| dc.description.abstracteng | 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. | de |