Shengbao Shi

Qualitative and quantitative analysis of dibenzothiophene, its methylated homologues, and benzonaphthothiophenes in crude oils, coal, and sediment extracts

Polycyclic aromatic sulfur heterocyclics (PASHs) consist mainly of thiophene class compounds, and are the most important organosulfur compounds in crude oils and sediment extracts. Dibenzothiophene (DBT) and its methylated homologues were identified on mass chromatograms by comparison with retention indices published in the literature. Some isomers of dimethyldibenzothiophene and trimethyldibenzothiophene that were tentatively identified in previous reports have been determined here by comparison with calculated retention indices and taking the substitution pattern of the methyl groups into account. The response factors relating that of dibenzothiophene to internal standards were obtained by GC-MS analyses of mixture solutions with different concentration ratios. We concluded that DBT-d8 (octadeutero-dibenzothiophene) is the optimal internal standard for quantitative analyses of the thiophene compound class in oils, coal, and sediment extracts. Calibration experiments for each class of compounds are absolutely necessary when quantifying polycyclic aromatic hydrocarbons and other heterocyclics with a stable isotope labeled internal standard.

Oil maturity assessment using maturity indicators based on methylated dibenzothiophenes

Aromatic fractions of 140 oils and condensates that originated from different types of source rocks (marine shale, terrestrial shale and marine carbonate) were analyzed using gas chromatographymass spectrometry (GC-MS) to investigate the relative distributions of methylated dibenzothiophenes with respect to thermal maturity. The positions of methyl groups of trimethyldibenzothiophene isomers (TMDBTs) including those used in the definition of maturity indicator TMDBT index in previous studies were firmly identified by co-elution of internal standards in GC-MS analysis and by comparing with reported retention indices. A new maturity ratio related to dimethyldibenzothiophenes (DMDBTs) is proposed on the basis of the differences in thermodynamic stability among different DMDBT isomers. Another maturity index (TMDBT-I2) based on TMDBTs is also suggested on the basis of our empirical observations and presumed thermodynamic stability of TMDBT isomers. These two newly proposed (2,6 + 3,6)-/l,4-DMDBT ratio and TMDBT-I2 correlate well with MDR (4-/l-methyldibenzothiophene) and 2,4-/1,4-DMDBT ratios, suggesting their common chemical reaction mechanisms and similar behavior with increasing maturity. Therefore, they can be effectively applied for maturity assessments. Furthermore, the TMDBTs related maturity parameters are more reliable for over-mature oils and condensates due to the relatively higher concentrations of thermodynamically unstable TMDBT isomers, i.e. 1,4,6-, 1,4,8- and 3,4,6-TMDBT in this study than those of 1-methyldibenzothiophene (1-MDBT) or 1,4-DMDBT. In contrast with 4,6-/1,4-DMDBT, the newly proposed (2,6 + 3,6)-/1,4-DMDBT ratios for oils that originated from different types of source rocks have approximately same relationship with the oil maturity (Rc %). This suggests that the lithology and organic facies may have relatively less influence on (2,6 + 3,6)-/1,4-DMDBT ratio compared to 4,6-/1,4-DMDBT. The maturity parameters based on methylated dibenzothiophenes are particularly useful in the maturity assessments of post- and over-mature oils and condensates and can complement maturity indicators based on steranes and terpanes.

Geochemical characteristics and families of the Paleozoic oil seepage and solid bitumen in the Southern Guizhou Depression, SW China

Fifteen oil seepage and solid bitumen samples in the Southern Guizhou Depression were analyzed with GC–MS. Characteristics of molecular markers and carbon isotopes are discussed systemically. The results showed that the oil seepage and solid bitumen samples in the Southern Guizhou Depression could be divided into two families: Ordovician and Siluric samples, and Permian samples. The two families are different in alkanes distribution, biomarkers, aromatic hydrocarbon composition, and stable carbon isotopes; differences mainly caused by source rock variation.