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- Title
Understanding the effects of heating rate, stress and fluids in maturation of carbonaceous materials by Raman spectroscopy.
- Authors
Schito, Andrea; Muirhead, David; Hackley, Paul; Parnell, John; Galimberti, Roberto; Mascheroni, Luca
- Abstract
Raman spectroscopy of carbonaceous material (CM) has recently become one of the most commonly used geothermometers in Earth Science studies. However, the effects of heating rate, stress, and fluid composition on the aromatization path are not yet fully understood. A recent review suggested that different maturation paths could be detected according to their heating rate and stress/fluids conditions. However, this result needs to be systematically tested using additional sample types and the same fitting approach for spectral deconvolution. In this work, the Raman spectra of two sets of coals and a type III kerogen matured under different natural (regional metamorphism vs. around shallow intrusion) or artificial (isothermal closed-system hydrous vs. high heating rate open-system anhydrous) conditions, were analysed and compared. Natural samples that matured around a shallow intrusion were from vitrinite-rich nearshore marine siltstones deposited during the middle Jurassic and then intruded by a 7-m thick doleritic dyke in the Paleogene in the Skye Island (UK). Vitrinite reflectance (%RO) of these samples increases from about 0.6% to about 4.5% at distances corresponding to 100% down to 10% of the intrusion thickness (i.e., from 7 to 0.7 m). A background (0.36% Ro) sample unaffected by the intrusion was used to perform open-system pyrolysis under anhydrous conditions while coals from the Paleocene-Eocene Wilcox group in Texas (USA) were analysed after hydrous pyrolysis experiments. Maximum temperatures in the open-anhydrous system vary between 300 and 550°C and were reached between 2 and 15 minutes and kept for 30-120 seconds, while hydrous pyrolysis maximum temperatures vary between 320-360°C and were held between 1 and 100 days. The maturity of the samples post-pyrolysis ranged from immature up to the wet gas window. Finally, several coals from different Carboniferous basins in the UK represent CM matured under a diagenetic/regional metamorphism regime in a thermal range between 0.4% up to 3.27% Ro. One naturally matured solid bitumen-rich shale and one graptolitic-shale from the Southern Uplands Paleozoic accretionary prism were also used. When compared against Ro values, the Raman band separation, full-width at half maximum height of the G band and the R1 Raman parameters of the different sample series follow a similar path up to a thermal maturity of-1.0% Ro. Beyond that, a mismatch occurs, with samples matured under regional metamorphism conditions showing Raman parameters associated with greater aromatization than the intruded or pyrolyzed samples, probably due to lithostatic pressure, as previously observed in literature. Hydrous pyrolysis seems to better reproduce the maturation path around intrusions compared to open-system pyrolysis, suggesting that the hydrous pyrolysis technique could be used to develop a Raman-based kinetics model for CM maturation at low maturities and/or high heating rate conditions.
- Subjects
RAMAN spectroscopy; CARBONACEOUS chondrites (Meteorites); PALEOGENE; PYROLYSIS; CARBONIFEROUS paleontology
- Publication
Bulletin of the Geological Society of Greece, 2023, p79
- ISSN
0438-9557
- Publication type
Article