Geology of the Ketzin Anticline

Geohistory of the Ketzin Anticline

In northern Germany, upper Permian salt formed pillows, walls, and diapirs giving rise to the deformation of its Mesozoic overburden into anticlines and synclines (Figure 1). The Ketzin site is located in the eastern part of a double anticline named Roskow-Ketzin anticline. The anticline formed above an elongated salt pillow situated at a depth of 1500-2000 m. The axis of the anticline strikes NNE-SSW, the flanks gently dip about 15°. The immediate overburden above the salt pillow is constituted by geologic formations of the Triassic (Buntsandstein, Muschelkalk, and Keuper) and the Lower Jurassic.

In Figure 2 the extent of the Roskow-Ketzin anticline is illustrated by a structure map of the depth of the seismic K2 (Keuper) reflector. This reflector picks up the "Heldburg-Gips", a 10-20 m thick gypsum or anhydrite bed forming the top of the Weser Formation. The K2 reflector is about 80 m above the top of the Stuttgart Formation, which is the target formation for the CO2 injection at Ketzin. The geology of the anticline is well known by many boreholes and seismic lines.

A first gentle uplift of the Ketzin anticline may have started in the early Triassic. The total eroded thickness amounts to about 500 m, from which a maximum burial depth for the Upper Keuper sediments of about 1200 m can be inferred. The Upper Cretaceous probably was never deposited in the area, which at that time was part of a structural high. Sediments of the Oligocene (Rupelton) form the first formation unaffected by anticlinal uplift resting above Jurassic sediments. These transgressive sediments are the first indicators of regional downwarping of the central parts of the Northeast German Basin that lasts until present.

The structural outline of the Ketzin anticline is shown on two structure maps (Fig. 3). A typical feature of the subhorizontal base of the Quaternary (Fig. 3a) are two glacial erosional troughs incised down to the Upper Jurassic and filled with Quaternary sediments. The structure map of the Stuttgart Formation (Fig. 3b) resembles that of the K2 horizon (Fig. 2).

Lithology and Facies of the Triassic Middle Keuper

The Middle Keuper sediments in Ketzin are mainly of continental playa type consisting of fine-grained clastics (clay/silt) that alternate with lacustrine sediments (carbonates), evaporites, and shallow marine carbonates. The playa-type sedimentation starting with the Grabfeld Formation (Unterer Gipskeuper) (Fig. 4) was interrupted by the fluvial, incised-valley deposition of sandstone of the Stuttgart Formation, but continued in the Weser (Oberer Gipskeuper) and Arnstadt (Steinmergelkeuper) formations, and finally ceased with the deposition of the shallow marine Exter Formation (Rhätkeuper) sandstones. The incised-valley sandstones of the Stuttgart Formation (formerly termed Schilfsandstein) occur basin-wide and consist of shoe-string-like sandstone bodies that demonstrate S-W-oriented paleocurrents with transport from northern and eastern Europe across the German Keuper basin.

The Reservoir and Caprock System at Ketzin

The playa-type rocks of the Weser and Arnstadt formations form an approximately 210 m thick caprock section above the Stuttgart Formation (Fig. 4) . This section consists mainly of claystone, silty claystone, and anhydrite. The caprocks contain a high fraction (>50 weight %) of clay minerals and some fraction of quartz and dolomite. Clayey, fine-grained sandstones intercalated with claystones exhibit smaller clay-mineral contents but higher contents of quartz and no dolomite. They also show higher contents in feldspar than claystones. From these studies and pore-space imaging it becomes evident that the caprocks exhibit an excellent sealing performance.

Farther up in the sequence, the caprock comprised of mudstone, siltstone, and anhydrite is interlayered with several sandstone units (Fig. 4), which form a multi-aquifer system. The Jurassic sandstones of this system were used by the VNG AG for industrial storage of gas until 2000. The caprock of the gas storage is a Tertiary clay (the Rupelton), about 80-90 m thick.

The CO2SINK storage formation, the Stuttgart Formation, is on average 80 m thick and lithologically heterogeneous: s andy channel-(string)-facies rocks of good reservoir properties alternate with muddy, flood-plain facies rocks of poor reservoir quality. The thickness of the sandstone interval may attain several tens of meters where subchannels are stacked (Fig. 5).

The sandstones of the Stuttgart Formation consist of varying amounts of quartz, feldspar, and rock fragments, classifying them as graywacke. They are fine-grained to medium-grained, well-sorted, and weakly cemented by silicates and clay as well as sometimes also by anhydrite. The sandstones have an average porosity of 23% (weighted mean from log data of the Ktzi 163 borehole in the Ketzin anticline). The permeability determined in this well during hydraulic well tests ranged between 500 and 1000 mD. The reservoir temperature was 33-36°C at a depth between 600-700 m. This is also the depth interval in which the reservoir sandstones are expected at the site of the CO2SINK boreholes. Formation pressure at 700 m depth is determined to range between 70 and 75 bar.

Hydrogeology of the Ketzin Anticline

The Aquifer-Aquitard System

The Mesozoic succession in the larger Ketzin area hosts several sandstone aquifers. The sandstones of the Exter Formation (Fig. 4) comprise the first major aquifer (about 20 m thick) above the Stuttgart Formation and the sandstone layers of the Jurassic a second major aquifer system, respectively. The Tertiary clay (the Rupelton) acts as a major aquitard separating the saline waters (brines) in the deeper aquifers from the non-saline groundwater in the shallow Quaternary aquifers. At the western and eastern flanks of the Ketzin anticline, two glacial, NNE-SSW oriented erosional troughs are incised into the Tertiary clay aquitard and into the Jurassic. These troughs, the "Nauen-Havelländische Rinne" to the west and the "Falkensee-Oranienburger Rinne" to the east (Fig. 3a), are filled with Quaternary sediments. The local erosion of the Rupelton aquitard may allow saline waters to ascend and mix with fresh water in shallow aquifers. The lower part of the Quaternary sedimentary infill consists of two sand aquifers which are hydraulically connected with the Jurassic aquifer system and are separated from each other by clayey and silty units. The upper part of the trough fill predominantly comprises silty and clayey sediments mostly prohibiting an upward migration of saline water.

The main fresh water aquifer in the Ketzin area is Quaternary sand (about 30 m thick) that is widely distributed overlying the clayey trough sediments. This sand unit is covered by Quaternary till units acting as aquitards (Fig. 6). Locally, above these till units lies another, unconfined sand aquifer exhibiting a maximum thickness of about 12 m. The general flow of the groundwater in the Quaternary aquifers is approximately from the north to the south following the drainage towards the Havel river. The groundwater flow rate is between 30 and 240 m.

Groundwater Composition

Groundwater in the Ketzin area typically shows a distinct vertical zoning with respect to total mineralization and composition (Fig. 7). The content of total dissolved solids increases with depth step-wise, from 0.2-1.4 g/l in Quaternary formations and 47-50 g/l in Jurassic formations to 250-321 g/l in Triassic formations. The typical Quaternary groundwater, however, shows a mineralization in the order of 0.3 g/l. The waters change in composition from hydrogen carbonate-dominated waters of Ca(HCO3)2 -type in the shallow Quaternary aquifers to saline waters of NaCl-type in the deeper Jurassic and Triassic formations. In the areas of glacial incision troughs, a mixing of fresh water from shallow aquifers outside the trough with deeper water is likely. A detailed study of the complex Quaternary aquifer system and its hydraulic connection to the deeper aquifer system is part of the ongoing work of CO2 SINK. It comprises the modelling of the shallow groundwater flow as part of the baseline geology (WP 2.1).

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