At temperatures and pressures encountered during thermal recovery, chemical reactions involving the oil, possibly the water and the mineral matrix, may lead to significant changes in the composition of the phases.

This work focuses on the thermal alteration of four crude oils with different geochemical compositions. The tests were performed in an autoclave at 350°C during 200 h. The oil was in the presence of water and a mineral matrix representative of reservoir rocks.

The formation of significant amounts of insoluble organic material was observed in all the tests.

Gaseous hydrocarbons (mainly methane) are formed by pyrolysis. CO2 evolution depends on oil and matrix compositions; moreover, water is found to be involved as a reactant in the formation of oxygen-containing components.

For the crude oils investigated, the oil phase is significantly enriched in lighter hydrocarbons (mainly in the saturates). However, the aromatic content is less affected by the thermal treatment. On the other hand, the asphaltenes and to a less extent the resins are the main precursors of the insoluble material.

These results provide a new insight on the mechanisms of the cracking reactions involved in thermal recovery processes.

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