Evaluation of Source Rock using EGA and Pyrolysis GC/MS

Significance of the Topic

Evaluation of organic content in sedimentary source rocks is a cornerstone of petroleum geochemistry. Determining both free and bound hydrocarbons provides critical insight into a rock’s potential to generate and expel hydrocarbons, guiding exploration decisions and resource assessment.

Objectives and Study Overview

This study demonstrates a combined approach using evolved gas analysis (EGA) and pyrolysis gas chromatography–mass spectrometry (GC/MS) to quantify and characterize hydrocarbons released from source rock under controlled thermal conditions. Two heating steps at 300°C and 600°C distinguish migratory hydrocarbons from thermal cracking products.

Methodology and Instrumentation

Sample analysis was performed on a CDS Model 5200 Pyroprobe interfaced to a GC/MS system. The protocol comprised:

• Step 1: Heating to 300°C to release free and sorbed hydrocarbons, with GC/MS separation and identification.
• Step 2: Heating to 600°C to induce thermal cracking and capture heavier cracking products.
• EGA mode: Direct transfer of evolving gases through a 1 m × 0.1 mm fused silica column at 300°C to generate composite peak profiles and determine Tmax (the temperature of maximum hydrocarbon evolution).

Main Results and Discussion

Heating at 300°C yielded a profile of migratory hydrocarbons, indicating the quantity of free compounds present. The second stage at 600°C produced a distinct set of heavier cracking fragments. Composite EGA peaks revealed a Tmax of 508°C, reflecting the most active thermal cracking interval. Fixed gases released during pyrolysis were quantified via a CDS 5500 Fixed Gas Analyzer.

Benefits and Practical Applications

• Rapid quantification of free versus bound hydrocarbons for resource screening.
• Detailed chemical fingerprinting of both migratory and cracked products.
• Informed assessment of thermal maturity and hydrocarbon generation potential.
• Applicability in exploration, reservoir characterization, and quality control workflows.

Future Trends and Potential Applications

Advances may include coupling with high-resolution MS for compound-specific isotopic analysis, integration of automated sample handling for high throughput, and application of machine learning to interpret complex pyrolysis datasets. Developing in-situ field deployable pyrolysis-EGA instruments could further accelerate exploration and environmental monitoring.

Conclusion

The combined EGA and pyrolysis GC/MS approach provides a robust toolkit for comprehensive source rock evaluation. By distinguishing free hydrocarbons from thermal cracking products and identifying Tmax, this methodology enhances precision in hydrocarbon potential assessment and supports decision-making in petroleum exploration.

Instrumentation Used

• CDS Model 5200 Pyroprobe with autosampler
• Gas chromatograph–mass spectrometer (GC/MS)
• 1 m × 0.1 mm fused silica capillary for EGA composite peaks
• CDS Model 5500 Fixed Gas Analyzer