Organic matter-rich rocks are potential source rocks for oil and gas but are also valuable archives for past environmental conditions. In this thesis, Permo-Carboniferous and Lower Jurassic sediments from southern Germany and Switzerland were studied, addressing both aspects. To evaluate source potential and depositional environment, organic petrographic, palynofacies, Rock-Eval pyrolysis, and geochemical techniques (incl. biomarker analysis and compound-specific carbon isotopy) have been used.
Permo-Carboniferous sediments in the basement of the North Alpine Foreland Basin (NAFB) occur in post-Variscan graben structures. Upper Carboniferous coals probably charged Switzerland´s only gas deposit, while lower Permian lacustrine shales with high organic matter contents are potential source rocks for oil. The study of the Permo-Carboniferous sediments is based on 90 core samples from borehole Weiach-1 (northern Switzerland). Carboniferous coal seams, up to 10 m thick, were deposited in low-lying freshwater mires. In Weiach-1, they reached a maturity stage corresponding to the onset of gas generation. Organic matter-rich Permian (Autunian) shales were deposited in anoxic lakes with a stratified water column. The organic matter is of mixed aquatic and terrigenous origin. 13C values of pristane and phytane provide evidence for reworking of organic matter in the water column. Locally, the Autunian shales have a very good oil potential. The main shale interval in Weiach-1 is about 12 m thick. It reached early oil window maturity and will generate about 0.35 tHC/m² when mature.
Lower Jurassic rocks with high organic matter contents (Posidonia Shale) represent the Taorcian anoxic event (T-OAE). A prominent negative carbon isotope excursion (CIE; ~183 Myr) is present in the lower part of the T-OAE. Posidonia Shale from an active quarry (Dormettingen; Swabian Alb) and a borehole core, recovered from the basement of the NAFB (Salem; Germany), has been investigated.
The Posidonia Shale at Dormettingen is about 10 m thick and was investigated using a total of 56 samples. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. Biomarker data suggest enhanced activity of diazotrophic cyanobacteria. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated organic compounds replicate the negative CIE. The shift for short-chain n-alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0 ‰. This is the highest value observed so far for any Toarcian section. 13C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction. In Dormettingen, the Posidonia Shale is immature to marginal mature.
In the Salem borehole, the Posidonia Shale is also about 10 m thick. The lower 7.5 m and the uppermost part of the underlying Amaltheenton Shale were studied using a total of 62 core samples. In contrast to other Toarcian sections, TOC contents are high in sediments deposited during the CIE, but even higher in post-CIE sediments. Biomarker ratios suggest that this reflects strong oxygen-depletion during the CIE (elegantulum to lower elegans subzones), but also during the falciferum Subzone. While the lower part of the Posidonia Shale (tenuicostatum Zone to elegans Subzone) is significantly thinner than in neighbouring sections (e.g., Dormettingen), its middle part (falciferum Subzone) is unusually thick. This suggests that increased subsidence contributed to anoxia and indicates major variations in basin geometry. Although the CIE interval is very thin, δ13C values of n-alkanes confirm the time lag between the maximum negative isotope shift (below “Unterer Stein”) and the strongest basin restriction (above “Unterer Stein”). Moreover, the onset of the CIE is recorded earlier for aquatic than for terrigenous organisms. In Salem, the Posidonia Shale is thermally mature and highly oil-prone. The residual source petroleum potential is about 0.8 tHC/m².