Magdalena Scheck-Wenderoth

Severity and timing of Cenozoic exhumation in the southwestern Barents Sea

The juxtaposition of the North Atlantic rift system with the borderlands of the Eurasian Arctic shelf has resulted in an unusual basin evolution in the southwestern Barents Sea. Exploration has concentrated on the Mesozoic sandstone plays of the Hammerfest Basin. These have yielded abundant gas and very little oil. Such findings are typical of peripheral North Atlantic margin basins that have undergone Cenozoic exhumation. This is generally thought to have occurred during the Palaeocene, Oligocene–Miocene or Pleistocene. Hammerfest Basin erosion is widely estimated to be in the range of 500–1500 m. However, the relative severity of individual episodes and impact on fluid dynamics within the basin are poorly constrained. We apply basin modelling tools to establish the sensitivity of the petroleum system to three cited scenarios for Cenozoic exhumation. Vitrinite reflectance and temperature data constrain burial and thermal histories. Calibration is not possible using heat flow variations alone; erosion timing and magnitude are required to match available data. The calibrated model indicates a significant thermal disequilibrium in the Hammerfest Basin at the present day resulting from Late Cenozoic exhumation. Forward modelling of Pleistocene ice sheets allows for an estimation of pressure and temperature fluctuations in response to glacial–interglacial cycles.

The transition from the continent to the ocean: a deeper view on the Norwegian margin

We present a regional, crustal-scale, 3D structural model of the Norwegian continental margin integrating sedimentary and crustal layers from the continental and the oceanic domain. The model includes six sedimentary units, underlain on the continental side by a thinned crystalline crust and a lower-crustal high-velocity body. In the oceanic domain, three crustal layers (2AB, 3A and 3B), thickened at the continent–ocean transition (COT), are modelled below the post-breakup deposits. Two major rift phases with different rift axes (Late Jurassic–Early Cretaceous and Late Cretaceous–Early Tertiary) have caused post-Jurassic subsidence and post-depositional deformation of the pre-Cretaceous units. The modelled COT suggests that the pre-breakup rifting event was related to differential stretching focused at the outer margin and that breakup took place in a ‘base-up’ magmatic process as a continuation of underplating. For the earlier rift event, stretching was distributed over the entire margin and led to accumulation of up to 12 km of Cretaceous deposits. The large sediment thickness of the Cretaceous units requires deep-water conditions and abundant sediment supply and thus coeval offshore subsidence and onshore uplift. All layers indicate a sinistral offset along the Jan Mayen Fracture Zone and its continentward continuation.

Modelling of fractured carbonate reservoirs: outline of a novel technique via a case study from the Molasse Basin, southern Bavaria, Germany

Fluid flow in low-permeable carbonate rocks depends on the density of fractures, their interconnectivity and on the formation of fault damage zones. The present-day stress field influences the aperture hence the transmissivity of fractures whereas paleostress fields are responsible for the formation of faults and fractures. In low-permeable reservoir rocks, fault zones belong to the major targets. Before drilling, an estimate for reservoir productivity of wells drilled into the damage zone of faults is therefore required. Due to limitations in available data, a characterization of such reservoirs usually relies on the use of numerical techniques. The requirements of these mathematical models encompass a full integration of the actual fault geometry, comprising the dimension of the fault damage zone and of the fault core, and the individual population with properties of fault zones in the hanging and foot wall and the host rock. The paper presents both the technical approach to develop such a model and the property definition of heterogeneous fault zones and host rock with respect to the current stress field. The case study describes a deep geothermal reservoir in the western central Molasse Basin in southern Bavaria, Germany. Results from numerical simulations indicate that the well productivity can be enhanced along compressional fault zones if the interconnectivity of fractures is lateral caused by crossing synthetic and antithetic fractures. The model allows a deeper understanding of production tests and reservoir properties of faulted rocks.

Sensitivity of 3D thermal models to the choice of boundary conditions and thermal properties: a case study for the area of Brandenburg (NE German Basin)

Based on newly available data of both, the structural setting and thermal properties, we compare 3D thermal models for the area of Brandenburg, located in the Northeast German Basin, to assess the sensitivity of our model results. The structural complexity of the basin fill is given by the configuration of the Zechstein salt with salt diapirs and salt pillows. This special configuration is very relevant for the thermal calculations because salt has a distinctly higher thermal conductivity than other sediments. We calculate the temperature using a FEMethod to solve the steady state heat conduction equation in 3D. Based on this approach, we evaluate the sensitivity of the steady-state conductive thermal field with respect to different lithospheric configurations and to the assigned thermal properties. We compare three different thermal models: (a) a crustal-scale model including a homogeneous crust, (b) a new lithosphere-scale model including a differentiated crust and (c) a crustal-scale model with a stepwise variation of measured thermal properties. The comparison with measured temperatures from different structural locations of the basin shows a good fit to the temperature predictions for the first two models, whereas the third model is distinctly colder. This indicates that effective thermal conductivities may be different from values determined by measurements on rock samples. The results suggest that conduction is the main heat transport mechanism in the Brandenburg area.

How warm are passive continental margins? A 3-D lithosphere-scale study from the Norwegian margin

We model the three-dimensional (3-D) conductive thermal field of the Norwegian margin and evaluate its lithospheric configuration, which is consistent with two independent observables: temperature and gravity. Here, we show that knowledge of the sediment and crustal configuration of the passive continental margin provides constraints for the configuration and thermal structure of the lithosphere. We find that the thickness of the oceanic lithosphere adjacent to the stretched continental margin controls, to a large degree, the shallow conductive thermal field of the entire margin. Our results confirm estimates of lithospheric thickness from seismology but contradict estimates from cooling models, which assume an equilibrium thickness of the oceanic lithosphere of 125 km. We find that the crust is colder in the oceanic than in the continental domain, whereas this trend is reversed downward. We obtain lateral temperature differences of ~100 °C at 5 km depth across the margin that increase to ~400 °C at 50 km depth. Higher temperatures and heat flows for the oceanic lithospheric mantle compared to the continent indicate that reduced upper-mantle P-wave velocities and densities are thermally induced.

Influence of fluid flow on the regional thermal field: results from 3D numerical modelling for the area of Brandenburg (North German Basin)

We analyse the effect of fluid flow on the recent thermal field for the Brandenburg region (North German Basin) which is strongly affected by salt structures. The basin fill is modified by a thick layer of mobilized salt (Zechstein, Upper Permian) that decouples the overburden from deeper parts of the lithosphere and is responsible for thermal anomalies since salt has a distinctly higher thermal conductivity than the surrounding sediments and is impermeable to fluid flow. Numerical simulations of coupled fluid flow and heat transfer are carried out to investigate the influence of fluid flow on the shallow temperature field above the Zechstein salt, based on the finite element method. A comparison of results from conductive and coupled modelling reveals that the temperature field down to the low-permeable Triassic Muschelkalk is influenced by fluids, where the shallow low-permeable Tertiary Rupelian-clay is absent. Overall cooling is induced by forced convective forces, the depth range of which is controlled by the communication pathways between the different aquifers. Moreover, buoyancy-induced effects are found in response to temperature-dependent differences in the fluid density where forced convective forces are weak. The range of influence is controlled by the thickness and the permeability of the permeable strata above the Triassic Muschelkalk. With increasing depth, thermal conduction mainly controls the short-wavelength pattern of the temperature distribution, whereas the long-wavelength pattern results from interaction between the highly conductive crust and low-conductive sediments. Our results provide generic implications for basins affected by salt tectonics.

Characterization of main heat transport processes in the Northeast German Basin: Constraints from 3-D numerical models

To investigate and quantify main physical heat driving processes affecting the present-day subsurface thermal field, we study a complex geological setting, the Northeast German Basin (NEGB). The internal geological structure of the NEGB is characterized by the presence of a relatively thick layer of Permian Zechstein salt (up to 5000 m), which forms many salt diapirs and pillows locally reaching nearly the surface. By means of three-dimensional numerical simulations we explore the role of heat conduction, pressure, and density driven groundwater flow as well as fluid viscosity related effects. Our results suggest that the regional temperature distribution within the basin results from interactions between regional pressure forces as driven by topographic gradients and thermal diffusion locally enhanced by thermal conductivity contrasts between the different sedimentary rocks with the highly conductive salt playing a prominent role. In contrast, buoyancy forces triggered by temperature-dependent fluid density variations are demonstrated to affect only locally the internal thermal configuration. Locations, geometry, and wavelengths of convective thermal anomalies are mainly controlled by the permeability field and thickness values of the respective geological layers.

Impact of single inclined faults on the fluid flow and heat transport: results from 3-D finite element simulations

The impact of inclined faults on the hydrothermal field is assessed by adding simplified structural settings to synthetic models. This study is innovative in carrying out numerical simulations because it integrates the real 3-D nature of flow influenced by a fault in a porous medium, thereby providing a useful tool for complex geothermal modelling. The 3-D simulations for the coupled fluid flow and heat transport processes are based on the finite element method. In the model, one geological layer is dissected by a dipping fault. Sensitivity analyses are conducted to quantify the effects of the fault’s transmissivity on the fluid flow and thermal field. Different fault models are compared with a model where no fault is present to evaluate the effect of varying fault transmissivity. The results show that faults have a significant impact on the hydrothermal field. Varying either the fault zone width or the fault permeability will result in relevant differences in the pressure, velocity and temperature field. A linear relationship between fault zone width and fluid velocity is found, indicating that velocities increase with decreasing widths. The faults act as preferential pathways for advective heat transport in case of highly transmissive faults, whereas almost no fluid may be transported through poorly transmissive faults.

3D reconstruction of salt movements within the deepest post-Permian structure of the Central European Basin System - the Glueckstadt Graben

The Glueckstadt Graben is a prominent structure of the Central European Basin System, where the sedimentary patterns are extensively affected by Permian salt movements. The relations of the sedimentary patterns to salt structures have been analyzed through present-day distributions of sediments. In addition, a three-dimensional backward modelling approach has been applied to determine the original salt distribution in response to the unloading due to sequential backstripping of the stratigraphic layers. The results of the modelling reveal the thickness distribution of the Permian salt for 5 time intervals from the end of the Triassic to present day. Spatial agreement has been found between the development of the depleted zone of the Permian salt through time and the observed distribution of the maximum subsidence for the different stratigraphic units above the salt. The sedimentation centres for each time interval are always located above the zone of reduced or depleted Permian salt. In the central part of the Glueckstadt Graben, the depletion occurred already in the Triassic and perfectly correlates with the thickest Triassic. During the Jurassic, Cretaceous and Tertiary, the areas of depleted Permian salt shifted towards the basin flanks, and the same occurred with the centres of maximum sediment deposition. Thus, the results of the modelling strongly support the conclusion that salt withdrawal has played a major role during the Meso-Cenozoic evolution of the Glueckstadt Graben and that the progressive depletion of the Permian salt layer, from the central part towards the margins, created the large part of the accommodation space for sedimentation in addition to tectonic subsidence. Furthermore, our study has several important implications for salt behaviour in different tectonic settings. In general, the results of modelling indicate a good correlation between the main phases of salt movements and tectonic events in the area under consideration. During the Triassic, the first stage of diapirism in the Glueckstadt Graben occurred within the central part of the basin. Regional extension may have triggered reactive diapirism and caused the formation of the deep primary rim synclines. Once the salt structures had reached the critical size, buoyancy forces supported their continued growth until the Jurassic when extension-induced regional stresses once more affected the Glueckstadt Graben. The results of the modelling indicate very little salt activity during the late Early Cretaceous-early Late Cretaceous when the area of the Glueckstadt Graben was tectonically silent. Therefore, our study supports the concept of tectonically induced salt movements which can be interrupted during the absence of tectonic forces. Salt movements were reactivated in the marginal troughs by compressional forces during the latest Late Cretaceous-Early Cenozoic. Paleogene-Neogene salt withdrawal led to the growth of N-S oriented salt structures mainly at the margins of the basin. This phase of salt tectonics correlates temporally with almost W-E extension. This indicates a renewed change in tectonic regime after Late Cretaceous-Early Cenozoic compression.

Controls on the deep thermal field: implications from 3-D numerical simulations for the geothermal research site Groß Schönebeck

The deep thermal field in sedimentary basins can be affected by convection, conduction or both resulting from the structural inventory, physical properties of geological layers and physical processes taking place therein. For geothermal energy extraction, the controlling factors of the deep thermal field need to be understood to delineate favorable drill sites and exploitation compartments. We use geologically based 3-D finite element simulations to figure out the geologic controls on the thermal field of the geothermal research site Groß Schönebeck located in the E part of the North German Basin. Its target reservoir consists of Permian Rotliegend clastics that compose the lower part of a succession of Late Carboniferous to Cenozoic sediments, subdivided into several aquifers and aquicludes. The sedimentary succession includes a layer of mobilized Upper Permian Zechstein salt which plays a special role for the thermal field due to its high thermal conductivity. Furthermore, the salt is impermeable and due to its rheology decouples the fault systems in the suprasalt units from subsalt layers. Conductive and coupled fluid and heat transport simulations are carried out to assess the relative impact of different heat transfer mechanisms on the temperature distribution. The measured temperatures in 7 wells are used for model validation and show a better fit with models considering fluid and heat transport than with a purely conductive model. Our results suggest that advective and convective heat transport are important heat transfer processes in the suprasalt sediments. In contrast, thermal conduction mainly controls the subsalt layers. With a third simulation, we investigate the influence of a major permeable and of three impermeable faults dissecting the subsalt target reservoir and compare the results to the coupled model where no faults are integrated. The permeable fault may have a local, strong impact on the thermal, pressure and velocity fields whereas the impermeable faults only cause deviations of the pressure field.