Thermal stability of the asphaltene/kerogen vanadyl porphyrins

Thermal stability of vanadyl porphyrins incorporated into the asphaltene/kerogen structurés of the Dead Sea asphalt “float”, the La Luna Mara limestone and the Serpiano marl was studied by electron spin resonance (ESR). Comparative studies were also made on alkyl vanadyl porphyrins such as deoxophylloerythroetioporphyrins and (extracted and commercial) etioporphyrins and tetraphenylporphyrin. According to these studies, alkyl vanadyl porphyrins predominate in the asphaltene/kerogen framework. These porphyrins are stable up to 400°C during laboratory heating, but at and above this temperature their content sharply decreases. At 500°C and 600°C, the asphaltene/kerogen vanadyl porphyrins content follows a similar pattern to that at 400°C, except that at 600°C it was reduced to traces in less than 12 h. In addition, it is implied that a high proportion of the asphaltene/kerogen vanadyl porphyrins (< 50% of total amount) are incorporated into the asphaltene/kerogen matrix with relatively higher activation energies. The asphaltene/kerogen vanadyl porphyrins kinetics during laboratory thermolysis are characterized by a step-wise pattern. This is attributed to thermolytic decomposition/demetallation of the asphaltene/kerogen alkyl vanadyl porphyrins with various energy barriers mediated by free radical intermediates. A rough extrapolation of pyrolytic data to longer times, based on both the Arrhenius equation and the often-quoted assumptions for the reaction rate decrease, infer that static laboratory pyrolysis of kerogen is irrelevant to natural geological processes characteristic of petroleum genesis.