Geochemical and petrographic evaluation of hydrous pyrolysis experiments on core plugs of Lower Toarcian Posidonia Shale: Comparison of artificial and natural thermal maturity series

Abstract

Semi-closed hydrous pyrolysis (HP) of whole-rock cuboids is a relatively novel technique aimed at improving the knowledge of the geochemical and petrographic alteration of petroleum source rocks. This study evaluates the comparability of observations on petroleum generation and migration in a natural maturation sequence and after HP in the same source rock. Two artificially matured samples of the Lower Jurassic (Toarcian) Posidonia Shale from the Hils Syncline (Lower Saxony Basin) were subjected to 24 h-HP experiments at 280 °C, 300 °C, 320 °C, 330 °C and 340 °C. The samples were subsequently analyzed with respect to changes in Rock-Eval pyrolysis parameters, molecular organic geochemistry, and organic petrography. After HP at 280 °C and 300 °C, organic geochemical composition and organic petrographic characterization show only minor changes. Significant geochemical and petrological changes occur at 320 °C: Tasmanales and Leiosperidales phytoclasts show weakened fluorescence and volume loss, accompanied by a pronounced decrease in the Rock-Eval S2 yield of the sample, indicating conversion of kerogen to petroleum products. Optical changes are even more pronounced at 330 °C and 340 °C, when very high transformation ratios are reached, exceeding those under natural conditions. The majority of aliphatic molecular geochemical proxies for thermal maturation show systematic changes with increasing vitrinite reflectance, similar to maturation trends observed in the natural maturation sequence. However, some hopanoid thermal maturity proxies (e.g. moretane/C₃₀ hopane) show unexpected inverse trends, whereas aromatic hydrocarbon ratios hardly change with increasing HP temperatures. These observations suggest that the reactions leading to changes in these parameters require considerably more time than CC bond breaking (cracking) within the kerogen structure. A large part of the organic carbon remaining in the cuboids after HP at 330 °C and 340 °C is soluble in dichloromethane and should, therefore, be classified as bitumen rather than kerogen.