Ronghui Fang

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.