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Five crude oil samples from five wells and 33 oil-containing sandstone reservoir rock samples from six wells of Chang 7 sub-unit were systematically studied to determine hydrocarbons in these oil r...

A series of anhydrous closed-system pyrolysis experiments were performed on pure pristane and octacosane and a sample from the Maoming oil shale to measure the effect of thermal maturation on pristane and phytane carbon isotopic fractionation compared with individual n-alkanes. The result showed that pyrolysis of pure pristane and octacosane caused 1.8‰ and 5.1‰ increase in δ13C values, respectively. Experiments on the oil shale sample showed 2.0‰ and 2.2‰ variations in pristane and phytane δ13C values, respectively, but an average 5.5‰ enrichment in 13C for the individual n-alkanes. This indicates that the effect of thermal maturation on pristane and phytane δ13C values is weaker compared with n-alkanes. The results further suggest that the stable carbon isotopic distribution of pristane and phytane can be used as a correlation tool.

Anhydrous closed-system pyrolysis experiments were conducted on the pure alkyl naphthalenes, n-alkanes and three crude oils at the highly to over mature stage in order to investigate compositional and isotopic changes of individual aromatic hydrocarbons during laboratory maturation. On the whole, aromatic hydrocarbons have better thermal stability than n-alkanes. While the carbon isotopic composition of substituted aromatic hydrocarbons exhibits a positive correlation with increasing thermal maturity, there is no significant fractionation in the carbon isotopic composition of unsubstituted aromatic hydrocarbons. This finding suggests that the carbon isotopic distribution of unsubstituted aromatic hydrocarbons can be used as an effective tool for oil-oil and oil-source correlation, especially for highly-over mature fields.

Aromatic hydrocarbons are an important component in crude oil and sediments. Furthermore, the structure of macromolecular organic matter (such as kerogen) is also based on aromatic nuclei. The earlier methylation and later demethylation of various aromatic nuclei would affect the chemical components and carbon isotopes of aromatics as well as saturated hydrocarbons and gaseous hydrocarbons. The scope of the present study was to investigate the role of aromatic nuclei in the formation and geochemical evolution of natural gas in complex geological systems. The use of model compounds could simplify the complex system, and n-tricosane (n-C23), n-tetracosane (n-C24), and naphthalene (N) were selected as model compounds for paraffins and aromatic hydrocarbons and subjected to pyrolysis experiments. The results showed that aromatic nuclei had a slight influence on the gas generation without using a catalyst. However, the gaseous components were substantially affected by aromatic nuclei under the catalysis of clay, when the temperature was lower than 400 °C. The ratios of isoparaffin to n-alkane of gaseous hydrocarbons were reduced by the aromatic nuclei. As carbon isotopically lighter methyls preferred to combine with the aromatic nuclei, the gas productions were reduced, and there was a 13C depletion of methane with increasing temperature, which is contrary to the case of paraffin cracking alone. This trend can also be observed in the pyrolysis gases formed from the type II and III kerogens. The macromolecular aromatic nuclei played a similar role with the small aromatic hydrocarbons. These effects would mainly occur in the mature to highly mature stage of the organic matter (the paleo-geothermal temperature is ≤ 150 °C). The above explorative experiments suggested that a large number of aromatic nuclei and their methylation reactions could exert a significant influence on the distributions and the carbon isotopic compositions of gaseous hydrocarbons in the evolution of sedimentary organic matter. This research also provided a new explanation for the carbon isotopic abnormality (the 13C-depleted methane and the increasing δ13C2-δ13C1 values) of deep natural gas of the Tarim Basin, in northwest China.