Carbon Isotopic Balance of Kerogen Pyrolysis Effluents in a Closed System

Tocqué, Eric¹, Françoise Behar¹, François Lorant¹, Helene Budzinski² (1) Institut Français du Pétrole, Rueil-Malmaison, France (2) Université de Bordeaux 1, CNRS, Talence, France

We devised an experimental method for calculating isotopic mass balances for the ¹²C and ¹³C in the pyrolysis products from a type II kerogen during artificial maturation in a closed system. This method also enables a study of the isotopic evolution of cracking prod­ucts versus kerogen transformation ratios.

The type II kerogen (Paris Basin) selected was at the onset of catagenesis. Pyrolysis was carried out in a confined closed system under isothermal conditions, at several tem­peratures (275°C to 350°C) and times with a constant pressure of 10 Mpa. Pyrolysis efflu­ents were separated into hydrocarbon gases, non-hydrocarbon gases, C₆-C₁₄ products soluble in pentane (C₆-C₁₄ saturated, C₆-C₁₄ aromatics), C₁₄₊ products soluble in pen­tane (C₁₄₊ saturated, C₁₄₊ aromatics, resins 1), C₁₄₊ products soluble in dichloromethane (asphaltenes and resins 2) and residue insoluble in dichloromethane.

Mass balances on pyrolysis products were higher than 97.7% of the initial kerogen amount. The ?¹³C fractionation observed between different pyrolysis products reached 18 ‰, but only 2‰ between different C₁₄₊ fractions within the range of the pyrolysis condi­tions.

This experimental method offers new possibilities for following carbon isotopic fraction­ation during kerogen cracking, and therefore for calibrating isotopic fractionation models more rigorously. Most of the fractionations observed here seem to be in agreement with the phenomena observed in natural systems. However, we noticed less ¹³C fractionation of gases in our pyrolysis temperature, compared to natural conditions. This is due to the tem­perature dependence of isotopic fractionation.