Predicting the generation of heavy oils in carbonate/evaporitic environments using pyrolysis methods

A series of low maturity kerogen and asphaltene samples of a sulphur-rich Mediterranean carbonate source rock sequence, as well as two associated oil samples were analysed by different pyrolysis methods. The source rock was characterised by the occurrence of four different lithotypes, three of which possessed source potential. Elemental analysis revealed that each source lithofacies contained a distinct kerogen type comprising Type I-S for shale intercalations, Type II-S in the laminated lithofacies and low sulphur Type II in the pure carbonate portions of the source rock. Pyrolysis-gas chromatography using coupled FID and HECD detectors provided a more detailed insight into kerogen/asphaltene composition than afforded by Rock-Eval pyrolysis. Bulk kinetic analysis demonstrated that asphaltenes and Type I-S kerogens were the most labile species present in the source rocks. Type II-S kerogens, often quoted as the major source of heavy oils, were the most stable of the sulphur-rich organic matter types. The occurrence of various kerogen types with differing stabilities in the carbonate source rock analysed, as determined by bulk pyrolysis-FID, implies that peak hydrocarbon generation in the source rock sequence is not confined to a single temperature, but rather to a relatively large temperature range of roughly 20°C. Combination of bulk hydrocarbon generation kinetics and a multistep pyrolysis approach demonstrated that for carbonate evaporitic environments it is necessary to differentiate between early and late maturity products. This is because the lower energies (46–49 Kcal/mol), which may be attributed to S---C bond cleavage reactions, are empirically of greater importance, especially in view of the fact that they may represent the main generation stage under natural conditions.