Abstract

Although shale gas systems constitute a new target for commercial hydrocarbon production, only a little attention has been paid to the evolution of these unconventional systems with increasing thermal maturation. This study reports the characterization of samples of the Lower Toarcian (Lower Jurassic) Posidonia Shale from northern Germany at varying levels of thermal maturity (0.5–1.45%R_o [vitrinite reflectance]). Observations were made using an original combination of focused ion beam–scanning electron microscopy (FIB-SEM) and transmission electron microscopy (TEM). We document the formation of microfracture-filling bitumen in close association with kerogen residues with increasing maturity. Porosity evolves from mostly submicrometric interparticle pores in immature samples to intramineral and intraorganic pores (irregular-shape pores of about 1 to 200 nm occurring within the macromolecular structure of pyrobitumen masses) in overmature (gas mature) samples. This intraorganic nanoporosity has most likely come about by the exsolution of gaseous hydrocarbon and been hydrocarbon wet during the thermal maturation processes. The mineralogical assemblage of the investigated samples strongly evolves with increasing thermal maturity. The formation of most of the mineral phases within the oil and gas mature samples is interpreted as resulting from the percolation of sulfate-rich evaporite-derived brines at temperatures of about 140 to 180°C. Although FIB-SEM and TEM images are small compared to field size, the present study emphasizes the need for nanoscale imaging to better constrain hydrocarbon generation processes in gas shale systems.