Yanqing Xia

Aliphatic and polycyclic aromatic hydrocarbons (PAHs) in soils of the northwest Qinling Mountains: Patterns, potential risk and an appraisal of the PAH ratios to infer their source

ABSTRACT Surface soils from the tourist areas of the northwest Qinling Mountains were analyzed to determine the concentrations, probable sources and potential risks of hydrocarbons. Concentrations of aliphatic and aromatic hydrocarbons ranged from 4.18 to 3240 ng g−1 and 0.0462 to 101 ng g−1 dry weight, respectively. The extent of soil contamination by hydrocarbons was generally typified by unpolluted to slightly polluted levels. The incremental lifetime cancer risks (ILCRs) for exposure to soil-borne PAHs indicated complete safety for tourists. Early diagenesis of natural products, bacteria activities and petroleum were the three main sources of aliphatic hydrocarbons, while the transport of air pollutants from pyrolytic processes was the main origin of PAHs. Because the photochemical reaction of PAHs in the atmosphere would produce lower ratios for Ant/(Ant + Phe), BaA/(BaA + Chr) and IcdP/(IcdP + BghiP), but a higher ratio for Fla/(Fla + Pyr), the source classification highly depended on the diagnostic ratios chosen. The plot of ΣCOM/Σ13PAH vs. ΣLMW/ΣHMWPAH provide additional information to distinguish the origins of PAHs, and it showed a cluster of pyrogenic sources except for sample JFS-8. Four sources were resolved by principal component analysis: (1) a low temperature pyrogenic process related to the use of fossil fuel and biomass, such as charcoal, straw and wood, which contributes 63.1% of the measured PAHs; (2) the potential contribution of diagenetic processes, contributing 18.4%; (3) traffic emissions, contributing 9.27%; and (4) bioconversion/bacterial action, contributing 5.82%. Additionally, there was a good exponential relationship (r2 = 0.969) between the natural n-alkanes ratio (NAR) and carbon preference index for C23–C35 (CPI23–35) for all samples, which is of great use for the determination of the origins of aliphatic hydrocarbon.

Possible origin of high molecular weight n-alkan-2-ones in jurassic bitumens from the Sichuan Basin in Southwest China

Lipid extracts from two Sichuan (China) bitumens have been systematically analyzed using gas chromatography—mass spectrometry (GC-MS). These extracts contain C25-C35n-alkan-2-ones with predominantly odd-carbon number homologs in the Yangba (YB) extracts and with exclusively odd-carbon number homologs in the Penglai (PL) extracts. We suggest that the n-alkan-2-ones with odd numbers of carbon atoms originated due to degradation (cleavage) of fatty acid salts containing one less carbon atom to form methyl and carbonyl radicals, followed by reaction between the latter. The distribution of n-alkan-2-ones with respect to number of carbon atoms can be used to identify the organic matter in sedimentary rocks from which the fatty acid salts were formed.

Volcanic reservoir characteristics and hydrocarbon genesis of Jiamuhe formation in Jinlong 2 wellblock, Junggar Basin

Abstract Due to deeper buried, multi-stage tectonic influences, rapid changing, and unclear originate of oil and gas in Jinlong 2 wellblock, we analyzed the volcanic reservoir characteristics and the genesis of hydrocarbon by cores, thin slices, loggings, physical properties, oil testing data, and carbon isotopes. Results demonstrated that the major lithology were pyroclastic rocks and lavas, the reservoir was generally poor and strongly homogeneous, the main reservoir space were primary pores, and the favorable reservoir mainly developed in the pyroclastic rocks of the third phase. Furthermore, the results of carbon isotopes indicated that the crude oil and dissolved gas originated from Fengcheng formation, while the condensate gas derived from Jiamuhe formation.

Oil-source correlation and the impact of oil-filling process on geochemical composition of the different states of oils in the Upper Triassic Chang 7 Formation, Ordos basin, China

ABSTRACT 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 reservoirs whether are the mixtures of oil components derived from different source rocks or from the same source rock during oil filling process over geological times. Sequential extraction was applied to the oil-containing reservoir rocks to deserve the free and adsorbed oils. The distribution of alkanes, hopanes and steranes and the correlation diagram of Pr/n-C17 versus Ph/n-C18 show that these oil components and crude oils have similar parent materials. And on this basis we compared the thermal maturity of the crude oils, the free oils and adsorbed oils and found that the thermal maturity of these oils is different. The cross plot of C29αα-20S/(20S+20R) versus C29ββ/(αα+ ββ) and the correlation diagram of Pr/n-C17 versus Ph/n-C18 both show that the crude oils have highest thermal maturity, followed by the free oils and then the adsorbed oils. The ratios of ∑C21−/∑C22+ for the crude oils and free oils are greater than the adsorbed oils, indicating the crude oils and free oils have suffered more thermal stress and extensive cracking than that of the adsorbed oils. These geochemical data reveal that hydrocarbons in these oil reservoirs and crude oils were derived from the same source rock with different thermal maturity over geological times.

Development of a novel method for the enrichment of unsubstituted naphthalene and phenanthrene from crude oils for compound-specific stable isotope analysis

A novel, effective and easy-to-operate method to enrich unsubstituted naphthalene and phenanthrene directly from crude oils was developed based on closed-system pyrolysis. The pyrolysis at 500 °C for 12 h was found to be sufficient for the enrichment of unsubstituted aromatics that were free of unresolved complex mixture (UCM) and poly-substituted aromatics. This allowed further accurate compound specific isotope analysis, and GC-IRMS data showed that the δ13C values of unsubstituted naphthalene and phenanthrene had no obvious fractionation during the enrichment process. This method provides an easy-to-operate pre-concentration technique for unsubstituted aromatics suitable for compound-specific isotope analysis (CSIA), laying the foundation for developing new oil–oil correlation indicators.

Improvement of saponification extraction method for fatty acids separation from geological samples

The conventional saponification method could result in lower recoveries and artificial changes of long-chain fatty acids. The main reason is the error judgment of the intermediate layer suspended between the aqueous and organic layer during the liquid–liquid extraction process. This study shows that the intermediate layer consists of lots of medium- to long-chain carboxylic salts for their special physical and chemical properties. An improved saponification extraction method is also developed and the results show that the carboxylic salts distributed in the intermediate layer could be obtained completely, which greatly enhances the authenticity and accuracy of fatty acid analysis. Additionally, the possible reasons of formation of the intermediate layer are also discussed.