Verlag des Forschungszentrums Jülich
JUEL-3567
Petmecky, Ralf Stephan
Numerische Simulation der Entwicklungsgeschichte des zentralen Niedersächsischen Beckens unter besonderer Berücksichtigung der Erdgaslagerstätten-Bildung
242 S., 1998
The burial, erosion and temperature history of the central part of the Lower Saxony basin (west of the Weser river) was reconstructed applying 1D and 2D basin modeling. Vitrinite reflectance data, Rock-Eval pyrolysis data, the results of apatite fission track analysis as well as fluid inclusion measurements and an organic-geochemical maturity indicator (Methylphenantrene index) were used as calibration parameters.
Within the study area the observed coalification increases towards the south. In the southern part of the Lower Saxony basin mean vitrinite reflectance values greater than 4 %Rr were established for the uppermost Carboniferous. The increasing maturity of the Carboniferous gas source rocks towards the south is accompanied by a decrease of the coalification gradients. Hence, the observed increase of coalification with depth is lower in the south than in the north. The combination of the maturity pattern for the top Carboniferous with a structural map of the subsurface of the Lower Saxony basin suggests that single tectonic blocks, which are bordered by deep reaching faults, are characterized by a specific maturity spectrum. Most likely there are no isolated areas of high reflectivity. The thickness distribution of upper Jurassic and Cretaceous strata indicates a strong increase of accumulation rates (up to 170 m/m.y. during Wealden) towards the southern rim ot the study area.
The results of the 1D simulations (PDI/PC) as well as the outcome of the 2D modeling (PetroMod 3.1) performed on the basis of two seismic cross sections - indicate that a satisfying match with the calibration data can only be realized if deep burial of the southwestern and central parts of the study area is assumed. The amounts of eroded Cretaceous sediments in these areas vary between 4200 and 3200 m with decreasing tendency towards the north. The calculated values of uplift match the amount derived from structural restoration of the western cross section. Estimated heat flow values during the time of maximum burial vary between 64 and 79 mW/m2. However, an upper Cretaceous intrusion south of the study area cannot be excluded with certainty, since it is not possible to establish a coalification gradient for the southeastern-most well (only 1050 m deep). The northern part of the study area experienced maximum thermal stress during Tertiary times (additional burial of a few hundred meters in combination with heat flow values around 75 mW/m2), followed by a period of cooling, which is also indicated by the results of the apatite fission track analysis. Present day heat flow values amount to 75 mW/m2 in the south and decrease gradually to 60 mW/m2 in the north.
Based on the calibrated temperature and burial history, the timing of methane generation from the Carboniferous source rocks, the gas migration and accumulation was simulated. The major phase of thermal gas generation within the southern part of the Lower Saxony basin occurred during upper Jurassic and Cretaceous times. The onset of the inversion (beginning at upper Cretaceous times) terminated the methane generation, which was never reactivated again. In the north, gas generation continued at least during early Tertiary times. Gas migration occurred mainly along deep reaching and active fault zones. The enormous tectonic stress within the southern parts led to the remigration of the methane, while in the north, due to much smaller movements along the faults and due to the ongoing gas generation during Tertiary times, potential traps were preserved and filled with methane.
The southern part of the Lower Saxony basin is locally characterized by high CO2 contents (up to 95 %) within the Zechstein carrier rocks. The [delta]13C signature of this gas varies between -1 and -5 ‰, which points to an inorganic origin. CO2 containing springs located south of the Lower Saxony basin revealed [delta]13C values of -6,62 to -9,88 ‰ in Bad Oeynhausen and of -18,67 to -21,74 ‰ in Bad Münder. The former values could indicate a juvenile origin of the CO2, whereas the latter data point towards organic material as precursor. Because of the insufficient data base, a genetic correlation between CO2 in the Lower Saxony basin and the CO2 containing springs is - at this time - not possible. Due to the overall geological evolution of the Lower Saxony basin as well as the spatial distribution of the CO2 accumulations it is most likely that the generation of the CO2 started during latest geological history and is still continuing.
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