The chemical composition of graptolite periderm in the gas shales from the Baltic Basin of Poland

Silurian graptolite shales from an exploration borehole within the Baltic Basin (N-Poland) were examined by the electron microprobe (for the first time) and micro-FTIR spectroscopy. The graptolite periderm of mean reflectance R_r ≈ 1.30–1.80% (VRE ≈ 1.10–1.50%) contains ~85–91 wt% of carbon, 2.5–8.5 wt% of oxygen, 1.5–3 wt% of nitrogen and (by difference) ~3.0–4.5 wt% of hydrogen. Aromatic structures predominate, with minor amounts of the aliphatic and carbonyl groups. Carbon content in the graptolite periderm increases and oxygen content decreases with the increasing depth and mean reflectance (R_r). This is accompanied by the increase in aromaticity of the structure and shortening of the aliphatic chains. The studied graptolites represent mostly kerogen type II and, partly, type III. Chemical structure of the graptolite periderm reveals many similarities to that of vitrinite of the adequate rank (considering the Vitrinite Reflectance Equivalent values).