Analysis of synthetic fluid inclusions based on Raman spectroscopy - Master's thesis - Dissertation

Introduction

Fluid inclusions are the original geological fluids trapped within mineral lattice defects and cavities. These tiny pockets of ancient fluid provide critical insights into the conditions under which minerals formed, the composition of the fluid, and the sources of materials during different geological periods.

Artificial inclusions, as analogs to natural ones, have become an effective tool for solving various issues related to fluid inclusions. As a standard for testing instruments and methods, synthetic fluid inclusions are increasingly recognized for their reliability and practicality. The most mature artificial inclusion technology involves creating inclusions in aqueous environments, with the H2O-CO2 (±NaCl) system being the most commonly used for encapsulation.

Raman spectroscopy is a technique based on inelastic scattering caused by molecular and lattice vibrations. It offers advantages such as rich information, high analytical efficiency, low sample consumption, and non-invasive detection. Laser micro-Raman spectroscopy integrates spectroscopy, chemometrics, detection, and computer technologies. In the field of fluid inclusions, it provides non-destructive, contactless analysis, high sensitivity, detailed spectral information, and the ability to study phase equilibrium, mineral dissolution, and deep rock partial melting in situ.

Current Status of Inclusion Raman Spectroscopy

Fluid inclusions contain trace amounts of the original ore-forming fluid, making them a relatively closed system. This characteristic allows them to be studied as reliable representations of the original fluid. The formation pressure of fluid inclusions plays a crucial role in understanding oil and gas migration, accumulation history, and tectonic movements. Various methods are currently used to measure this pressure, including isothermal diagrams, salinity-temperature methods, sodium chloride-water solution inclusion density, isovolumetric methods, and PVT simulation techniques.

Since the 1970s, Raman spectroscopy has been applied to fluid inclusions. Rosasco et al. (1975) were among the first to publish results on natural fluid inclusions using Raman. Beny et al. (1982) and Tourary et al. (1985) later contributed comprehensive studies on fluid systems and Raman methods. Their work not only demonstrated the potential of Raman in this field but also laid the foundation for quantitative analysis. Pasteris et al. (1988) discussed the limitations of Raman instruments and optimized analytical conditions, paving the way for broader applications.

In China, researchers like Huang Weilin (1990) and Xu Peicang (1996) have extensively used Raman spectroscopy for fluid inclusion analysis, contributing significantly to the development of quantitative methods in this area.

Laser Raman Detection Principle

Raman spectroscopy, with nearly 90 years of research and application, was theoretically predicted by A. Smekal in 1923 and experimentally discovered by C.V. Raman in 1928. It involves inelastic scattering caused by molecular vibrations, charge density fluctuations, spin density fluctuations, and electronic transitions. When a sample is excited by a laser, elastic (Rayleigh) and inelastic (Raman) scattering occurs. Most molecules undergo elastic scattering, while a small portion undergoes inelastic scattering, leading to frequency changes in the scattered light.

The Raman effect can manifest as Stokes or anti-Stokes scattering. The Raman shift depends on the energy difference between vibrational levels, not the excitation wavelength. This makes Raman spectroscopy highly useful in fields such as biotechnology, mineralogy, environmental monitoring, food safety, forensics, and medicine.

Experimental Procedure

The experimental sample consisted of a fluid inclusion prepared using a fusion silica capillary technique, with a diameter of 8–12 mm and a set of natural fluid inclusions. The minimum size of fluid inclusions that can be analyzed by Raman depends on factors like the microscope system, laser source, detector type, fluid density, and background signal from the substrate.

To reduce test time and improve signal quality, samples were ground to a thickness of 50–200 µm and mounted using beeswax. High reflectivity minerals like calcite can cause double images, making sample identification difficult. It's recommended to analyze inclusions close to the surface, ideally at depths of 30–70 µm, to avoid air interference. Confocal Raman spectrometers generally do not suffer from such issues, so they were selected for this experiment.

Fluorescence from surfaces, matrix minerals, and inclusions can interfere with Raman signals, so careful consideration of experimental conditions is essential before testing.

The experimental equipment included the Finder Vista microscopic confocal Raman spectrometer system, developed by Beijing Zhuoli Hanguang Instrument Co., Ltd. It featured a CCD backscatter detector, a 532 nm laser (10 mW), a 1800 g/mm grating, a slit width of 100 µm, an integration time of 20 seconds, and a 10× objective lens.

Experimental Analysis

The Raman spectra of natural and synthetic inclusions are shown in Figure 2. The experimental sample contained COâ‚‚ gas inclusions, providing important data for studying mineralization, hydrocarbon migration, fluid evolution, and tectonic dynamics.

Figure 2: Raman spectra of natural and synthetic inclusions

Conclusion

Raman spectroscopy offers a simple, direct, fast, and precise method for analyzing capillary samples. It does not interfere with fluid signals and allows accurate focusing on each phase, resulting in strong Raman signals. Synthetic inclusions provide clear insights into phase change processes, laying the groundwork for more accurate observation of natural inclusions. COâ‚‚ synthetic inclusions serve as standard samples for verification and analysis, offering technical feasibility and practicality for Raman spectroscopy in natural fluid inclusions.

References

[1] Li Jiajia, Li Rongxi, Liu Haiqing. Research progress in the determination of fluid inclusion pressure by laser Raman spectroscopy [J]. Physical and Chemical Testing - Chemistry, 2016, 52(7): 859–864.

[3] Ni Pei, Ding Junying, I-Ming Chou et al. A new type of artificial "fluid inclusion": fusion silica capillary technology [J]. Geoscience Frontiers, 2011, 18(5): 132–139.

[4] Chen Yong, ERNSTA A. J. Burke. Principles, methods, existing problems and future research directions of laser inclusion Raman spectroscopy for fluid inclusions [J]. Geological Review, 2009, 55(6): 851–861.

[5] Chen Jinyang, Zheng Haifei, Zeng Yushan et al. In situ analysis of fluids at high temperatures using inclusions as cavities[J]. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2003, 23(4): 726–729.

[6] Chen Yong. Principles, methods, existing problems and future research directions of laser inclusion Raman spectroscopy for fluid inclusions [J]. Geological Review, 2009, 55(6): 851–860.

[7] Ding Junying, Ni Pei, Guan Shenjin. H₂O-CO₂ In-situ Micro-laser Raman Spectroscopy Study of System Fusion Silica Capillary Samples[J]. Geoscience, 2011, 18(5): 140–146.

[8] Li Jiajia, Li Rongxi, understand, etc. Microscopic laser Raman quantitative analysis of CO₂ Gas Carbon Isotope Composition Method[J]. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2016, 36(8): 2391–2398.