Methane and Ethane Generation from Oil Secondary Cracking in High Pressure-High Temperature Reservoirs: Isotopic Modelling Based on ¹³C Labelled Compounds

FUSETTI, Luc^*,1, BEHAR, Françoise¹, GRICE, Kliti², and DERENNE, Sylvie^3
1IFP, Geochemistry Dpt, France; *[email protected]
²Stable Isotope and Biogeochemistry Group, Centre for Applied Organic Geochemistry, Curtin University of Technology, Perth, Australia
³Laboratoire BIOEMCO, Ecole Nationale Supérieure de Chimie de Paris, Paris, France

Basin modelling is used to estimate the presence and quantity of hydrocarbon fluids in the subsurface. At the same time, it is also used to predict the significance of some important ratios such as gas-oil ratio (GOR), condensate-gas ratio (CGR) and gas dryness. However, the quality of this prediction strongly depends on the kinetic scheme which has been used in basin modelling to simulate the generative potential of hydrocarbons including gas formed via thermal cracking. For example, the unexplained variation of gas dryness between natural gases and the ones obtained by artificial thermal maturation of sedimentary organic matter may be due to an incomplete mass balance. In fact not all possible sources of gas in a sedimentary basin are taken into account, particularly those gases which originate from aromatic compounds via thermal cracking. Indeed, attempts to predict gas generation from oils secondary cracking in high pressure and high temperature reservoirs were found to be unsuccessful because existing models did not take into account the thermal cracking processes of aromatic components (Vandenbroucke et al., 1999). Aromatic compounds are in fact much more sensitive to thermal cracking than saturated compounds under high pressure and high temperature (HP-HT) conditions. Therefore the focus of the present work includes a molecular and isotopic study of secondary methane and ethane production from aromatic compounds present in the low-molecular-weight fraction (C6-C14) of petroleum found in HP-HT reservoirs. In addition the kinetic scheme used for basin modelling will illustrate the isotopic fractionations taking place at temperatures ranging from 160°C to 220°C over several millions of years. Laboratories pyrolysis experiments have been carried out at temperatures ranging from 400 to 450°C for periods of several hours to several months. A series of aromatic model compounds were selected for representing the overall composition of the aromatic fraction of the oil. A preliminary complete kinetic study on non-labelled compounds has been performed in order to establish the kinetic scheme for methane and ethane generation. In addition an isotopic study has been carried out on at least one synthesised ¹³C labelled aromatic compound especially synthesised for the present work. In order to get a better constrained isotopic model, a complete mass balance was carried out for the ¹³C of the gaseous, liquid and solid fractions. Stoïchiometric reactions of this scheme have enabled us to calculate the kinetic parameters, also taking into account carbon isotopic fractionation. The results of the present study have established the first kinetic model of carbon isotope fractionation during methane and ethane generation from a given oil cracking in high pressure and high temperature reservoirs.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands