FT-IR measurements of petroleum fluid inclusions:
methane, n-alkanes and carbon dioxide quantitative analysis

(Geofluids, 2001, v. 1: 2-10)

Jacques PIRONON1, Régis THIERY2, Mohamed AYT OUGOUGDAL3, Georges BEAUDOIN3 and Frédéric WALGENWITZ4

1UMR G2R-CNRS, Univ. H. Poincaré, BP 239, F-54506 Vandœuvre-lès-Nancy, France
2UMR 6524, Univ. B. Pascal, 5, rue Kessler, F-63038 Clermont-Ferrand, France
3MEDEF, Université Laval, Québec, Qc, G1K 7P4, Canada
4CSTJF, ElfTotalFina, Avenue Larribau, F-64018 Pau, France

Abstract

New tendencies in petroleum geochemistry is to model fossil oil composition using microthermometric and volumetric data acquired from individual fluid inclusion analysis. FT-IR microspectroscopy has shown its ability to record composition information related to gas (CH4 and CO2) and alkane contents of petroleum inclusions. The objective of this work is to reconsider FT-IR data in order to acquire quantitative information to constraint the thermodynamic models. In spite of limitations of FT-IR microspectroscopy applied to fluid inclusion analysis, a quantitative procedure has been defined to obtain mole % concentrations of methane, alkanes and carbon dioxide. Standard petroleum inclusion, from Quebec City Promontory nappe area, gave us the opportunity to record a reference spectrum of methane. The analytical procedure is based on the measurement of CH4/alkane and CH4/CO2 band area ratios. CH4/alkane infrared band area ratio is obtained after spectral subtraction of the reference methane spectrum. This area ratio, affected by absolute absorption intensities of methane, methyl and methylene, gives us a molar CH4/alkane ratio. Methyl/methylene ratio (CH2/CH3) ratio is obtained following procedures established in previous works. CO2/CH4 concentration ratio is estimated from their relative absolute absorption intensities. Applications to natural inclusions from different environments show good correlation between FT-IR quantification and PIT (Petroleum Inclusion Thermodynamic) modelling.