Raman spectroscopy of graptolite periderm and its potential as an organic maturity indicator for the Lower Paleozoic in southwestern China

The determination of organic maturity for Lower Paleozoic sediments is challenging due to the lack of vitrinite. Many studies have been completed on the morphology and optical properties of graptolite periderm, but little is known about vibrational changes in its structure with increasing maturation (e.g., Raman spectroscopy). In this study, the vibrational structure of graptolite periderm was determined using the Raman spectrum. Raman spectra were obtained from natural samples with a wide range of organic maturity and from two artificially matured sample sets (El and AS-DD-LG). As graptolite reflectance increases, the position of the disorder band shifts to lower wavenumbers and the graphite band becomes narrower. Due to these alterations, the width ratio (FWHM-D/FWHM-G), area ratio (AD/AG) and band separation (RBS) increase. Graptolite reflectance displays negative correlations with the maximum position of the D1 peak (W-D) and the full width at half maximum of the G band (FWHM-G). These observations demonstrate that the structural order and aromatization in graptolite periderms increase as organic maturity increases. A positive correlation between RBS and graptolite reflectance is shown in this study. Based on the relationship between vitrinite reflectance and graptolite reflectance proposed by Luo et al. (2018), a conversion equation of the equivalent vitrinite reflectance on the basis of RBS values of graptolite periderm is proposed to assess the thermal maturity for the Lower Paleozoic.