Distribution of alkylated dibenzothiophenes in petroleum as a tool for maturity assessments

Aromatic fractions of 64 oil and condensate samples from various basins of the Former USSR were analyzed using GC with a flame-photometric detector (FPD), and GC/MS to investigate relative distributions of aromatic sulfur compounds with respect to maturity. Gas chromatography with FPD has shown that thermal stress dramatically changes distributions of aromatic sulfur compounds resulting in total disappearance of thermodynamically unstable benzothiophenes (BTs) and the unresolved envelope at the end of the chromatogram. Relative concentration of dibenzothiophene (DBT) seems to be organic facies controlled and does not show systematic change with increasing maturity. Relative concentrations of alkylated dibenzothiophene homologues of methyl-, dimethyl-and trimethyl-dibenzothiophenes measured by GC/MS appeared to be sensitive to maturity. Several new maturity parameters have been proposed based on the assumption that alkyl-DBT isomers with the methyl substituent in the 4-position are the most thermodynamically stable, whereas isomers with the methyl substituent in the 1-position have low stability. The newly proposed maturity parameters the 4,6-DMDBT/1,4-DMDBT, 2,4-DMDBT/1,4-DMDBT and TMDBT index correlate well with the 4-MDBT/1-MDBT (MDR) indicator and can be used effectively for maturity assessments. (C) 1997 Elsevier Science Ltd.