Application of Micro-FT-IR Spectroscopy to Individual Hydrocarbon Fluid Inclusion Analysis

Abstract

Infrared spectra of fluid inclusions are obtained with a Fourier transform infrared miscrospectrometer. The experimental conditions and the analytical use are discussed. Interferograms are recorded in the transmission mode. The quality of spectra is good for fluid inclusions with diameters larger than 30 μm, the spectral resolution being 4 cm⁻¹ and the recording time 200 s. Interferograms made of inclusions with diameters of less than 20 μm are reached in the same amount of time, but with worse spectral resolution. Infrared spectra are usually only suitable above 2000 cm⁻¹, because of the absorption of most of the inclusion-bearing host minerals, the thickness of which should never exceed 1 mm. This fact further limits their interpretation, as the inclusion composition is complex. However, complementary information can be obtained in the 4600–4000-cm⁻¹ range for hydrocarbon inclusions with a thickness that is larger than some tens of micrometers. This shows that it is necessary to use a detector which is as sensitive as possible towards high frequencies. A comparison of the current performances of infrared and Raman spectroscopies as analytical methods for investigating fluid inclusions is presented. The important improvement of better spatial resolution and the corresponding possibility of being able to characterize heterogeneities, in comparison to limitations with classical dispersive infrared spectrometry, are discussed. Methane, carbon dioxide, liquid water, aromatic ester, and linear or branched alkanes are identified in several samples. It is also possible to estimate the mean ratio of alkane CH₂/CH₃ groups. In some cases, the intensity of the absorptions indicates the inclusion effective thickness and the mole fraction ratio.