STRUCTURAL-ANALYSIS BY NMR AND FIMS OF THE TAR-SAND BITUMEN OF BEMOLANGA (MALAGASY)

The detailed characterization of the tar-sand bitumen of Bemolanga (Malagasy) was performed with the aid of NMR and FIMS techniques. After precipitation of the asphaltenes an elaborate separation of maltenes was carried out by HPLC. The number average molecular weight measurements indicate that this bitumen is much heavier than American bitumens (MWBAR = 900 against 534 for the Athabasca bitumen for example). The modeling shows that the average length of the alkyl chains and the ratio of the number of naphthenic rings to that of aromatic rings increase from the aromatic fractions to the resin fraction and then decrease for the asphaltenes. Moreover, the average length of the alkyl chains is always larger than that in the corresponding fraction of American bitumens. All the results indicate that the saturated fraction does not contain many linear paraffins and is constituted of naphthenic rings on which alkyl chains are branched. The change from the saturated fraction to the aromatic fraction with the same molecular weight is made by substituting the naphthenic rings by aromatic rings. The more saturated character of the Malagasy bitumen with respect to other bitumens is explained by the difference in their geological origin; the Bemolanga bitumen, coming from a Rift basin, is relatively protected and, consequently, less subjected to degradation by microorganisms.