The organic petrology of graptolites and maturity assessment of the Wufeng-Longmaxi Formations from Chongqing, China: Insights from reflectance cross-plot analysis

The Wufeng Longmaxi Formations (Ordovician-Silurian) contain abundant graptolite-bearing shales, which have proven to be one of the most successful shale gas exploration targets in China and can provide an opportunity to study the organic petrology of graptolites. Organic petrology analysis has been conducted on these organic-rich shales in order to depict the shape of graptolite reflectance indicating surface (GRIS) and their organic maturity. Granular and non-granular graptolites have been observed in the studied sediments, and non granular graptolites were the major graptolite species. The strong positive correlation between the maximum reflectance (R-max) and bireflectance of graptolites indicates that the graptolites display stronger anisotropy with maturation. A linear coalification path of graptolite fragments was observed in the studied sediments. A remarkable decrease in the minimum reflectance (R-min) values of graptolites was observed when R-max exceeds 6%. The samples with lower thermal maturity (R-max <= 2%) tend to be biaxial neutral, whereas higher rank samples display biaxial negative characteristics. The GRIS shape becomes more oblate with maturation. The Wufeng Longmaxi shales from the southeastern Chongqing and Wuxi areas of the Chongqing municipality, China, are post mature and in the dry gas zone, whereas samples from the Chengkou area of the Chongqing municipality, China, are still in the oil window. Their different burial and thermal histories controlled their thermal maturity.