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| Microscope-based Fluid Inclusion Techniques: Applications to E & P | |||
 Introduction
Fluid inclusions are small samples of pore fluid crystallographically trapped
in rocks during diagenesis or fracture healing processes. They contain
composition and density information that can be translated to temperature,
pressure and compositional constraints. These data are useful for understanding
petroleum migration, reservoir filling, diagenesis and constraining basin
models. To accomplish these goals, transparent, polished slabs of rock
material are prepared and studied optically with a petrographic microscope.
Samples are viewed under transmitted plane-polarized white light as well
as under reflected ultraviolet or blue-violet illumination. Aromatic species
within natural oils and condensate inclusions render them fluorescent under
UV light. Hence, aqueous inclusions, non-fluorescent gas and fluorescent
condensates and oils can be identified and their relationship to each other,
diagenetic features (e.g., physical and chemical compaction) and the rock
matrix can be resolved. Once these relationships are clear, samples are
placed into a gas-flow temperature stage (manufactured by Fluid Inc.) And
individual inclusions in selected petroleum and aqueous inclusion populations
are viewed optically during heating and cooling (-196oC to +200oC
or higher). Phase equilibria within the trapped fluids reflect their composition
and bulk density, which, in turn, is a function of trapping temperature,
pressure and fluid composion for each inclusion.
Temperature Constraints Each fluid inclusion forms a self-contained
geothermometer which potentially records a subsurface temperature at some
time in API Gravity Two Salinity, Cation Composition and Dissolved Gas In aqueous inclusions, the temperature at which initial melt is generated, termed the eutectic, and the final melting temperature (generally of ice) are the most commonly measured phase transitions, They provide, respectively, information on the nature of the salts in solution, and the total salinity. Aqueous inclusions containing dissolved gas will form crystalline gas-H2O compounds termed "clathrates" upon cooling. Most organic and inorganic gases form clathrate compounds. The melting of a clathrate is a function of the particular gas, the salinity of the aqueous phase, and the internal pressure (density) of the bulk fluid. |
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the
geologic past. By determining the homogenization temperatures of different 
inclusion
populations within a sample (that is, the temperatures at which the inclusions
become single-phase fluids during laboratory heating) and then applying
appropriate corrections, fluid entrapment temperatures can be estimated.
From this, thermal and diagenetic histories can be investigated. Additionally,
cementation temperature relationships are useful for evaluating processes
which enhance or destroy reservoir quality, and investigating the relative
timing of these processes with respect to hydrocarbon migration. Finally,
the mode and temperature of homogenization of petroleum inclusions can
be used to evaluate product type and saturation state, particularly when
integrated with data from coeval aqueous inclusions.
techniques 
are
available for determining the API gravity of liquid petroleum inclusions.
The first involves quantifying fluorescence color with a microspectrophotometer
and referencing the fluorescence spectra to those of a calibrated suite
of oils. Aromatic hydrocarbons and NSO compounds are thought to be the
major contributors to petroleum fluorescence, and multi-ring compounds
are interpreted to fluoresce at progressively longer (redder) wavelengths
as the number of rings increase. Hence, it has been demonstrated that fluorescence
color tends to change from yellow to blue as maturity or API gravity increases.
The second technique is a patented, microscope-based method, which allows
a direct evaluation of liquid petroleum inclusion density within most minerals,
and referencing back to API gravity through calibration with oil standards
of known composition. Each of these techniques has its own strengths and
limitations, and is capable of producing API estimates to an uncertainty
of +/- 2 units, particularly when used in tandem.