THERMAL MATURITY AND HYDROCARBON GENERATION IN ROCKS FROM THE SEDIMENTARY BASINS OF MADAGASCAR

Abstract

Oil exploration in Madagascar has been concentrated on the Morondava Basin in the SW of the island, which is well‐known for its large deposits of tar sand and heavy oil at Bemolanga and Tsimiroro. Petroleum shows have frequently been encountered in exploration wells, but source‐rock identification has remained inconclusive. Thus, the present study of a wildcat was originally related mostly to geochemical prospect appraisal. Due to the inform oraganic matter type and wide range of thermal evolution stages sampled, the case history has more general implications, however, as regards relationships between thermal maturation and hydrocarbon generation.Evaluation of eleven rock samples taken at depths of between 1,100 and 2,500m by Rock‐Eval, solvent extraction and vitrinite reflectance (Rr) methods revealed only poor source rocks for oil with marginal through to post‐mature Type III kerogens (Rr between 0.5% and 1.4%) Discrepancies in yield profiles of total C15+ ‐hydorcarbons peaking at higher Rr in the Morondava and Dauala Basin (Cameroon) wells than in a Deep Basin (West Canada) well are tentatively attributed to their different geothermal histories. However, yield trends of specific saturated and aromatic hydrocarbon families, as determined by capillary gas chromatography, did not testify to the suspected effect of geothermal rate variations on generation curves, which hence remain unproven.Based on systematic changes with depth in the distribution of methylated di‐ and tri‐aromatics, the interpretration of maturation levels with respect to hydrocarbon generation was refined. When converted to vitrinite reflectance equivlents (Rc), molecular parameters such as the Trimithylnaphthalene Ration (TNR 1) and the Methylphenanthrene Index (MPI I) were able to disclose gradual marurity changes.In contrast to the large Morondava Basin, the smaller Majunga Basin in NW Madagascare has been the site of little explortion activity until recently. Based on organic‐geochemical analysis of seven rock samples from two exploration wells by the above‐mentioned methods, it was found that good source rocks for oil are absent. Some of the samples may have a considerable gas generation potential, which at best has been partially reasised. Certainly, maturities inadequate for substantial gas generation are indicated by aromatic parameters.