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

Abstract

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.