INFLUENCE OF MATURITY ON DISTRIBUTIONS OF BENZOTHIOPHENE AND DIBENZOTHIOPHENE IN TITHONIAN SOURCE ROCKS AND CRUDE OILS, SONDA DE CAMPECHE, MEXICO

The Sureste Basin is the most prolific oil-producing area of Mexico, e xtending from the mainland into the Sonda de Campeche offshore region. The Tithonian sedimentary sequence is the most important source of hy drocarbons. It contains sulphur-rich kerogen which progressively crack s to form petroleum beginning at 0.53% R-r. Maturity increases from no rtheast to southwest across the region. Sulphur-compounds in the Titho nian source rocks and in the petroleums they have generated have been studied by gas chromatography. Alkylbenzothiophenes are present in hig hest abundance at low maturity whereas the alkyldibenzothiophenes are most abundant at higher stages of maturity. The relative abundance of specific C-3-benzothiophene isomers are especially sensitive to change s in maturity of sulphur-rich Type-II organic matter. A novel maturity parameter, the C-3-benzothiophene index (C3BTI), shows a very good co rrelation to vitrinite reflectance values (R-2 congruent to 0.96, n = 10). The methyldibenzothiophene ratio MDR' also has a wide dynamic ran ge. 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 ailing history of reservoirs in th e southwest part of the Sonda de Campeche. (C) 1998 Published by Elsev ier Science Ltd. All rights reserved.