PACROFI VI - Electronic Program

Synchrotron Infrared Microspectrometry of Single Fluid Inclusions Beyond the Diffraction Limit

Nicole Guilhaumou*, Paul Dumas** and Gwyn Phillip Williams***

*Ecole Normale Supérieure, Laboratoire de Géologie, URA 1316 du CNRS, Paris (France).
**LURE-CNRS, Centre Universitaire Paris-Sud, F91405 Orsay and LASIR-CNRS, F94320 Thiais (France).
***National Synchrotron Light Source, Brookhaven National Laboratories, NY11973 Upton (USA).

The microanalysis of individual fluid inclusion by infrared microspectrometry is currently limited by the brightness of the conventional black body thermal emission source. The spatial resolution and signal is limited and allows only the analysis of elements with significantly high absorption in inclusions greater than 15 micron. The use of the infrared Beam line of the National Synchroton Light Source (U2B, Brookhaven National laboratory) overcomes these problems. The U2B line provides a high brightness (increase of 103) source of IR radiation with a low noise level. When coupled with the IR spectrometer, high signal/noise is obtained at or below the system's optical diffraction limit and in short acquisition time. In order to evaluate the new possibilities of this technique, we have performed infrared microscopy measurements using a confocal system and a SpectraTech IRuS microspectrometer on hydrocarbon and acqueous fluid inclusions in flourite and quartz matrix. A 3x3 micron2 aperture was used in this study as optimal conditions. The system is completely purged by dry nitrogen allowing a significant detection of CO2 in vapour and liquid phase of both types of inclusions.

The bending and stretching CH vibrations of aliphatics (Figure 1a) are easily detected in 5 to 10 micron hydrocarbon fluid inclusion in a flourite matrix. In addition the intense 2335 band indicates a high proportion of dissolved CO2 under pressure (stretching vibration). H20 is not detected. In the same sample, coeval aqueous fluid inclusions of 10 microns in size are well confirmed by the OH vibration centered at 1600 and 3454 cm-1. Large amounts of dissolved CO2 are indicated by the intense stretching band at 2345cm-1 (Figure 2a and 2b). Shifts of the CO2 stretching bands were noted and could be probably related to different densities. Such measurements will allow the rapid characterization of very small sized fluid inclusions representative of diagenetic events at the limit of detrital quartz and overgrowth in reservoir sandstones. Joint analyses by Raman spectrometry in aqueous inclusions aim to completely define the fluids in the reservoir when diagenesis started. In the Brent samples from the North sea, for example, the main siliceous diagenetic event (Cordon et Guilhaumou 1995) probably took place in an heterogeneous melt composed of oil, gas and concentrated seawater.

Semiquantitative measurements of the integrated intensities of each vibrator allow to establish a chemical image of each component inside the fluid inclusion. We have mapped the oil and the CO2 chemical images of fluid inclusions in fluorite from Tunisia and in quartz overgrowth from North Sea core drill. One example of fluid inclusion in fluorite is shown in figure No 3. In figure 3a, we show a CO2 map derived from the intensities of the 2335cm-1 absorption band, corresponding to the stretching mode. In figure 3b, we show a map of the intensities of the 2850 cm-1 band corresponding to the symmetric C-H stretching vibrations. Although the beam is going partly through the oil surrounding the bubble, we clearly see that the CO2 is mainly present in the gas phase. The relative intensity at a function of the location inside the fluid has to be related to the local density. As the oil is supposed to be homogeneous, the various intensity can be related to the thickness of the area probed. This opens up the possibility of quantitative analysis of hydrocarbon fluid inclusions.

In conclusion, the use of a storage ring, as an infrared source, associated with a microscope, is a promising tool for the analysis of very small sized fluid inclusions (<10 microns), particularly those which are representatives of diagentic events at the edge of detrital quartz and overgrowth in reservoir sandstones. The mapping of the chemical elements open up the possibility of quantitative analysis and thickness measurements in oil and aqueous fluid inclusions. The spatial resolution is not strictly limited to 3 microns, but can be further improved using brightest synchrotron source. This program is under accomplishment at the National Synchrotron Light Source, and spatial resolution of 1 mm can be achieved.

The NSLS is funded by the U.S. Department of Energy-Contract DEACO1-76CH00016.