Diagenetic controls on sandstones of the Stuttgart Formation – consequences for the porosity evolution of hydrothermal reservoirs in the North German Basin
by Sebastian Niegel
Date of Examination:2023-05-09
Date of issue:2023-07-05
Advisor:Dr. Matthias Franz
Referee:Prof. Dr. Inga Moeck
Referee:Prof. Dr. Reinhard Gaupp
Referee:Dr. Matthias Franz
Persistent Address:
http://dx.doi.org/10.53846/goediss-9977
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Abstract
English
The present dissertation is part of the R&D project GeoPoNDD, which deals with the potential use of the six main Mesozoic geothermal reservoir complexes in Northern Germany. The hydrothermal reservoirs of the Stuttgart Formation, which have been poorly explored, were evaluated in the previous Sandsteinfazies project using an extensive database of cores, wireline logs and seismic data. This subsurface facies and thickness mapping of the Lower and Upper Schilfsandstein Members revealed a fluvio-deltaic environment prograding from the northern and southern margins toward this subsidence center. The resulting subsurface facies maps and samples of different facies types are the basis of this dissertation. This study focused on petrography and diagenesis in the reservoirs of the Stuttgart Formation (Middle Keuper). Therefore, an extensive petrographic analysis of the Lower and Upper Schilfsandstein Members was carried out, leading to a mineralogical and diagenetic characterization of different facies types. Associated to the deposition of sandstones in distinct fluvio-deltaic environments, four diagenetic pathways – Delta Channel, Delta Plain, Fluvial Channel and Floodplain Diagenesis Types – are indicated by systematic variations of detrital and authigenic assemblages. In response to the evolution of the North German Basin, high permeabilities of sandstones of the Delta Channel and Fluvial Channel Diagenesis Types are related to the subsequent control of depositional and diagenetic regimes on porosity evolution. The proposed model of cementation-controlled burial diagenesis, followed by dissolution-controlled retrograde diagenesis (uplift) and low to moderate reburial corresponds to the Mesozoic–Cenozoic evolution of larger areas of the North German Basin. Due to the documented diagenetic pathways, parageneses and the low to intermediate grain packaging seems to be not in agreement with the burial history of the Stuttgart Formation in the North German Basin. To reconstruct the basin evolution, i.a. isotopic and microthermal investigations were done. A reactive transport modelling for the possible formation of the analcime was also realized. The calculated formation temperatures of authigenic dolomite and quartz of up to 200 °C, as evidenced by fluid inclusions, indicate greater maximal burial depths of the Stuttgart Formation than previously known. To explain this discrepancy, different hypotheses are discussed. The subsequent uplift from deep to shallow burial depth, associated to latest Jurassic–Cenozoic structural differentiation and inversion of the North German Basin, triggered substantial dissolution processes that mainly affected carbonate cementations. A possible second prograde diagenesis was documented by U/Pb dating of carbonate cementations, followed by a second reburial phase. Further dissolution and cementation processes were associated with this basin evolution. This opened pore spaces and contributed to the evolution of secondary porosity. Moderate post-inversion reburial rates resulted in only negligible mechanical compaction and reduction of secondary porosity. Improved knowledge of diagenetic pathways and burial history, exemplified by sandstones of the Stuttgart Formation, will contribute significantly to improved predictions for other Mesozoic hydrothermal reservoir, i.e. Rhaetian reservoirs, in the North German Basin. Thus, better predictions regarding to the reservoir quality also minimize the exploration risk. Geothermal exploitation of the Lower or Upper Schilfsandstein Members is limited due to locally highly variable reservoir quality (thicknesses, porosities, and permeabilities) resulting from different facies types and their characteristic diagenetic processes. The stratigraphic, lithological, petrographic as well as petrophysical data finally led to aquifer maps of the Lower and Upper Schilfsandstein Members, which were made available to the public in the Geothermal Information System (GeotIS). This set of maps is intended to minimize exploration and financial risks for deep, geothermal reservoirs in the North German Basin in the future.
Keywords: Triassic; Keuper; Schilfsandstein; Sandstone reservoir; Diagenesis; Secondary porosity
