Application of Raman-Infrared spectroscopy in the evaluation of organic matter characteristics and thermal maturity of highly- to overmature source rocks

With the increasing depletion of conventional oil and gas resources, deep to ultra-deep petroleum resources have become a critical direction for global energy strategies. The scientific evaluation of highly- to overmature source rocks developed in deep to ultra-deep strata is crucial for understanding hydrocarbon generation and accumulation. However, traditional geochemical evaluation methods are generally ineffective in this case. This study integrates Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR) to effectively analyze source rocks from the Permian Fengcheng, Xiazijie, and Urho formations in the western Peng-1 West Sag, establishing a comprehensive evaluation system suitable for highly-to over-mature source rocks. The results indicate that: (1) Raman spectral parameters can effectively calibrate the maturity of highly evolved source rocks and estimate the vitrinite reflectance ( R o ); (2) FTIR spectral parameters clearly distinguish organic matter (OM) types of source rocks from different sedimentary facies, i.e., Type II 1 in deep lacustrine facies of the Fengcheng Formation → Type II 1 –II 2 in fluvial-deltaic facies of the Xiazijie Formation → Type II 2 –III in semi-deep lacustrine facies of the Urho Formation); (3) The comprehensive evaluation shows that the hydrocarbon generation potential of source rocks in the study area follows the order: deep lacustrine facies > semi-deep lacustrine facies > fluvial-deltaic facies. This technical system provides a new approach to address the limitations of traditional methods and offers important insights for the assessment of gas potential in similar basins with highly- to over-mature strata. • Integrated Raman-FTIR spectroscopy effectively evaluates the thermal maturity and hydrocarbon potential of highly- to over-mature source rocks in the Junggar Basin. • Raman spectral parameters reliably calibrate maturity and estimate vitrinite reflectance ( Ro ​) in high- to over-mature stages, with a strong correlation ( R 2 > 0.85) between calculated and measured Ro ​. • FTIR spectral parameters clearly distinguish organic matter types across different sedimentary facies, revealing a progressive improvement in OM quality from semi-deep lacustrine to deep lacustrine facies. • A novel A–H–I ternary diagram based on molecular structural parameters enables accurate discrimination of hydrocarbon generation potential, ranking it as: deep lacustrine > semi-deep lacustrine > fluvial-deltaic facies. • A comprehensive workflow combining Raman and FTIR spectroscopy is established, providing a robust and convenient alternative to traditional geochemical methods for evaluating highly evolved source rocks.