Conference Agenda

Deep geothermal energy is a key to lower local and global CO₂ emissions caused by the burning of fossil fuels. Different initiatives aim at establishing deep geothermal energy production at the Weisweiler coal-fired power plant near the city of Aachen in order to replace district heat generated as a side product of coal burning^1,2. But how much information do we actually have about the subsurface to carry out such a project?

The conducted investigations will provide a 3D geological and probabilistic subsurface model created with the open-source package GemPy³ developed at RWTH Aachen University. This model is in contrast to established models by [4] and [5], discussed by [6] and constructed in more detail for the Weisweiler area by [7].

The geological structures between Aachen and Weisweiler represent a SW-NE striking syncline, the Inde Syncline, embedded in the Aachen fold-and-thrust belt (AFTB)⁸. The syncline is offset by Cenozoic normal faults of the Lower Rhine Embayment (LRE)⁹. The target layers comprise of karstic Lower Carboniferous Kohlenkalk platforms and Upper/Middle Devonian Massenkalk reef carbonates outcropping along the flanks and down faulted within the LRE¹⁰.

Results show that the AFTB and the down faulted fault blocks can be modeled integrating the available surface and shallow subsurface data (Fig. 1 + 2). The probabilistic modeling provides information about uncertainties of the target layers in the subsurface. It can be shown that a planned exploration well will reduce uncertainties in the subsurface in the vicinity of the target layers enabling improved economic decisions.

Modelling of subsurface temperatures is based on various approaches, which typically focus on interpolation or numerical simulation procedures. Mostly, they are based on few single measurements and do not take into account the geological structure or lithological and thermal conductivity conditions of the subsurface.

Herein, we present a new approach for the parameterization and modelling of subsurface temperatures for areas with sparse available data. We used temperature log measurements from 26 spatially distributed boreholes drilled by oil and gas exploration companies. Corrected temperatures were transferred to each (stratigraphic) layer in the borehole, which were correlated with the structural model's stratigraphy. Afterwards, mean temperature gradients could be derived for each model formation. Statistical analysis of derived gradients shows the dependence from a) the dominant lithology of a formation (e.g. salt and clay) corresponding with high differences in the thermal conductivities and (b) the total thickness of the evaporitic (dominantly salt) Zechstein formation. We noticed higher gradients (R² = 0.69) and thus a heat accumulation effect in supra-salt layers above salt pillows and diapirs and an exponential gradient decrease with increasing depth (R² = 0.76) (heat extraction) within the Zechstein formation. Both effects are caused by the high thermal conductivity of the evaporites.

These significant correlations allowed for an empirical adjustment of temperature gradients, which were the basis for calculating subsurface temperatures. For that purpose temperature gradients were applied to a 3d structural SKUA-GOCAD model of northern Saxony-Anhalt comprising 31 stratigraphic horizons from the Late Permian (Zechstein) to the Holocene. We calculated temperatures for each voxel of the volume model, starting at a depth of 13 meters, where temperatures are unaffected by seasonal variations and can be derived from weather data. The effects caused by salt structures and variable thermal conductivities are well represented in the resulting model. In further course of the project, methods for the consideration of tectonic faults and deep aquifers have to be developed. Furthermore other important parameters like porosity or clay fraction will be integrated in our correlations.

Even though the Late Jurassic Arab-D reservoir is the most prolific oil-producing and one of the most studied intervals in the world, it is a major challenge resolving sub-seismic interwell-scale heterogeneities. In the subsurface, in addition to depositional heterogeneities, diagenetic alteration of depositional fabric developed five distinct dolomite types. They vary highly in reservoir properties ranging from flow baffle to Super K further adding complexity to reservoir architecture. Earlier studies focused on petrographical, geochemical and petrophysical analysis to understand genesis and reservoir properties of these dolomites using well log and core data. But very little is known about the geometry, connectivity and inter-well scale heterogeneity of these dolomites. Understanding of these parameters is crucial for development of a realistic subsurface reservoir model and fluid flow heterogeneity scenarios. Outcrop analogues may help in getting insights about these parameters, however, earlier studies reported little about dolomite and dolomite body geometries in outcrops.

The present study introduces a new outcrop analogue for the Arab-D reservoir which is located approximately 100 km North of Riyadh. The outcrop is unique as it contains multiple layers of stratiform dolomites with facies succession similar to subsurface Arab-D reservoir. In addition to the traditional methods (sedimentary log, sample collecting, thin-section petrography), we have also collected drone based photogrammetry (1x1 km²), spectral gamma ray logs (~100 m) and ground penetrating radar (~2 km). The preliminary studies focus on outcrop to subsurface correlation and characterization of dolomite layers to provide insight about subsurface reservoir architecture and associated fluid flow heterogeneities.

Interwell meter-scale depositional heterogeneity is a key geologic factor behind un-even fluid advance in carbonate reservoirs undergoing waterflooding and enhanced oil recovery developments. Subsurface data (seismic and well data) are limited in both resolution and data density to resolve this level of heterogeneity. Outcrop analogues are commonly utilized to fill this data gap on interwell heterogeneity. However, most outcrop analogue studies are limited to vertical sections and two-dimensional interpolated profiles. This study builds from outcrops high-fidelity 3D depositional models using 3D drone photogrammetry, cores and 3D geophysical data.

We applied our methodology to the top 40 m of the Late-Jurassic Hanifa Formation outcrop in Wadi Birk, Saudi Arabia. Datasets include a multi-resolution 16 km² drone photogrammetry survey over criss-crossing wadi systems, measured vertical sections from 10 different locations including spectral gamma-ray, 160 hand samples and thin sections, three 50 m-long cores taken from behind the outcrops, a 19 km-long network of 2D and 3D Ground Penetrating Radar, and 1.9 km of 2D seismic.

The overall cleaning upward succession contains shallow marine wackestones/packstones at the bottom, followed by stromatoporoid/coral complex (with m-scale lateral and vertical heterogeneity) that is capped by oncolithic and cross-bedded peloidal grainstones at the top. The stromatoporoid/coral buildups display biostromal/biohermal morphology with inter-buildup space filled by wackestones/floatstones/grainstones. The smaller stromatoporoid/coral complexes (< 10 m) are more circular, while the larger are pseudo-ellipsoid (30-50 m long, 5-10 m wide), indicating preferential alignment with local/regional currents.

The primary reservoirs present in the Southern Chotts Basin, Central Tunisia, are located within Triassic-, Permian- and Ordovician units. They are mainly sourced by the Silurian - Lower Devonian Fegaguira Fm. and its Hot Shale member. The Hercynian Orogeny has reworked the Palaeozoic package, removing the Devonian - Carboniferous and most of the Permian deposits in the Southern Chotts Basin. This resulted in a diachronous unconformity in the present-day stratigraphy.

Constraining estimated sediment deposition and subsequent erosion at unconformities is crucial to predict timing of source rock maturation. This study aims to qualify and quantify the effects of several simplified burial- and thermal histories on the timing of source rock maturation, understand migration pathways to predict sweet spots and gain insight in Mesozoic burial.

Thermal history calibration was performed and allowed modelling source rock maturation in the basin's kitchen area. Hydrocarbon generation occurs in two phases separated by a phase of stable maturity during Hercynian exhumation. Pre-Hercynian generation is unlikely to result in preserved accumulations in post-Hercynian traps. Renewed maturation in Jurassic - Cretaceous times likely sources present-day hydrocarbon accumulations in post-Hercynian traps. The number of phases- and maximum burial depth associated with the Hercynian Orogeny determine the amount of missed pre-Hercynian hydrocarbons and the timing of renewed maturation in the Mesozoic.

Sensitivity analysis on estimated Hercynian erosion further constrained associated maximum burial and -hydrocarbon generation. 2D basin modelling captures migration pathways and helped predicting sweet spots. Subsidence analysis shows Mesozoic subsidence occurs as a result of pulsed rifting with possible short-lived uplift phases, indicating the diachronous nature of tectonics in the Southern Chotts Basin.