Organic geochemistry and petrology of Posidonia Shale (Lower Toarcian, Western Europe) – The evolution from immature oil-prone to overmature dry gas-producing kerogen

Kerogen concentrates as well as whole rock samples of the Lower Jurassic Posidonia Shale, representing a complete maturity series from immature to highly overmature, were analyzed using organic petrography, palynology, bulk geochemical analysis, FT-IR and Curie Point-Pyrolysis-GC–MS (CP-Py-GC–MS) in order to investigate changes in kerogen composition with increasing maturation. Vitrinite reflectance (VRr) ranges from 0.5 to 3%. HI values reflect the maturity trend very well, with little scatter. In contrast, Tmax, OI and PI values show more scatter and greater differences between kerogen concentrates and whole rock samples. FT-IR spectra show a decrease in functional group bands with increasing maturation which has been quantified. CP-Py-GC–MS analyses indicate an increase in hydrocarbons, especially for short- and medium-length n-alkanes, with a strong predominance of short-length alkanes at highest maturities (about 3% VRr). This effect, however, depends also on the applied pyrolysis temperature, where higher temperatures (920 °C, as compared to 650 °C) favor generation of short-length n-alkanes, especially for samples at high levels of maturation. Ratios of aromatic over aliphatic hydrocarbons or alkenes over alkanes show a non-steady evolution, depending also on the applied pyrolysis temperatures, indicating that higher pyrolysis temperatures are needed for pyrolytic alkene formation, with a maximum alkene formation for samples within early overmature maturities (~ 1.4% VRr). Aromaticity first decreases, but increases at high levels of maturation. Finally, weathering of the highly overmature sample (3% VRr) resulted in strong oxidation of organic matter with impact on Rock-Eval, FTIR and Curie Point-pyrolysis data.