Geochemical evolution of organic-rich shales with increasing maturity: A STXM and TEM study of the Posidonia Shale (Lower Toarcian,northern Germany)

Hydrocarbon generation and retention processes occurring within gas shales as a response to increases in thermal maturation are still poorly constrained. While efforts have been directed at unravelling the resource potential, composition and textures of these economically important unconventional systems, their spatial variability in chemistry and structure is still poorly documented at the sub-micrometer scale. Here, we have characterized samples of the Lower Toarcian Posidonia Shale samples from northern Germany at varying stages of thermal maturation using a combination of compositional organic geochemistry and spectromicroscopy techniques, including synchrotron-based scanning transmission X-ray microscopy (STXM). We document geochemical and mineralogical heterogeneities down to the nanometer scale within the investigated samples as a function of their level of thermal maturity. In particular, authigenic albite crystals containing nanometric halite inclusions have been documented within the investigated mature and overmature samples. The presence of such tracers of palaeobrine–carbonate interactions supports a maturation scenario for the Lower Toarcian Posidonia Shale intimately related to ascending brine fluids rather than a maturation scenario solely resulting from high heat flows. In addition, various types of asphaltene- and NSO-rich bitumen have been detected within the same samples, very likely genetically derived from thermally degraded organic precursors. Furthermore, the formation of nanoporous pyrobitumen has been inferred for samples of gas window maturity, likely resulting from the formation of gaseous hydrocarbons. By providing in-situ insights into the fate of bitumen and pyrobitumen as a response to the thermal evolution of the macromolecular structure of kerogen, the results reported here constitute an important step towards better constraining hydrocarbon generation processes during natural shale gas maturation.