenes, were retained in the skeleton of asphaltenes during the early stage from kerogen to oil.…”

Experimental Studies on the Adsorption/Occlusion Phenomena Inside the Macromolecular Structures of Asphaltenes

“…n-C 20 D 42 released by H 2 O 2 / CH 3 COOH treatment belonged to the occluded compounds inside the macromolecular structures of asphaltenes according to the previous reports. 1,16,17,26 Detection of n-C 20 D 42 in the oxidized products of the acetone-extracted pyrolysates (Figure 8) suggests that the targeted compound indeed occluded inside asphaltenes during the experiments.…”