Abstract
Diamondoids in crude oil are useful for assessing the maturity of oil in high maturation. However, they are very difficult to separate and accurately quantify by conventional geochemical methods due to their low abundance in oil. In this paper, we use comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) to study the compounds in condensates from the Junggar Basin of northwest China and address their geological and geochemical applications. GC×GC-TOFMS improves the resolution and separation efficiency of the compounds. It not only separates the compounds that coelute in conventional GC-MS (e.g., 4, 8-dimethyl-diamantane and trimethyl-diamantane) but also allows the identification of compounds that were not previously detected (e.g., trimethyl-diamantane (15A)). A reversed-phase column system improves the separation capabilities over the normal phase column system. The diamondoid indexes indicate that a representative condensate from Well DX 10 is highly mature with equivalent Ro being approximately 1.5%.
Highlights
Diamondoids are rigid, three-dimensional cyclohexane-ring alkanes that have a diamond-like cage structure [1,2]
Analytical results show that the diamondoid compounds identified in our work mainly include adamantanes and diamantanes
The carbon number of adamantanes that were identified in the condensate from Well DX 10 ranged from C10 to C14
Summary
Diamondoids are rigid, three-dimensional cyclohexane-ring alkanes that have a diamond-like cage structure [1,2]. They are generally more stable than other hydrocarbon compounds in geological evolution [3,4] Because of these factors, diamondoids have important geochemical implications in studying source rocks and oils (especially with high degree of thermal evolution), including maturity, biodegradation and organic facies. In comparison with conventional GC-MS analysis, the method of comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) is effective for separation and identification of complex mixtures, being characterized by high resolution, high sensitivity, large peak capacity, rapid analysis speed and regular qualitative analysis [11,12,13,14,15,16,17] Using this method, diamondoids can be effectively distinguished simultaneously with conventional hydrocarbon compounds [18], providing good conditions for the study of diamondoids.
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