In a recent study published in the International Journey of Coal Geology, the U.S. Geological Survey and the Wyoming-Montana Water Science Center present a simpler way to gauge the thermal maturity of organic matter in shale and coal samples. Researchers used a portable All-In-One Process Raman Analyzer with a 785 nm laser to examine whole-rock samples that have been crushed. The study compares traditional indicators like organic matter reflectance (Ro) and programmed temperature pyrolysis (Tmax) to Raman band separation (RBS) values obtained with this method. The goal is to create connections between these measurements, making it easier to determine thermal maturity in different geological contexts.
Summary:
The study thoroughly examined 32 coal and shale samples, covering a broad range of thermal maturities (0.40–4.62% Ro), kerogen types (Types I, II, and III), and depositional ages. Reflectance (Ro) and Tmax data were either collected from existing sources or reported for the first time. Different sample preparation methods were tested, with the most effective method involving washing, drying, and hand crushing to pass through a 40-mesh sieve. The results show a strong connection between RBS values and both Ro and Tmax values, up to 3% Ro and 551 °C Tmax, suggesting that whole-rock Raman analysis holds promise for assessing thermal maturity in field or laboratory settings.
“Data were collected using a MarqMetrix All-In-One portable Raman probe which is equipped with a handheld wand connected by fiber optic cable to a combined computer-spectrometer that measures 25 × 25 × 7.5 cm. A computer screen is required to control the instrument and view data. The total weight of the instrument is ~9 kg and can be easily transported via external frame backpack or hard case,” the authors wrote. 1
Study at a Glance:
- Objective: Develop a simple method to assess thermal maturity in shale and coal using portable Raman spectroscopy and establish links between RBS values and traditional indicators (Ro and Tmax).
- Methodology: Collected 32 coal and shale samples of varying thermal maturities, kerogen types, and ages. Used a portable All-In-One Raman system with a 785 nm laser, 1000 Wh portable power station that allowed for several days of intermittent use in the field without having to recharge to measure RBS values. Also measured Ro and Tmax. Tested various sample preparation methods to reduce RBS value variations.
- Results: The results indicate a strong correlation between RBS values and both Ro and Tmax values, up to 3% Ro and 551 °C Tmax, respectively, demonstrating the potential utility of whole-rock Raman analysis for thermal maturity assessment in field or laboratory settings.
Results
The study results indicate that whole-rock Raman analysis can serve as a viable proxy for thermal maturity assessment in shale and coal samples. The strong correlations observed between RBS values and traditional thermal maturity indicators (Ro and Tmax) up to 3% Ro for both shale and coal samples, as well as up to 551 °C Tmax for shale samples, highlight the potential of this approach for generating thermal maturity data from minimally processed samples.
Furthermore, the use of a portable 1000 Wh power station provided an invaluable advantage in the field, allowing for several days of intermittent use of the All-In-One (AIO) Raman system without the need for recharging. This innovation significantly extended the researchers’ capability to bring Raman analysis to the point of need, enhancing their efficiency and productivity during data collection in remote or challenging environments. The ability to operate the Raman system for extended periods without worrying about power constraints not only streamlined the research process but also contributed to more comprehensive and robust data collection. This integration of portable power technology into the research workflow underscores the adaptability and practicality of the whole-rock Raman analysis approach, making it a valuable tool for thermal maturity assessment in diverse geological settings.
Citations
1 p.2, M.R. Stokes, A.M. Jubb, P.C. Hackley, J.E. Birdwell, E.P. Barnhart, C.T. Scott, J.L. Shelton, M.M. Sanders, J.J. Hatcherian, Evaluation of portable Raman spectroscopic analysis for source-rock thermal maturity assessments on bulk crushed rock, International Journal of Coal Geology, Volume 279, 2023, 104374, ISSN 0166-5162, https://doi.org/10.1016/j.coal.2023.104374. (https://www.sciencedirect.com/science/article/pii/S0166516223001921)
