Sebastian Homuth , Annette E. Götz , Ingo Sass

Lithofacies related thermo-physical characterization of the
Upper Jurassic geothermal carbonate reservoirs
of the Molasse Basin, Germany
1
2
1
Sebastian Homuth , Annette E. Götz , Ingo Sass
1
Institute of Applied Geosciences, Department of Geothermal Science and Technology, TU Darmstadt, Germany
2
Rhodes University, Department of Geology, Grahamstown, South Africa
Introduction
Material and Methods
In the early stages of hydrothermal reservoir exploration, the
thermo-physical characterization of the reservoir is accomplished
by evaluating drilling data and seismic surveys. Especially in
carbonate reservoirs the distinction of different facies zones or
heterogeneities in general is very complex and often simply not
possible. For reservoir predictions, geothermal parameters such as
permeability, thermal conductivity/diffusivity, specific heat capacity
and reservoir heat flow have to be quantified. These thermophysical parameters show facies-related trends. Therefore,
applying a thermofacies classification to the Upper Jurassic
limestones is helpful to understand the heterogeneities and to
identify production zones. In addition, for economic reasons a
sufficient high flow rate to the production well is necessary. This flow
rate is mainly controlled by the reservoir permeability. To
characterize those fracture-controlled reservoirs the orientation of
fractures, fracture width, surface roughness of fractures as well as
the connectivity of fractures and possible secondary mineralization
in the fracture system is important. These analyses enable to
assess the natural or due to stimulation measures artificially
generated possible reservoir permeability in advance of drilling
operations.
The outcrops of the Swabian and Franconian Alb (Fig. 1) represent
the target formations of Upper Jurassic carbonate reservoirs in the
adjacent Molasse Basin (Fig. 3). These limestone formations
contain the main flow paths through tectonic elements and
characteristic of limestone formations also through karstification.
The type and grade of karstification is also facies related (Fig. 8).
Finally, the correlation of distinct sedimentary facies and their
thermo-physical parameters may contribute to establish integrated
structural 3D reservoir models.
Fig. 1: Study area with location of
studied outcrops and drillings in
the Swabian and Franconian Alb
north of the Molasse Basin.
Fig. 2: Scales of Investigation.
Fig. 3: Cross section of the
western Molasse Basin (modified,
Clauser et al. 2002).
Fig. 4: Palaeogeography of the Upper
Jurassic in central Europe with study
area (modified, based on Meyer and
Schmidt-Kaler 1990).
Outcrop analogue studies enable the determination and correlation
of thermo-physical parameters and structural geology data with
regional facies patterns. An outcrop analogue investigation
examines the same rock formations (stratigraphy, lithology, facies;
Fig. 4) as the potential reservoir formations from which fluids at
according depth are discharged. Outcrop analogue studies of the
target formation Upper Jurassic, which is the most promising
formation for deep geothermal projects in the German Molasse
Basin, have to include facies studies following a thermofacies
concept (Sass & Götz, 2012). Seventeen outcrops located in the
Swabian and Franconian Alb and three research drillings (see Fig. 1)
were investigated to assess the whole accessible stratigraphic
section spanning Malm α to Malm ζ3 strata. The upper Malm ζ4-6 is
almost completely eroded and only known from a few deep drillings
in the Molasse Basin. According to the Dunham classification of
carbonate rocks the following facies types are detected in the study
area: mudstones, wackestones, grain-/packstones and rud/floatstones (Fig. 7).
The investigations are carried out on three different scales (Fig. 2):
(1) The macro scale including an outcrop mapping to detect the
lithotypes, structural elements and facies patterns in the outcrop; (2)
the meso scale, to determine thermo-physical properties of different
lithotypes in the laboratory; and (3) the micro scale, to analyze
microstructures, cements, porosities, etc. in thin sections.
To determine the thermo-physical properties of the sampled
formations a thermal conductivity scanner (optical scanning method
after Popov et al. 1985) and a air driven permeameter (Jaritz 1999)
and porosimeter are used. For direct correlation in this study all
parameters are determined at the same sample.
d
[a] Rud-/Floatstone
a
b
[b] Grainstone
c
Fig. 4: Example of an outcrop analogue study (open pit Kinding, Franconian Alb); a: thick-bedded
limestone, b: micritic limestone, c: marly limestones, d: dolomitic limestone; Abbreviations used: cr.=
crussaliensis Marls, pl.= platynota Marls
Fig. 6: top left: A stratigraphic trend of increasing thermal conductivity is detected within
the Malm from Malm alpha to Malm zeta; top right: Stratigraphic porosity trend with
inferred hydraulic conductivity conditions; bottom left: Porosity/permeability relation for
different lithotypes (only mean values are displayed); bottom right: Grain desity distribution
by rock type.
[c] Mudstone
[d] Wackestone
Fig. 5: Schematic sketch of the marine accumulation area of carbonates of the Purbeck and Malm and
corresponding thermo- and petrophysical matrix properties and intensity of dolomitization; estimation of
dolomitization intensity is based on Wolfgramm et al. (2011); values marked with * of the Purbeck are
from Koch et al. (2007, 2009); TC: Thermal Conductivity, cp: specific heat capacity, n.d.: no data.
Fig. 8: left: Karstification along fault in thick-bedded and platy limestones (Solnhofen);
right: karstification on larger scale (karst cavities) in reefal limestone (Hülen).
Fig. 7: Thermofacies classification of Upper Jurassic limestones of southern Germany based on the
international Dunham classification and the regional nomenclature introduced by Pawellek (2001)
with according rock samples.
Results and Outlook
A high variation of thermo-physical parameters is recognized within one facies zone or stratigraphic unit (Fig. 5 & Fig. 6); variations even occur within one outcrop. However,
general trends indicate that the hydraulic flow patterns are related to tectonically created weak zones in the formation and that the matrix permeability has only a minor effect on
the reservoir's sustainability. The matrix permeability of all measured carbonates is quite low except for some grainstones with higher permeabilities and porosities. Mud- and
wackestones show thermal conductivities around 2 W/mK, characteristic of limestones. Permeabilities range from 0.001 to 100 mD (Fig. 5 & Fig. 6). Mudstones have lower
thermal conductivities than wackestones. The permeabilitiy range of mud- and wackestones is about the same. The thermal conductivities of the rudstones show values of 1.8
to 3.8 W/mK. Reefal structures show the highest values of thermal conductivity, due to the higher content of secondary mineralized silicates. The comparatively high thermal
conductivity corresponds to low permeability. The parameters are determined on oven dried samples. These values have to be corrected for water saturated rocks under the
according temperature and pressure conditions. These calculated parameters can be validated in a Thermo-Triax-Cell simulating the existing temperature and pressure
conditions in the reservoir. Furthermore the cell induces a pore pressure on the rock sample and measures the permeabilty. Based on the investigation of the matrix parameters
the sustainable heat transport into the geothermal reservoir can be assessed. Thus, the long term capacities for different utilization scenarios can be calculated more precisely.
Investigations on the lateral extension and facies heterogeneity will give insight on the transmissibility of different target horizons. The facies related characterization and
prediction of reservoir formations is a powerful tool for the design, operation, extension and quality management of geothermal reservoirs. Facies related petro-physical data
can be used for detailed numerical simulations of geothermal carbonate reservoirs.
References
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seiner Eignung zur Endlagerung radioaktiver Stoffe. – Endbericht zum Auftrag 9X0009-8390-0 des Bundesamtes für Strahlenschutz (BfS), Applied Geophysics and Geothermal Energy E.ON Energy Research Center, 159 S., RWTH Aachen (2002).
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Pawellek, T. (2001): Fazies-, Sequenz- und Gamma-Ray-Analyse im höheren Malm der Schwäbischen Alb (SW-Deutschland) mit Bemerkungen zur Rohstoffgeologie (Hochreine Kalke). Tübinger geowissenschaftliche Arbeiten, Reihe A, 61.
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Sass, I. & Götz, A.E. (2012): Geothermal reservoir characterization: a thermofacies concept. Terra Nova, 24, 142-147
Wolfgramm, M., Dussel, M., Koch, R., Lüschen, E., Schulz, R., Thomas, R.,: Identifikation und Charakterisierung der Zuflusszonen im Malm des Molassebeckens nach petrographisch-faziellen und geophysikalischen Daten. – Proceedings, Der
Geothermiekongress 2011, Bochum, (2011).
Contact:
Dipl.-Ing. Sebastian Homuth, M.Sc.
Technische Universität Darmstadt
Institute of Applied Geosciences
Chair of Geothermal Science and Technology
Schnittspahnstrasse 9
D-64287 Darmstadt
Germany
email: [email protected]

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