The German Central Graben (North Sea)

Tectonostratigraphic evolution and hydrocarbon systems

authored by: Simon Maximillian Müller
supervised by: Jutta Winsemann
Abstract: The German Central Graben is part of the Central Graben in the southern North Sea, which is essentially a Mesozoic half-graben structure. The interaction of active rifting and salt tecton-ics formed its geology and deeply influenced sedimentation, which resulted in a differentiated basin architecture with various sub-basins. The Central Graben is also a major hydrocarbon province for both The Netherlands and Den-mark. Therefore, and due to a more liberal data publication policy in The Netherlands, most geoscientific studies concentrate on the Danish and Dutch part of the graben. One aim of this thesis is hence to advance research in this rather neglected part of the Central Graben regarding its hydrocarbon systems and geological evolution The studies that are included in this thesis were mostly realized within the projects “Subsurface potentials for storage and economic use in the North German Basin (TUNB)” and “Establishing the European geological surveys research area to deliver a geological service for Europe (Ge-oERA)”. These projects provided the opportunity to interpret reflection seismic data (3D and 2D) and well log data in regard of shallow and conventional hydrocarbon systems as well as the tectonostratigraphic evolution of the German Central Graben. Amplitude anomalies, which are indicative for gas accumulations, were mapped on reflection seismic data within the northwestern German North Sea, including the German Central Graben. These anomalies occur mainly within unconsolidated Cenozoic sediments above Zechstein salt domes. Three exemplary seismic profiles with anomaly clusters, including bright spots, seismic attenuation, and velocity pull-downs, are discussed and compared to the seismic image of cur-rently producing Dutch shallow gas fields. Seafloor methane seep sites were mapped in the northwestern German North Sea, above the German Central Graben, using a ship-based multibeam echosounder. Most detected seep sites occur above salt structures that feature amplitude anomalies in their overburden on reflection seismic data. Abandoned offshore wells show no correlation with seep sites. Analysis of bottom and surface waters at the seep sites revealed increased methane concentrations and indicated a contribution to the atmospheric greenhouse gas inventory. The potential for generating hydrocarbons of six Uppermost Triassic to Lowermost Cretaceous formations was investigated. These formations include important source rocks of the southern Central Graben, like the Lower Jurassic Posidonia Shale Formation and Upper Jurassic to Lower Cretaceous “Hot Shales”, which were both mapped anew on reflection seismic data. Additionally, other marine or terrestrial formations with mudstones or coal layers were inves-tigated in a petroleum system model. The results reveal that some of these not-in-the-spotlight formations are likely to have generated hydrocarbons and are the most likely sources of the potentially in part thermogenic shallow gas accumulations in the area. The tectonostratigraphic evolution of the German Central Graben was reconstructed. Therefore, nine laterally traceable horizons from the Lower Jurassic to the Lower Cretaceous were mapped on reflection seismic data, including the base horizons of tectonostratigraphic mega-sequences (TMS) of the tectonostratigraphic concept from offshore The Netherlands. The results reflect the evolution of the basin from rift to salt dominated, to thermally induced subsidence. The findings were integrated in the Dutch and Danish tectono- and sequence stratigraphic concepts and discussed regarding the influence of tectonics on (litho-)stratigraphy.
Organisation(s): Institute of Geology
Type: Doctoral thesis
No. of pages: 206
Publication date: 2023
Publication status: Published
Sustainable Development Goals: SDG 14 - Life Below Water
Electronic version(s): https://doi.org/10.15488/13228 (Access: Open)