Fluid inclusions - optical analysis

Application

The aims are to provide information on one or more of the following:

  • timing of reservoir charging
  • P & T conditions during reservoir filling
  • composition of fluids

The methods that can be employed are:

  • petrography
  • fluorescence spectroscopy
  • microthermometry
  • confocal laser scanning microscopy

Petrography determines the textural relationships between the different types of fluid inclusions present and in the context of diagenetic processes. This permits selection of suitable inclusions for microthermometry. A review of methods has been published by Munz (2001).

Fluorescence spectroscopy determines the composition of HCFIs and identifies different populations, as well as aiding the basic differentiation HC from aqueous fluid inclusions. It is possible to estimate API gravity from the measurements taken.

Microthermometry determines homogenization temperatures, which again allows different populations of HCFIs to be recognized and is an important step in reconstructing PVT conditions during trapping and also the timing of reservoir filling.

Confocal laser scanning microscopy (CLSM or LSCM) is a technique for obtaining high resolution optical images of thick specimens. This optical sectioning technique constructs a 3D representation from point images using a computer. It is used to determine the volumetric ratios of liquid:vapour in HCFIs at room temperature (Pironon et al. 1998; Aplin et al. 1999). Combining this information with fluid composition and average molecular weight allows the molar volume to be calculated (Munz 2001).

Sample requirements

Core or core plug samples are required from the interior of the core.

Prior to undertaking detailed fluid inclusion petrography, fluorescence spectroscopy and and microthermometry on slide-mounted sections, which is time consuming and expensive, it is possible to perform a screening assessment of presence, abundance and suitability of fluid inclusions for such detailed examinations. Pre-cut slabs are required.

When making conventional thin sections for petrography, all but the most minute fluid inclusions (<< 20 µm) will survive, the rest will decrepitate, which might bias the overall assumption towards an actual fluid inclusion under-representation. IFE has circumvented this by using a special optically inert (high refractive index/non-fluorescing/non-toxic) liquid bath microscope system into which a sawed sample is immerged.

With subsequent use of 366/405 nm light the presence of populations of hydrocarbon fluid inclusions (HCFI) are readily detected before any further preparative steps are required. Hence samples devoid of HCFI are not further analysed.

Analytical method

Once samples have been selected, two types of doubly polished sections are required. Conventional 30 µm doubly polished thin-sections, preferably vacuum impregnated with blue or green epoxy for porosity to be readily identified, are prepared to establish diagenetic characteristics and general petrography.

For fluorescence spectroscopy of HCFIs, a mirror of the first section is made, but ~200 µm thick; the optimum thickness (100–400 µm) being determined by sample grain size. It is again doubly polished. The fluorescence study permits compositional signatures of the HCFIs to be determined and API gravity to be estimated.

A specially developed vacuum impregnation non-fluorescing epoxy resin method is used to enhance the S/N of HCFI versus artificially fluorescing matter. A unique, specially adapted microscope setup is used. The basics of fluorescence spectrometry and a discussion of potential errors are presented by Burruss (1991).

For microthermometry, rectangular 12x12 mm wafers are cut from the original thick-section using a guillotine device, mounted by an alphacyanoacrylate glue at the extreme corners onto a quartz cover glass. Heating allows homogenisation temperatures to be determined, permitting establishment of minimum trapping temperatures.

It is recommended that homogenization temperatures of HCFIs are determined before those of aqueous inclusions, because the latter are generally higher, and may induce detrimental decrepitation or fracturing and subsequent leakage of HCFIs. Details of the procedure for microthermometry of HCFIs is provided by Shepherd et al. (1985).

Confocal laser scanning microscopy (CLSM or LSCM) is based on the laser causing fluorescence in the liquid but not in the vapour of a HCFI, enabling the two phases to be distinguished easily. Focusing is very precise and the analysis is performed by visually locating the top and bottom of the HCFI and scanning through it in 1 micron slices.

This produces a series of two dimensional images, from which a 3D model can be created on a computer (Pironon et al. 1998). Total volumes of HCFIs and the volumetric ratio between liquid and vapour phases can then be determined.

Potential problems

  • insufficient inclusions
  • leakage or distortion of inclusions – invalidates PVT data
  • imprecise relations between aqueous and hydrocarbon filled inclusions

References

Aplin A.C., Macleod G., Larter S.R., Pedersen K.S., Sørensen H., Booth T. (1999) Combined use of confocal laser scanning microscopy and PVT simulation for estimating the composition and physical properties of petroleum in fluid inclusion. Mar. Pet. Geol. 16, 97−110.

Munz I.A. (2001) Petroleum inclusions in sedimentary basins: systematics, analytical methods and applications. Lithos 55, 195−212.

Pironon J., Canals M., Dubessy J., Walgenwitz F., Laplace-Builhe C. (1998) Volumetiric reconstruction of individual oil inclusions by confocal scanning laser microscopy. European Journal of Mineralogy 10, 1143−1155.

Shepherd T.J., Rankin A.H., Alderton D.H.M. (1985) A Practical Guide to Fluid Inclusion Studies. Blackie, Chapman & Hall, 239 pp.