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Xiong’s research group revealed the effect of evaporation on the concentration and distribution of diamondoids in oils TEXT SIZE: A A A

Diamondoids are commonly found in petroleum and sediments and have an inherent resistance to thermal and biological destruction, which means they can provide useful information in situations where conventional biomarkers cannot. The different evaporation behavior of individual diamondoids and other components in oils can cause changes in the concentrations and distribution of hydrocarbons, further affecting diamondoid parameter-based interpretations. The majority of previous research on the effects of evaporation merely identified the general volatility of diamondoids, rather than the exact evaporation behavior of these compounds. 

Here, we present the results of an investigation of the effects of atmospheric evaporation on the concentration and distribution of low molecular-weight diamondoids in four petroleum fractions (gasoline, condensate, diesel, and fuel oil). These experiments indicate that both adamantanes and diamantanes evaporate with the other light hydrocarbons from oils, and that variations in the concentrations of these compounds during evaporation are controlled by the type of petroleum fraction, the extent of evaporation, and the boiling point of the diamondoid compounds within the oil. Evaporation has a significant effect on adamantane concentration ratios, whereas no changes in diamantane concentration ratios occur, suggesting that diamantane-based concentration and distribution indices can be used for the correlation of oils and determination of maturity even if even if oils have undergone evaporation. Some diamondoid concentration ratios, such as adamantane/1-methyladamantane, 1-methyladamantane/2-methyladamantane, 1-methyladamantane/1-ethyladamantane, 1-methyladamantane/4-methyldiamantane, adamantane/diamantane, and 1,3-dimethyladamantane/4,9-dimentyldiamantane,, progressively decrease with ongoing evaporation and are independent of petroleum fraction type, indicating that given the original unaltered index value, these indices can be used to deduce the relative extent of oil evaporation. The study also indicates that slight to moderate evaporation of oils leads to an increase in diamantane concentrations that is nearly proportional to the extent of oil evaporation, indicating that these compounds can be used as indices to estimate the extent of oil evaporation.

The results suggest that the influence of evaporation probably interferes with the interpretation of some diamondoid indices, a factor that needs to be considered when these indices are applied to natural examples. For example, a DMAI-1 vs. DMAs/MDs cross plot has been suggested as a possible tool for the identification of oil maturity. However, both DMAI-1 and DMAs/MDs ratios progressively decrease with increasing evaporation, thus, the experimental result can explain that evaporation prior to pyrolysis is one likely reason that causes the difference between measured and calculated data in the plot of DMAI-1 vs. DMAs/MDs.

This work was recently published in Organic Geochemistry (Yun Li, Yongqiang Xiong, Yuan Chen, Youjun Tang. The effect of evaporation on the concentration and distribution of diamondoids in oils. 2014, 69: 88-97).

(Supplied by the State Key Laboratory of Organic Geochemistry & Science and Technology Department, CASGIG)

 

 

 

 

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