Lower Cretaceous Geothermal Reservoirs in the North German Basin
by Sandra Franke
Date of Examination:2023-05-23
Date of issue:2023-10-02
Advisor:Dr. Matthias Franz
Referee:Prof. Dr. Jonas Kley
Referee:Dr. Thomas Voigt
Referee:Dr. Bettina Wiegand
Referee:Prof. Dr. Inga Moeck
Referee:Prof. Dr. Martin Sauter
Sponsor:Bundesministerium für Wirtschaft und Energie
Persistent Address:
http://dx.doi.org/10.53846/goediss-10068
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Abstract
English
This thesis contributes to the strategic effort to make Germany’s energy supply more sustainable. The primary focus of this study was to enhance and update the geological knowledge of the Mesozoic deep geothermal sandstone aquifer complexes. The economically important Mesozoic deposits have been studied for decades, for example, the Buntsandstein, the Keuper, the Jurassic and the Lower and Upper Cretaceous. A comprehensive data base is provided by the data resulting from intensive hydrocarbon exploration since the last century. However, previous detailed work on Lower Cretaceous deposits had been done over 50 years ago, making new investigations applying new methods and techniques necessary. Therefore, lithofacies analysis and determination of depositional environments were required to enhance the understanding of sand body architecture, sediment distribution and basin-scale configuration. Granulometric and petrophysical properties of lithofacies types and depositional environments were determined correlations and dependencies were differentiated according to available data. This work will contribute to reconstructing the primary basin extension in the eastern North German Basin and the basin history to predict the location, thickness and facies of Lower Cretaceous geothermal reservoirs. The results of this study show the importance of evaluating how tectono-stratigraphic and eustatic features influence the present-day petrophysical characteristics, distribution and thickness of Lower Cretaceous sandstone aquifers. The eastern North German Basin was well connected to boreal and tethyal waters, although regressive conditions temporarily hampered faunal migration. Dominating terrestrial and shallow-marine conditions do not enable completely the same biostratigraphic time control as applied in the western North German Basin. However, characteristic ostracod, spore, pollen, foraminifera and dinocyst species could be identified combined with lithostratigraphic characteristics a time control on the Lower Cretaceous strata is enabled. Despite the dominating extensional tectonic regime assigned to Lower Cretaceous times, the study area revealed widespread erosional patterns implying differential uplift and erosion. The formation and activity of the swell regions and salt movement contributed much to basin differentiation, the formation of local sub-basins and strata thickness. Three main unconformities could be figured out. Generally, the stratigraphic most complete successions kept preserved in syn-tectonic graben structures in northeast Germany and the rim synclines of salt plugs. Petrographic analysis shows that the investigated sandstones are highly mature and influenced by diagenetic features, which enable high porosities and permeabilities depending on the depositional environment. The grain fabric was stabilised by an eodiagenetic carbonate cement, which was dissolved during telodiagenetic uplift. The analysis of the grain-size distribution shows a constant trend, which suggests a relatively stable sediment source area. Corresponding to the heavy mineral assemblage, characterised by an ultrastable heavy mineral assemblage located on the Fennoscandian High. The analysis of the depositional environments revealed deposits from a lagoonal or tidal flat, deltaic channel belt facies and alternating sand-, silt- and claystone successions from an upper and lower delta plain. Furthermore, delta front, prodelta and shoreface deposits were analysed. The ongoing subordinated basin extension and flooding are characterised by the formation of shoreface sands, fine clastics from the shelf transition zone, and pelagic limestones and mudrocks combined to a fully marine microfauna. Finally, all information was combined to develop cumulative sandstone thickness maps, a schematic facies map for the Berriasian (Wealden 3-horizon) and the proposal of three potential exploration sites to minimise financial and exploration risks for deep geothermal aquifer exploration in the North German Basin.
Keywords: North German Basin; Lower Cretaceous; Reservoir Potential; Geothermal Reservoirs; Sandstone Reservoirs
