Poster Session - IFP Invited Poster

Methane and Ethane Generation from Oil Cracking: First Isotopic Modelling based on 13C Labelled Compound

Luc Fusetti (1), Françoise Behar (1), Kliti Grice (2) and Sylvie Derenne (3)

(1) IFP, Geochemistry Department, 1 & 4, avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex, France.

(2) Stable Isotope and Biogeochemistry Group, Centre for Applied Organic Geochemistry, Curtin University of Technology, Kent Street, WA6845 Perth, Australia.

(3) LCBOP, UMR 7618 BioEMCo CNRS/ENSCP, 11 rue P. M. Curie, 75231 Paris cedex 05, France.

Recent papers have demonstrated that aromatic hydrocarbons are less stable than saturated ones during thermal cracking. Therefore the focus of the present work includes a molecular and kinetic isotopic study of secondary methane and ethane generated from aromatic compounds present in the low-molecular-weight fraction (C6-C14) of petroleum found in high pressure and high temperature (HP-HT) reservoirs. In addition the kinetic scheme will illustrate the isotopic fractionations taking place at temperatures ranging from 160°C to 220°C over several millions of years.

More than 200 isothermal pyrolysis experiments in gold sealed tubes have been carried out at three temperatures ranging from 395 to 450°C for periods of several hours to several months. Attention has been paid to mass balances using different fractions, such as a gaseous fraction (C1-C4), a pentane extract (C6-C14 and C15-C20), a DCM extract (C20+) and the residue, in order to get a better constrained isotopic model. A complete kinetic study on 10 non-labelled compounds has been performed first in order to establish the kinetic scheme for methane and ethane generation. These components indeed represent the overall composition of the C6-C14 aromatic fraction of a typical oil and also represent different group positions, chain lengths, ring substitutions and ring effects. The influence of the previous factors was quantified by calculating activation energies (Ea) and frequency factors (A) for methane and ethane generation, using an Arrhenius plot.

In addition, an extensive isotopic study has been carried out in order to get a holistic mass balance for the 13C/12C of the different fractions.

Synthesises of aromatic compounds, with a 13C label in specific positions, have also been carried out successfully and will allow us to study precisely the carbon isotope fractionations occurring during methane and ethane generation.

Key Words: Methane and Ethane Generation, Carbon isotope fractionations, Secondary Cracking, Kinetic and Isotopic Modelling, Deep Prospect.