Microstructure of graptolite periderm in Silurian gas shales of Northern Poland

Core samples of Silurian graptolite shales from an exploration borehole within the Baltic Basin were examined by optical microscopy, Raman spectroscopy and high resolution transmission electron microscopy. Graptolite periderm is composed of poorly ordered carbonaceous material of mainly aromatic structures. The dimension of the coherent domains is 1-2 nm. They are composed of two to three stacked layers. Raman spectral parameters of graptolite periderm are correlated with the mean random (Rr) and mean maximum (Rmax) reflectance of graptolites. The spectral ID1/IG ratio predicts thermal maturity in a mean reflectance of graptolites (Rr) range between 1.30% and 1.80% (Rmax ≈ 1.40-2.00%; Vitrinite Reflectance Equivalent VRE ≈ 1.10-1.50%). It is possible to calculate VRERaman from the ID1/IG ratio.