Organic geochemistry of Venezuelan extra-heavy oils: 1. Pyrolysis of asphaltenes: A technique for the correlation and maturity evaluation of crude oils

Abstract

The asphaltene fraction from Cerro Negro (Venezuela) oil has been used to test the effects of pyrolysis temperature on the distribution of alkanes generated by this technique. Particular attention has been paid to the biological marker compounds (steranes, triterpanes) released in these experiments and to the correlation and maturity parameters as normally applied in organic geochemical exploration. Pyrolysis temperatures up to 330°C have little effect on the distribution of these compounds, whereas higher temperatures cause thermal destruction of hopanes and steranes, thereby affecting the values of such parameters. The distribution of these compounds in the pyrolyzates was less mature than those normally found in crude oils. Four Venezuelan marine oils, from different regions and of different levels of biomarker maturity, but of generally similar depositional origin as identified by their triterpane and presumed sterane (in the case of those heavily biodegraded) distributions have been studied. These oils were readily distinguished as their respective asphaltene pyrolyzates on the basis of the hopane/sterane ratio and the distribution of C 27–C 29 5α(H), 14β(H), 17β(H)-steranes. The distribution patterns of steranes and hopanes obtained in the asphaltene pyrolyzates have been compared with those reported for pyrolysis of sediments. The data are compatible with an origin for the asphaltenes involving catagenic degradation of the kerogen in the source rock. The pyrolysis technique applied to asphaltenes is suited to the characterization of heavy biodegraded oils or weathered source rocks where most of the molecular information has been destroyed.