Thermal maturation as revealed by micro-Raman spectroscopy of mineral-organic aggregation (MOA) in marine shales with high and over maturities

Abstract

Laser Raman spectroscopy of organic matter in coals and sedimentary rocks has been increasingly investigated to assess thermal maturity. Pure organic matter such as vitrinite and solid bitumen, as well as isolated kerogen are definitely selected for this purpose, but there has been no systematic report on the Raman spectral parameters directly based on mineral-organic aggregations (MOA) in shales. In this paper, the Raman spectral characteristics and parameters of MOA in two sets of marine shale samples from southern China with either different TOC contents (TOC=0.10-4.59%) or different maturities (BR o =1.71-4.57%) were investigated. It was found that the Raman technique is very sensitive to the organic matter in MOA. As the TOC content of the shales reaches about 0.60%, perfect Raman spectra with D and G bands derived from organic matter in MOA were obtained, and the Raman parameters of MOA are in good agreement with those of its associated solid bitumen, and can indicate the thermal maturity just like those of solid bitumen. Using the Raman parameters of MOA is a potential alternative way to estimate maturity levels of high and over mature shales, especially those lower Paleozoic and Precambrian shales that lack macerals suitable for reflectance or Raman measurements.

Chemical Structure Transformations in Kerogen from Longmaxi Shales in Response to Tectonic Stress as Investigated by HRTEM, FTIR, and 13C NMR

“…However, there may be considerable uncertainty in the reflectance measurement owing to the distinction between inertinite and vitrinite as dispersed OM, because the standards to which one may resort are scarce and weak . Another problem is that the measured reflectance values are considered unsuitable for Lower Paleozoic and Precambrian marine shales due to the limitation of vitrinite origin, optical anisotropy, and dimension. , RS, as a rapid, nondestructive, and cheap technique, has been widely employed to assess the thermal maturity of geological carboniferous materials. The Raman spectra can provide some information on the short-chain structure and chemical properties of carboniferous matter, which are tightly related to the thermal evolution of OM in shale.…”

“…Raman experimentation has been broadly utilized to assess the thermal alteration of geological carboniferous materials. − Raman spectroscopy (RS) was conducted following a protocol of Li et al and Song et al First, the kerogen powders were ground below 200 mesh. All Raman spectra analyses were performed with a Bruker Sentera Raman spectrometer.…”

“…The effectiveness of RS for measuring the thermal maturity of OM has been proven in the literature, but the calculation formulas proposed by various scholars are different. ,,− In this work, we use the more widely utilized formula reported by Wilkins et al to determine the equivalent vitrinite reflectance (VR eqv ): V G represents the center point of the G band; V D indicates the center point of the D band; the specific value (height of the G band/the minimum value between the G and D bands) reflects the saddle index; fwhh D and fwhh G are the widths of half the height of the G band and D band, respectively.…”

Methods for Petrological and Petrophysical Characterization of Gas Shales

“…However, a formula that can convert the Raman parameters of shale macerals to the equivalent vitrinite reflectance value is needed. Researchers have proposed many different formulas, such as the relationships of vitrinite reflectance versus the Raman parameters of the solid bitumen/graptolite and mineral–organic aggregates. − Moreover, there are various deconvolution approaches for Raman spectra, such as two- and five-band deconvolutions, and even non-deconvolution methods, with no agreement on which one is the best (Table ). As shown in Table , the laser Raman spectroscopy method is a semi-quantitative and non-standard method because the conversion formulas or indicators obtained by Raman for assessing shale maturity have not yet been accepted by all researchers.…”

Foot scales in the Early Cretaceous bird Gansus yumenensis from China

“…The copyright holder for this preprint (which this version posted June 8, 2021. ; https://doi.org/10.1101/2021.06.07.447457 doi: bioRxiv preprint Raman spectrum of the scale to eumelanin here is supported by its wavenumber difference from the G-band in the spectrum of the sulcus. Raman spectral parameters of organic matter is indicative of the temperature experienced by host rocks and the thermal maturity of organic matter [37,38]. A G-band at 1585 cm -1 indicates a higher maturity level than a G-band at 1600 cm -1 band.…”