Microthermometry and Petrography Overview

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 (-196°C to +200°C 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.

Figure 1: Photo 1: Gas inclusions. Photo 2: Water inclusions. Photo 3: Liquid petroleum in plain light. Photo 4: Liquid petroleum in UV light.

Temperature Constraints

Each fluid inclusion forms a self-contained geothermometer which potentially records a subsurface temperature at some time in 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.

API Gravity

Two 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.

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.