Influence of maturity on distributions of benzo- and dibenzothiophenes in Tithonian source rocks and crude oils, Sonda de Campeche, Mexico

The Sureste Basin is the most prolific oil-producing area of Mexico, extending from the mainland into the Sonda de Campeche offshore region. The Tithonian sedimentary sequence is the most important source of hydrocarbons. It contains sulphur-rich kerogen which progressively cracks to form petroleum beginning at 0.53% Rr. Maturity increases from northeast to southwest across the region. Sulphur-compounds in the Tithonian source rocks and in the petroleums they have generated have been studied by gas chromatography. Alkylbenzothiophenes are present in highest abundance at low maturity whereas the alkyldibenzothiophenes are most abundant at higher stages of maturity. The relative abundance of specific C3-benzothiophene isomers are especially sensitive to changes in maturity of sulphur-rich Type-II organic matter. A novel maturity parameter, the C3-benzothiophene index (C3BTI), shows a very good correlation to vitrinite reflectance values (R2 0.96, n=10). The methyldibenzothiophene ratio MDR′ also has a wide dynamic range. Application of C3BTI and MDR′ to the study area strongly supports a model involving localised vertical migration avenues, though several possible scenarios may explain the filling history of reservoirs in the southwest part of the Sonda de Campeche.