Stratigraphy and diagenesis of the Zechstein 2 in the Southern Permian Basin, onshore northern Netherlands and NW Germany
Aachen (2016) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (122 Blätter) : Illustrationen, Diagramme, Karten
The presented thesis focuses on the Upper Permian Zechstein 2 sequence of the German part of the Lower Saxony Basin and onshore the northern Netherlands. The Dutch study area is further divided into three structural elements: the Groningen High in the east, the Lauwerszee Trough, and the Friesland Platform in the west. Both study areas belong structurally to the North German Basin, which is in turn part of the Southern Permian Basin. The Zechstein is characterised by several successive carbonate-evaporite cycles, of which up to seven can be identified in the studied areas today. The second cycle, named the Zechstein 2 (Ca2), is comprised of a carbonate unit at the base, which is overlain by anhydrite and rock salt deposits. The rock salt layer reaches a thickness of more than 500 m in the studied areas and is therefore acting as seal for hydrocarbons trapped in Zechstein rocks, also prohibiting a downward migration of e.g. meteoric fluids into the studied intervals. At least two intra-salt layers, composed of varying amounts of polyhalite and anhydrite, are completely embedded in the Dutch salt unit, but are too thin to be resolved on seismic images. With the help of geophysical well log data, these sub-seismic scale sulphate layers can be correlated and used as stratigraphic markers in the Dutch study area. In order to analyse the precipitation patterns and the diagenetic evolution of the Zechstein 2 sequences, rock samples of overall eight wells were analysed. Petrography methods such as transmitted light microscopy and cathodoluminescence were combined with a large set of analytical techniques, including XRD and XRF analysis, ion chromatography, stable isotope analysis, as well as fluid inclusion analysis. Bromine analysis confirmed that the intra-salt sulphate layers represent freshening events inside the salt basin. Furthermore, geochemical analyses revealed that the underlying Zechstein 2 carbonates in the Lower Saxony Basin were highly affected by late-diagenetic, partially iron-rich, hydrothermal fluids, from which hydrocarbon gases and CO2 were released and trapped in primary and secondary fluid inclusions. The inorganic CO2, derived from the thermal decomposition of underlying Devonian carbonates, is considered as one trigger for carbonate dissolution during burial. However, CO2 alone has no great impact on dissolution, but in combination with late dolomitization and pyrite formation, secondary porosity is created. Thermochemical sulphate reduction (TSR) is a common process in deeply buried sulphate-bearing carbonates and occurred to a great extent in one of the studied wells. In addition to the inorganic CO2 derived from ascending fluids, TSR is considered as an additional source for CO2 in the Lower Saxony Basin. Overall, this study gives new insights into the combined effects of salt tectonics, carbonate diagenesis, burial porosity evolution, and the migration of CO2 derived from inorganic sources.
Biehl, Bianca Coline
Kukla, Peter A.