Geochemical analysis of the marginal eastern basin facies of the German Wealden (Berriasian, Lower Cretaceous)

Abstract

Organic-rich shales of the German Wealden, Lower Cretaceous, are a target for gas and oil shale exploration in Germany. Results of published organic petrographic investigations as well as oil shows and hydrocarbon gases extracted at the historical Lüdersfeld shaft of the Stadthagen syncline confirm the hydrocarbon potential of the Wealden sediments at the eastern margin of the Lower Saxony Basin. These former observations suggest that the shales of the eastern subbasin that comprise several hundreds of meters of thickness have reached thermal maturities compatible of the oil and gas generation windows. We present results of a surface geochemical study in which seventeen outcrop samples collected along a profile across the Stadthagen syncline (Bückeberge to Rehburg anticline) have been used for re-assessment of the hydrocarbon potential of the Wealden sub-basin. The analytical methods comprise porosity measurements, elemental analysis (x-ray fluorescence, total and organic carbon and total sulfur analyses), Rock- Eval pyrolysis, hydrocarbon extraction, fractionation of extracted hydrocarbons and gas chromatography on aliphatic fractions as well as stable carbon isotope analyses and gas geochemical investigations . The investigated Wealden sediments comprise sandstones and shales and the latter contain high relative concentrations of carbon with an average of 3.2 wt.-%. Rock-Eval pyrolysis suggests variable contributions of different types of organic organic matter (land plant, phytoplankton). The depositional environment of the sediments was predominantly oxic with the exception of three samples that were deposited under dysoxic to anoxic conditions. Thermal maturities range from the oil to the wet gas window (samples from the basin center). Two samples from the Rehburg anticline contain phytoplankton dominated kerogens and show thermal maturities of the oil window, the latter also confirmed by stable isotope geochemistry of extracted hydrocarbon gases. Hydrogen indices suggest excellent generative potentials for algae-rich sediments; however, most samples have comparatively low hydrogen indices related to either high thermal maturities or unfavorable organic matter types. Thermal maturities and measured shale porosities indicate changes of probable maximum burial depths from about 1800 m at the Rehburg anticline (Wealden V/VI) to about 3600 m in the basin center (Wealden III/IV). The estimated maximum burial depth of the Bückeberge amounts to about 2600 m (Wealden I/II). Although, initial results suggest good shale gas potential in the Stadthagen syncline, our investigations indicate a horizontal and vertical variability of the hydrocarbon potential within the basin.