The organic petrology of graptolites in sections parallel and perpendicular to the bedding: Implication for thermal maturity evaluation

Graptolites are widely presented in the Ordovician–Silurian Wufeng–Longmaxi Formations and other Lower Paleozoic marine shales throughout the world, and their reflectance is regarded as an effective index for the thermal maturity assessment of the graptolite-bearing shales devoid of vitrinite. Graptolites display distinct optically biaxial features, and the optical characteristics and reflectance of graptolites reveal significant differences depending on orientation of respective graptolite fragments relative to the bedding planes. However, specific studies on these optical discrepancies remains limited, resulting in confusion and/or substantial errors in thermal maturity assessment of graptolite-bearing shales. In this study, the organic petrological method was employed to compare the optical characteristics of graptolites in sections parallel and perpendicular to the bedding. Graptolites and in-source solid bitumen (SB) are the dominant organic components in the Alum and Wufeng–Longmaxi graptolite-bearing shales, and the liptinites are only detected in low maturity samples. In the studied sediments, both non-granular and granular graptolites were identified in abundance, but non-granular graptolites, which exhibit higher reflectance and stronger anisotropy than granular graptolites, were the predominant species. The intense positive correlations between the mean random reflectance (GR_o) and true maximum reflectance (R_max) and bireflectance of non-granular graptolites indicate their increasing anisotropy with maturation. R_max values in sections parallel to the bedding are slightly lower than or equal to those in sections perpendicular to the bedding, and the mean maximum reflectance ($\overline{R}$_max), the intermediate reflectance (R_int), and bireflectance are lower than those in sections perpendicular to the bedding. GR_o values in sections parallel to the bedding display a strong positive correlation with GR_o in sections perpendicular to the bedding, and the former are higher than the latter. GR_o exhibits stronger positive correlations with R_max, $\overline{R}$_max, and bireflectance in sections perpendicular to the bedding than those in sections parallel to the bedding, revealing the stronger anisotropy of non-granular graptolites in sections perpendicular to the bedding. According to the reflectance discrepancies of non-granular graptolites in sections parallel and perpendicular to the bedding, several equivalent equations are proposed to evaluate the thermal maturity of Ordovician–Silurian graptolites-bearing sediments.