Sample maturation calculated using Raman spectroscopic parameters for solid organics: Methodology and geological applications

Abstract

The shape and shift of Raman peak of solid organics prove to be capable of revealing atomic and molecular level vibration information of aromatic ring structure and its relationship with sample maturation. Raman "D" peaks and "G" peaks of a series of continuously maturated coal samples were measured, and the inter-peak intervals (GD) and peak height ratios (Dh/Gh) were derived and correlated with the vitrinite reflectance (vR o %) of standard coal samples. As a result, two formulae were established by using the two Raman indices for calculation of Raman reflectance (RmcR o %), which is equivalent to vitrinite reflectance. The formula for calculating Raman reflectance indicative of organic maturation using Raman shift inter-peak interval (GD) is RmcR o %= 0.0537 d(GD)-11.21, which is mainly applicable to matured to highly matured carbonized samples of solid organics; The formula for calculating Raman reflectance indicative of organic maturation using Raman peak height ratio (Dh/Gh) is RmcR o %= 1.1659 h (Dh/Gh)+2.7588, which is mainly applicable to carbonized samples of solid organics that are over matured or going to be turned into granulated graphite. Preliminary applications indicate that Raman reflectance "RmcR o %" calculated based on results of Raman spectral analysis of solid organics can be used to characterize sample maturation at molecular level, so enjoying extensive prospects in geological applications. In petroleum geology and coal petrography, vitrinite reflectance (vR o %) is a generally universal index gauging maturation of hydrocarbon source rocks and coals. However, highly matured to over-matured samples would show extensive variations in vitrinite reflectance due to considerable inhomogeneity, which would affect accurate assessment of sample maturation. Recently, peak D and peak G in Raman scattering of carbon nanotubes and natural solid organics seem to reflect not only structures and performance of carbon nanotubes but also thermal evolution of carboniferous solid organics in geological samples, and temperature and pressure conditions for experimental samples, so attracting extensive attention from the academic communities in the 7] reported the relationships between Raman inter-peak interval (D-G) and peak height ratio (D/G) of pyrolytic products of bitumite and kerogen and maturation of samples, or the temperature and pressure conditions for experimental samples. These authors concluded that

The relationship between micro-Raman spectral parameters and reflectance of solid bitumen

“…5) established in this study. The Raman spectral data of the natural samples were drawn from Liu et al (2013). These samples were collected from some boreholes and bitumen mines from Sichuan Basin Fig.…”

“…In contrast, Raman spectral parameters of solid bitumen could solve this problem. The advantages of Raman spectral method include: (1) requirement of a small amount of sample (dozens of mg), (2) very fine-grained particles (as small as 1 μm) that could be used (Guedes et al, 2010;Schopf et al, 2005;Schopf and Kudryavtsev, 2009), and (3) less dispersion of the data compared with reflectance; hence, the reliable Raman spectral parameters can be obtained from only a few measured points (Liu et al, 2013).…”

Potential Raman parameters to assess the thermal evolution of kerogens from different pyrolysis experiments

“…The measurement conditions were calibrated according to Liu et al [13]: wavelength 532 nm, filter 1%, hole 300, Grating 1800 T, Raster slit 100um, exposure time 20-40 s, objective 100×, eyepiece 10×, each sample was scanned from 400 cm −1 to 4000 cm −1 .…”

“…Liu et al [13] found that in a reasonable thermal maturity scope, the Raman spectra show much similarity regarding different maceral units in one sample. Therefore 6-8 points were measured on each sample to get the kerogen structure information as a whole, which is exemplified by the area of Raman peak G from the semiopen pyrolysis systems as shown in Fig.…”

“…Hu et al [7,11] demonstrated that at high levels of thermal evolution (Ro > 2.0%), the G-D band interval decreased with the increasing maturation level of sedimentary organic matter. Kelemen et al [12] and Liu et al [13] proposed a positive linear correlation between the inter-interval of peaks D/G and Ro. Beyssac et al [14,15] and Bernard et al [16] suggested that simple D/G band ratios are not adequate to evaluate the thermal maturity level of geologic carbonaceous materials by changes in Raman spectra.…”