Lithofacies and depth dependency of thermo- and petrophysical rock

parameters of the Upper Jurassic geothermal carbonate reservoirs of the

Molasse Basin

Lithofazies- und Teufenabhängigkeit thermo- und petrophysikalischer

Gesteinskennwerte der oberjurassischen geothermischen

Karbonatreservoire im Molassebecken

Dipl.-Ing. & M.Sc. Sebastian Homuth1, Prof. Dr. Annette E. Götz1,2, Prof. Dr. Ingo Sass1

1Technische Universität Darmstadt, Institute of Applied Geosciences, Geothermal Science and Technology,

Schnittspahnstr. 9, 64287 Darmstadt, homuth@geo.tu-darmstadt.de, sass@geo.tu-darmstadt.de

2co Rhodes University, Geology Department, Sedimentology, P.O. Box 94 Grahamstown 6140, South Africa,

a.gotz@ru.ac.za

Keywords: Reservoir characterization, outcrop analogue study, Upper Jurassic, Molasse Basin,

thermo-physical parameters, thermofacies, geothermal energy

Schlüsselwörter: Reservoircharakterisierung, Aufschlussanalogstudie, Oberer Jura, Molassebecken,

thermophysikalische Kennwerte, Thermofazies, Geothermie

Abstract

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 and heterogeneities in general is very complex. For

economic reasons a sufficiently high flow rate toward the production well and an according high fluid

temperature is necessary. For reservoir predictions and modelling, geothermal parameters such as

permeability, thermal conductivity/diffusivity, and specific heat capacity have to be quantified. The

thermo-physical parameters are facies related. The application of a thermofacies classification to

Upper Jurassic limestones serves to understand the heterogeneities and to identify production zones.

Outcrop analogue studies enable the determination and correlation of facies related thermo-physical

parameters and structural geology data and thus the geothermal exploration concept becomes more

precise and quantitative. The analogue outcrops of the Swabian and Franconian Alb represent the

target formations of Upper Jurassic carbonate reservoirs in the adjacent Molasse Basin. These

limestone formations contain the main flow paths through fractures, faults, and characteristic of

limestone formations also through karstification. The type and grade of karstification is also facies

related. In general the matrix permeability has only a minor effect on the reservoir’s sustainability

except for some grainstones and dolomitized zones with higher porosities and permeabilities.

Permeabilities range from 10-18 to 10-13 m² (0.001 mD to 100 mD). The permeability range of mud-

and wackestones is about the same. A high variation of thermo-physical parameters is recognized

within individual facies zones or stratigraphic units. Mud- and wackestones show thermal

conductivities around 2 W/(mK), while mudstones have lower thermal conductivities than

wackestones. The thermal conductivities of massive reefal limestones show values of 1.8 to 3.9

W/(mK). Secondarily silicified reefal limestones and dolomites show the highest values of thermal

conductivity. These parameters determined on oven dried samples have to be corrected for water

saturated rocks under the according temperature and pressure conditions using transfer models. A

comparison of calculated reservoir properties with measurements from deep drill cores confirms a

good correlation. Based on the investigation of the matrix parameters in combination with reservoir

transfer models, the reservoir prognosis and numerical simulation can be improved. The facies related

characterization and prediction of reservoir formations is a powerful tool for the exploration,

operation, extension and quality management of geothermal reservoirs in the Molasse Basin.

Kurzfassung

In der Planungsphase einer hydrothermalen Reservoirerkundung erfolgt die thermophysikalische

Charakterisierung des Reservoirs durch die Auswertung von Bohrungsdaten und seismischen

Erkundungen. Im Falle von Karbonatreservoiren ist die Differenzierung von verschiedenen

Faziesbereichen bzw. Heterogenitäten im Allgemeinen sehr komplex. Aus wirtschaftlichen Gründen

ist eine hohe Förderrate der Produktionsbohrung und eine entsprechend hohe Fluidtemperatur

notwendig. Für eine Reservoirprognose und -modellierung müssen die geothermischen Kennwerte wie

Permeabilität, Wärmeleitfähigkeit, Temperaturleitfähigkeit und spezifische Wärmekapazität im

Reservoir quantifiziert werden. Diese thermo- und petrophysikalischen Kennwerte sind

faziesabhängig. Die Anwendung einer thermofaziellen Klassifikation auf die Karbonate des Oberen

Jura dient hierbei zum besseren Verständnis der Heterogenitäten und zur Identifikation von

Produktionszonen. Aufschlussanalogstudien ermöglichen hierbei die Bestimmung faziesabhängiger

thermophysikalischer Kennwerte und deren Korrelation mit strukturgeologischen Daten, wodurch die

geothermische Erkundung präziser und quantitativer wird. Die analogen Aufschlüsse der

Schwäbischen und Fränkischen Alb repräsentieren die Zielformation der oberjurassischen

Karbonatreservoire des angrenzenden Molassebeckens. Die Hauptzuflüsse in diesen

Kalksteinformationen erfolgen durch Klüfte, Störungen und charakteristisch für Kalksteine durch

Karststrukturen. Der Typ und der Grad der Verkarstung ist ebenfalls faziesabhängig. Prinzipiell hat

die Gesteinspermeabilität sämtlicher untersuchter Karbonate aber nur einen geringen Anteil an der

hydraulischen Reservoirnachhaltigkeit mit Ausnahme einiger Grainstones und dolomitisierter

Bereiche, die höhere Porositäten und Permeabilitäten zeigen. Die Permeabilitäten reichen von 10-18 bis

10-13 m² (0.001 mD bis 100 mD). Es wurde eine hohe Variabilität der thermophysikalischen

Kennwerte innerhalb verschiedener Faziesbereiche bzw. stratigraphischer Einheiten festgestellt. Mud-

und Wackestones zeigen Wärmeleitfähigkeiten um die 2 W/(mK), wobei Mudstones eine geringere

Wärmeleitfähigkeit als Wackestones bei gleichem Permeabilitätsbereich besitzen. Die

Wärmeleitfähigkeit der Massenkalke liegt zwischen 1,8 und 3,9 W/(mK). Sekundär silifizierte

Riffzonen und Dolomite weisen die höchste Wärmeleitfähigkeit auf. Die Kennwerte werden an

ofengetrockneten Gesteinsproben bestimmt und müssen für entsprechende wassergesättigte

Reservoirbedingungen korrigiert werden. Dies erfolgt mittels verschiedener Transfermodelle. Ein

Vergleich von prognostizierten Reservoirkennwerten mit gemessenen Kennwerten an Kernen aus

Tiefbohrungen zeigt eine gute Übereinstimmung der Prognosewerte mit den tatsächlichen

Bedingungen im Reservoir. Basierend auf der Untersuchung der Matrixkennwerte in Verbindung mit

Reservoirtransfermodellen kann sowohl die Reservoirprognose als auch die Grundlage für numerische

Simulationen verbessert werden. Die faziesbezogene Charakterisierung und Prognose einzelner

Reservoirhorizonte ist somit ein wirkungsvolles Instrument für die Erkundung, den Betrieb, die

Erweiterung und Qualitätsüberwachung geothermischer Reservoire im Molassebecken.

1. Introduction

To assess the potential and productivity of a hydrothermal and petrothermal reservoir a comprehensive

and detailed knowledge of the thermo-physical, the hydraulic and mechanical rock and formation

properties of the system is mandatory. In general, the determination of these reservoir properties is

limited to time and cost-consuming exploration drillings, which reflect only a small portion of the

whole reservoir. Especially in terms of carbonates it is difficult to evaluate the heterogeneity of

different facies zones in seismic sections (Chilingarian et al. 1992), which applies to the Jurassic

(Malm) target formation of multiple planned geothermal power plant projects in the southern German

Molasse Basin. These carbonates, characterized by a karst-fractured aquifer system (Schulz et al.

2012), are located in 3,500 to 5,500 m below surface in the southern part of the Molasse Basin and

have, like in the Southern German Scarplands, a typical dip of 5° in south-east direction which results

in an outcrop area north of the Basin (Clauser et al. 2002, Fig. 2). The outcrop analogues of the

Swabian and Franconian Alb (Fig. 1) represent the reservoir formations and can be used for detailed

facies and thermo- and petrophysical investigations on a low cost basis. The integrated analysis of

lithology, facies, and corresponding thermo- and petrophysical rock properties as well as the

application of relevant reservoir transfer models, leads to an improved prognosis of the reservoir

properties and conditions. The planning and the geothermal exploration phase of multiple planned

combined heat and power (CHP) projects in the Molasse Basin can therefore be more cost effective

and less time consuming. An outcrop analogue study of the target formation Malm (Upper Jurassic)

which is the most promising formation for deep geothermal projects in the German Molasse Basin has

to include facies studies following a thermofacies concept (Sass & Götz 2012).

Outcrop analogue studies enable the determination and correlation of thermo- and petrophysical

parameters as well as structural geology data with regional facies patterns. An outcrop analogue

investigation examines the same rock formations (stratigraphy, lithology, facies) as the potential

reservoir formations from which fluids at according depth are discharged (Philipp et al. 2007). For the

economic utilization of deep geothermal reservoirs, a sufficiently high flow rate of thermal waters

throughout the reservoir to the production well is necessary. This flow rate is mainly controlled by the

reservoir permeability (Nelson 1985). In the Molasse Basin, the limestone formations of the Upper

Jurassic contain the main flow paths through tectonic elements such as faults, joints and fractures, and

characteristic of limestone formations, through karst structures.

To characterize such fractured 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 the assessment of natural or due to stimulation

artificially generated reservoir permeability prior to drilling operations. To forecast geometries and

further properties of existing fracture systems in three dimensions for geothermal appropriate depths,

the resolution of seismic analyses is not high enough (Philipp et al. 2007). However, in most cases the

main faults or structural trends, which are generally detectable with seismic surveys, control the

direction of the fractures. Furthermore the density of fractures increasing towards faults. Therefore, if

the data quality is sufficient enough this can be indirectly used to model the fracture direction and

estimate the density of fractures. Also drill cores from exploration drillings are often not available or

give only a limited set of information. Therefore, it is necessary to obtain thermo-physical and

structural geological data from outcrops, a well-established exploration tool in oil and gas industry

(Jahn et al. 2008). Finally, the correlation and the spatio-temporal development of distinct sedimentary

facies and their thermo-physical parameters may contribute to establish integrated structural 3D

reservoir models. The thermofacies concept was developed for low and mid enthalpy geothermal

systems linked to graben settings and deep sedimentary basins located in Central Europe (Sass & Götz

2012).

To consider all facies types of the Upper Jurassic in the subsurface of the Molasse Basin, eighteen

outcrops located in the Swabian and Franconian Alb were investigated to assess the whole accessible

stratigraphic section of the Upper Jurassic (Malm) spanning Malm α to Malm ζ3 strata (Fig. 1, Fig. 4).

The upper Malm ζ4-6 is almost completely eroded and only known from a few deep drillings in the

Molasse Basin. Drill cores of two shallow and one deep drilling (Moosburg SC4) were investigated to

cover the uppermost strata of the Malm formation. The aim is to identify lithofacies-related trends of

thermo- and petrophysical reservoir parameters. The results and inferred predictions from the

laboratory testing can be validated with drilling and pump test data from recent drill sites.

This paper gives a brief summary of the regional geological setting of the study area, including the

main facies zones and their diagenesis. Subsequently the outcrop studies and laboratory methods for

the determination of rock properties are introduced. Next, the thermofacies concept and facies-related

rock properties are presented. Then, the results and their transferability to reservoir conditions are

discussed by evaluating data from deep drill cores. In addition facies-controlled karstification

structures and the tectonic setting is presented and discussed. Finally, the application and access of

data is summarized.

2. Geological Setting

During the Mesozoic the study area was part of a broad epicontinental sea north of the Tethys Ocean

(Fig. 3). To the north the shelf area was confined by an island archipelago of changing dimensions. In

the southern, deeper part of this epicontinental sea, an extensive siliceous sponge-microbial reef belt

developed. With the burial of the Vindelician Ridge a direct connection of the south German Jurassic

Sea with the Tethys Ocean was established (Meyer & Schmidt-Kaler 1989). During the entire Upper

Jurassic a high carbonate production on the shallow shelf resulted in thick limestone series (Selg &

Wagenplast 1990). In the southern, deeper part of this epicontinental sea, a reefal facies, established in

the Middle Oxfordian, and was part of a facies belt characterized by silicious sponge reefs spanning

the northern Tethys shelf (Pieńkowski et al. 2008).

The main reef building organisms are microbial crusts, however, coral reefs are also present and

becoming increasingly important towards the end of the Upper Jurassic, mirroring the overall

shallowing sea level (Pieńkowski et al. 2008). In addition, clay-rich sediments from the Mid German

Swell were shed into the shelf area. During times of low carbonate production, the clay content of the

sediments increased which resulted in the sedimentation of marl (Meyer & Schmidt-Kaler 1990).

According to Meyer and Schmidt-Kaler (1989, 1990), the Swabian facies as the central part of the reef

belt formed a deeper-water area between the shallower Franconian-Southern Bavarian platform in the

East and the Swiss platform in the West. In the Southwest, the Swabian shelf facies deepens gradually

towards the pelagic facies of the Helvetian Basin. The Helvetic facies is characterized by dense,

bituminous limestones with in places interbedded oolith layers. This facies describes the transition of

the Germanic facies into the Helvetic facies, which is considered as sediments of a deeper shelf area of

bedded limestones with very low permeabilties. Also karstification is not observed, so the northern

boundary of the Helvetic facies is considered as the southern boundary of the Malm aquifer of the

Molasse Basin (Villinger 1988).

In general 400 to 600 m of carbonate rocks were deposited during the Upper Jurassic and two major

lithofacies-types can be distinguished (Geyer and Gwinner 1979, Pawellek & Aigner 2003):

(1) a basin facies, consisting of well-bedded limestones and calcareous marls (mud-/wackestones), and

(2) a reefal or massive facies, when bedding is either absent, indistinct or very irregular (rud-/float-

/grainstones).

The massive limestones are built by microbial crusts (stromatolites and thrombolites) and siliceous

sponges that have been interpreted by various authors as relatively deep and quiet water ‘‘reefs’’,

mounds or bioherms (Gwinner 1976, Leinfelder et al. 1994, 1996, Pawellek & Aigner 2003). The

basin facies may either interfinger with the reefs or onlap onto the reefs (Gwinner 1976; Pawellek

2001). In the upper parts of the Upper Jurassic, a coral facies developed locally upon the microbial

crust-sponge reefs. The abundance of reef facies differs regularly through time. Reef expansion phases

correlate with an increase in the carbonate content within the basin facies, while phases of reef retreat

correlate with increasing abundance of marls within the basin facies (Meyer and Schmidt-Kaler 1989,

1990, Pawellek 2001). A comparison of the different lithostratigraphic units in the Franconian and

Swabian Alb is given in Figure 4.

3. Materials and Methods

The investigations are carried out on three different scales: (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- and petrophysical properties of different lithotypes in the laboratory of

representative rock samples; and (3) the micro scale, to analyze microstructures, cements, porosities,

etc. in thin sections. This approach is based on the concept of Aigner (1985) and was applied to the

Upper Jurassic by Schauer (1998), Aigner & Schauer (1998) and Pawellek & Aigner (2004).

The rock samples were classified in different facies groups after the scheme developed by Pawellek

(2001). The following simplifications have to be considered: Pawellek (2001) differentiates within the

massive reefal limestones crust-dominated and sponge-dominated rocks. This detailed differentiation

is for the thermo- and petrophysical property investigation not necessary, because no significant

difference is observed in terms of thermo- and petrophysical properties of these rock types. In

addition, a simplified facies classification (Tab. 1, Fig. 7) with respect to the dominant reservoir facies

types is necessary, matching the resolution of reservoir formations by geophysical exploration

methods. According to the Dunham (1962) and Embry & Klovan (1971) classification of carbonate

rocks the following facies types are detected in the study area: mudstones, wackestones, grain-

/packstones and float-/rudstones. The rock classification is based on previous studies of the Malm

formations in Southern Germany (Schauer 1998, Pawellek 2001).

Reservoir characterization based on thermo-and petrophysical parameters including permeability and

thermal conductivity data is rarely available measured at the same sample. For direct correlation in this

study all parameters are determined at the same sample. More than 350 rock samples from 17 outcrop

locations as well as shallow and deep core drillings in Baden-Wuerttemberg and Bavaria (Fig. 2) are

collected and analyzed. For statistical purposes on each rock sample 3 to 10 single measurements of

different rock properties were conducted, i.e. in total over 1150 measurement pairs or triples are

collected.

To determine the thermo-physical properties of the sampled formations and to generate reproducible

results the samples were dried at 105°C to mass constancy and afterwards cooled down to 20°C in an

exsiccator. A thermal conductivity scanner (optical scanning method after Popov et al. 1985) and a gas

pressure permeameter (Jaritz 1999, Hornung & Aigner 2004) and porosimeter are used. With a

thermal conductivity scanner the thermal conductivity and diffusivity are determined. The

measurement is based on a contact free temperature measurement with infrared temperature sensors

(Bär et al. 2011). The measurement accuracy is stated by the manufacturer of about 3 %. The

determination of the grain and bulk density as well as the porosity was done by measuring the grain

and bulk volume of the samples, using a helium pycnometer and a powder pycnometer. The specific

heat capacity cp was calculated with the measured thermal conductivity λ, density ρ and thermal

diffusivity α (converted Debye-Equation):

𝑐𝑐𝑝𝑝 =𝜆𝜆

𝜌𝜌 ∙ 𝛼𝛼 Eq. 1

For the determination of the rock permeability a combined column and mini permeameter was used.

The method offers either the measurement of the apparent gas permeability which afterwards is

converted in permeability or the direct measurement of the intrinsic permeability. The basis for the gas

driven permeameter is the Darcy law which is enhanced by the terms of compressibility and viscosity

of gases. The maximal measurement error depending on the measurement setting is 10% while the

minimal error is about 1%.

4. Results and Discussion

4.1 Thermofacies

The matrix permeability of all measured carbonates is quite low except for some grainstones with

higher permeabilities and porosities. Thick-bedded and platy limestones show thermal conductivities

around 2 W/mK, characteristic of limestones. Permeabilities range from 10-18 to 10-13 m² (K in [m²] =

K in [D] · 9.8692 10-13). Marly limestones have lower thermal conductivities than thick-bedded and

platy limestones, showing the same range of permeabilities as the thick-bedded limestones. It seems

that the higher clay content of the marly limestones is decreasing the thermal conductivity by

insulating the heat conduction and at the same time showing only minor effects on permeability which

shows the same range for mud- and wackestones. The thermal conductivities of different reefal

limestones have values of 1.8 to 3.9 W/(mK), related to the higher content of secondarily silicified reef

bodies and due to dolomitization of reefal structures. The layers with increased silica content are

identified as silicified sponge layers (Leinfelder et al. 1994, 1996). The dolomitized carbonates show

the highest values of thermal conductivity of all investigated carbonates in this study. The herein

presented thermal conductivity and porosity values were in good accordance to results reported from a

limited number of samples in recent works (Koch et al. 2007). However, this study focuses on the

facies relation of thermo- and petrophysical rock parameters of Malm carbonates with respect to

geothermal reservoir characterization. Figure 5 shows that for some stratigraphic units trends are

detectable (increasing thermal conductivity from Malm α to Malm ζ, due to decreasing clay content

and increasing dolomitization (the maximum of the dolomitization is also found within the Malm δ by

Schauer (1996, 1998)). The peak of thermal conductivity observed in the Malm δ also correlates with

increased silica content supplied by silicified sponge layers). Thermo-physical correlations between

different reservoir properties are controlled by lithofacies types. A thermofacies classification based on

the rock parameters thermal conductivity and permeability and the related facies types is shown in

Figure 6, including the nomenclature after Pawellek (2001).

Diagenetic processes caused dolomitization and de-dolomitization of reef structures and their adjacent

transition zones to the basin facies, resulting in an increase of inter-crystalline porosity and therefore

increased matrix permeability. On the other hand, if de-dolomitization led to the formation of

saccharoidal limestone, permeability is decreasing due to reduced crystalline porosity.

With increasing dolomite content an increase of thermal conductivity is observed due to the higher

thermal conductivity of the dolomite crystal structure. The dolomitized areas related to the geometry

of the massive reefal limestone complexes can span over several stratigraphic units of the Malm

predominantly in vertical direction (Schauer 1998, Koch 2011). In the study of Wolfgramm et al.

(2011) an overview and estimation in percentage of the dolomitized areas of the Purbeck and Malm is

given. Figure 7 includes the measurement ranges of different thermo- and petrophysical matrix

properties to improve the reservoir assessment.

Based on the Debye-Equation (eq. 1) and the results of measurements on deep drill cores (Fig. 11), it

can be inferred that the thermophysical properties for tight carbonate rocks are density controlled (see

also section 5.2 Transferability). Density itself is strongly dependent on the lithofacies of the carbonate

rock, so the massive and basin facies has direct influence on the formations’ hydraulic conductivity. In

the transition zone of basin facies to reef facies sub-vertical fractures caused through differential

compaction between massive facies and adjacent basin facies can also be observed in the studied

outcrops. Due to the increased fracture density in this zone, the karstification process is favored, which

results in dissolution of carbonate. The increased hydraulic conductivity results in the disintegration

into dolomite sand or in the process of de-dolomitization (re-calcification) either. In this context it is

important to consider that zones of dolomites due to their primary facies interfingering and genesis

also on a small scale are laterally variable and developed across fractures and porous zones into

adjacent facies (Koch 2011). Therefore, the identification and location of dolomitized areas is of

special interest for the geothermal reservoir prognosis in terms of hydraulically prospective reservoir

formations.

The grain density of the outcrop samples ranges between 2.59 and 2.84 g/cm³ (Fig. 8), the bulk density

is between 2.31 and 2.75 g/cm³. Dolomites showing typically higher densities than limestones, while

dolomitized limestones also elevate the grain density of effected well bedded and reefal limestones.

The porosity calculation based on these values is accordingly less than 15%. The massive limestones

have porosities less than 8%, grainstones and dolomitized zones showing increased porosities up to

15%. From these measurements (Fig. 9) it can also be inferred that the lower Malm α to γ shows very

poor hydraulic conductivity due to average porosities around 5% or less and the upper Malm δ to ζ has

poor to good hydraulic conductivity due to a considerable share of extreme values which are showing

porosities up to 18%. These high porosity values are often correlated to increased permeability of

grain- and dolostones in the upper Malm strata. This observation also correlates with pump test data

from deep drill sites in the Molasse Basin (Schulz et al. 2012).

The permeability measurements state in general very low matrix permeabilties. Only grainstones,

reef/coral debris limestones and dolomitized zones show higher permeability ranges up to 10-14 m² (10

mD). A comparison of permeability and porosity indicates that high porosities occur in grain- and

dolostones and also cause higher permeability. But for all other lithofacies types no correlation

between porosity and permeability in regard to interconnected porosity can be inferred (Fig. 10).

Compared with other carbonate reservoirs mainly explored for hydrocarbons (Ehrenberg und Nadeau,

2005), the rock permeability and porosity of the Malm carbonates of the Molasse Basin is rather low.

Including fracture network, dolomitization and karstification, a positive shift of the permeability-

porosity relationship across several magnitudes can be observed (Fig. 11). This shift identifies a high

hydrothermal potential of the deep Malm aquifer system in the Molasse Basin.

The assumption of a positive 2-3 magnitude reaching permeability correction is based on pump test

data and comparisons of matrix and formation productivity from different deep drilling locations in the

Molasse Basin (Böhm et al. 2011, Schulz et al. 2012).

4.2 Transferability of Reservoir Characterization Parameters

The presented data of outcrop analogue studies are based on rock measurements on oven dried cores,

which are conducted under laboratory conditions with atmospheric pressure and room temperature of

20°C. This approach guarantees a very good reproducibility of the results but also requires a correction

of the measured data for reservoir conditions. It is assumed that the reservoir is completely saturated.

For the following analyses the temperature and pressure conditions of a 5000 m deep and 150°C hot

reservoir, which are realistic values for the Molasse Basin (Fig. 12), are estimated.

The thermal conductivity of water saturated rocks can be calculated following the model of

Lichtenecker and numerous other authors (Pribnow 1994, Clauser & Huenges 1995, Pribnow & Sass

1995, Williams et al. 1995, Schoen 1996, Popov et al. 2003, Hartmann et al. 2005, u. a.). The water

saturated geometric mean thermal conductivity (λgeo) is calculated with the thermal conductivity of the

rock matrix (λrock), the known or assumed thermal conductivity of the pore space filling fluid (λfluid),

and the porosity (Φ):

𝜆𝜆𝑔𝑔𝑔𝑔𝑔𝑔 = 𝜆𝜆𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝛷𝛷 ∙ 𝜆𝜆𝑟𝑟𝑔𝑔𝑟𝑟𝑟𝑟1−𝛷𝛷 Eq. 2

Respectively, the calculation of the specific heat capacity under water saturated conditions is done

using eq. 1.

For the temperature and pressure correction numerous transfer models for reservoir parameters exist.

Temperature dependency models of thermo-physical properties of different rock types can be found in

Tikhomirov (1968), Huenges et al. (1989), Somerton (1992), Pribnow (1994), Vosteen &

Schellschmidt (2003), and Abdulagatova et al. (2009). All authors provide different correction

functions based on comparisons of their own laboratory tests and published data. In general the

thermal conductivity decreases with increasing temperature and increases with increasing pressure

(Clauser & Huenges 1995). The fundamental effects are the reduction of pore space and the increasing

temperature with increasing depth (Clauser et al. 2002). Both parameters control the fluid and matrix

conditions, although in terms of carbonates the temperature depending porosity reduction is the

dominant factor. Also for carbonates the lithostatic pressure has only minor to negligible influence on

the porosity-permeability relation. The pressure depending increase of thermal conductivity is only

relevant for unconsolidated rocks.

For large-scale models of the upper earth crust and also for deep geothermal systems the function after

Zoth & Hänel (1988) for the correction of the thermal conductivity for different rock types is

commonly used, where λ(T) is the thermal conductivity at temperature T:

𝜆𝜆(𝑇𝑇) = 𝐴𝐴350+𝑇𝑇

+ 𝐵𝐵 Eq. 3

In this equation specific petrophysical coefficients are applied. For the Malm carbonates the

coefficients A = 0.13 and B = 1073 valid for a temperature range of 0 – 500°C are chosen. The

correction function of Vosteen & Schellschmidt (2003) is also valid for a temperature range of 0 –

500°C and the rock specific coefficients for carbonates are a = 0,0034 +/- 0,0006 and b = 0,0039 +/-

0,0014.

𝜆𝜆0 = 0.53 ∙ 𝜆𝜆25 +12�1.13 ∙ (𝜆𝜆25)2 − 0.42 ∙ 𝜆𝜆25 Eq. 4

𝜆𝜆(𝑇𝑇) = 𝜆𝜆00.99+𝑇𝑇∙�𝑎𝑎−𝑏𝑏𝜆𝜆0

� Eq. 5

Here λ0 is the thermal conductivity at 0°C, λ25 is the thermal conductivity at 25°C (laboratory

temperature) and T is temperature in °C.

The model of Sass et al. (1971) is given in equation 6 and 7:

𝜆𝜆(𝑇𝑇) = 𝜆𝜆01.007+𝑇𝑇 �0.0036−0.0072𝜆𝜆0

� Eq. 6

𝜆𝜆(0) = 𝜆𝜆25 ∙ �1.007 + 25 ∙ �0.0037−0.0074𝜆𝜆25

�� Eq. 7

With the parameters as defined above the heat capacity is inversely proportional (cf. eq. 1) to the

thermal conductivity and increases with increasing temperature. The thermal diffusivity depends on

the thermal conductivity, density and heat capacity and therefore decreases with increasing

temperature and increasing pressure. After Vosteen & Schellschmidt (2003) the thermal conductivity

of rocks decreases in a temperature interval of 1 to 300°C about 35 ± 10 %, while the thermal

diffusivity in the same temperature range decreases about 48 ± 6 %, showing that this process is

mainly temperature controlled. The same order of magnitude is stated by other authors as well

(Somerton 1992, Clauser et al. 2002). Table 2 shows the range of measured values and calculated

transfer values of different thermo-physical rock properties for different facies types of the Malm

carbonates. For a comparison also the calculated reservoir rock properties with respect to the

mentioned reservoir conditions and according correction functions are listed.

In terms of matrix porosity and permeability it is concluded that the low rock porosity measured on the

outcrop samples will not change significantly with increasing depth in regards to hydraulic

conductivity. In terms of the mean reservoir porosity the temperature of the carbonate systems is the

dominant factor with regard to the thermal expansion and carbonate chemistry and not the depth

function of the reservoir (Schmoker 1984, Bjørkum & Nadeau 1998). A comparison with other

carbonate reservoirs confirms this approach (Ehrenberg & Nadeau 2005).

Measurements of thermal conductivity and permeability of drill cores from greater depths (up to 1600

m, drilling: Moosburg SC4) in the Molasse Basin are shown in Figure 13. The measurements confirm

the above mentioned assumptions and correction functions applied on the near surface values. In terms

of permeability no significant change with depth is inferable. The permeability values obtained from

cores in depth of 1600 m show typical values of permeability comparable to the value range of outcrop

samples. Due to the slight temperature difference of maximum 40°C the thermal conductivity shows

only minor to negligible differences compared with the outcrop results, except for the depth range of

1300 to 1500 m where a dolomitized area of massive facies in the Malm ζ1 and ζ2 is encountered. The

dolomitization process results here in a significant increase (up to 2 W/(mK)) of thermal conductivity.

In general the thermophysical matrix parameters seem to be density controlled. Density increases with

depth typically and with dolomite content especially, therefore thermal conductivity increases but also

decreases with increasing temperature, which is the dominant factor. Specific heat capacity increases

with depth and temperature (Tab. 2).

4.3 Diagenesis

The Malm and Purbeck carbonates are diagenetically altered. In terms of the Molasse Basin three

phases are known: (1) the marine-phreatic early diagenesis, (2) the shallow burial diagenesis with (2a)

a hydrothermal phase and (3) the meteoric karstification diagenesis (Reinhold 1998) and deeper burial.

(1) In the early-diagenesis phase calcitic cements and microbiological pyrite was built.

Furthermore, the silicious yielding skeletons of sponges were dissolved (Reinhold 1998) and

afterwards lithified by different carbonate-based cements.

(2) During the shallow burial first fractures, grain margin dissolution and styloliths occur

(Wolfgramm et al. 2011). Next, dolomitization of the matrix starts from fractures and

styloliths (Schauer 1996, 1998). During the burial phase the carbonate partly dolomitized

completely, so that diagenesis generated dolomites, which have the same petrophysical

behaviour than grainstones (increased porosity and permeability). A selective early diagenetic

dolomitization occurred which was related to primary facies with probably increasing

alteration of the pore water chemistry (Liedmann & Koch 1990). Dolomite was formed in

residual pore spaces which were not closed during early diagenetic cementation-phases of the

sponge-algae reefs. The basinal facies dolomitization is associated with pre-formed pressure-

solution seams. The dolomitization fluids percolated predominantly through relatively

permeable packstones/floatstones and grainstones of the sponge-algal bioberms (Liedmann &

Koch 1990). Within these massive dolomites secondarily formed inter-crystalline pore space

is detected (Wolfgramm et al. 2011). Also during the burial the Ca-rich matrix dolomites can

be altered into Ca-poor dolomites. After tectonic phases (2a) dissolution (karstification) along

fractures occurred. Along the fractures Ca-rich dolomitic cements are attached. Due to the

supply of CO2-rich fluids these dolomitic cements were dissolved again. Afterwards blocky

calcidic cements precipitated.

(3) During emersion of the Malm formation related to the Alpine orogenesis, the effects of

meteoric and hydrothermal waters became more dominant. De-dolomitization and

karstification of the matrix dolomites started from the fracture network. Evidence for long-

term karstification can be found in the increasing opening width of fractures caused by

dissolution (Reinhold 1998) and karst structures in great depth observed in seismic sections

(Lüschen et al. 2011). Especially in fault zones increased opening widths of fractures and karst

structures, which were secondarily filled by bitumen, are recognized (Wolfgramm et al. 2011).

Due to different diagenetic processes and depending on primary porosity conditions, the intensity of

dolomitization varies in the different facies types and can result in substantially increased rock

permeability (cf. Böhm et al. 2013).

4.4 Tectonics and karstification

The present study shows that the rock permeability of the investigated carbonates in general is very

low. Thus, it is concluded that no significant matrix supported hydraulic influence on the short-term

production capacity of the Malm aquifer can be inferred, except for grainstone or dolomitized facies

zones, which are often associated with fault zones. But in terms of the long term sustainability of the

total reservoir system the matrix potential has to be considered. Accordingly, numerical simulations

under consideration of facies-related variations of the matrix potential should be applied (cf. Schulz et

al. 2012), in which the compiled data of this study can be an important contribution. However, in

places the Malm aquifer is very productive, producing discharge rates of over 100 l/s. So the main

hydraulic flow paths must be related to dense hydraulically open fracture networks and associated

karstification. By looking at karst structures two general facies-related karstification types can be

observed (Fig. 14): (1) In the case of thick-bedded or platy limestone (basin-) facies (mudstone,

wackestone) karstification occurs only along major fractures and faults; (2) in the case of reefal

limestones and bioclastic limestones (grainstone, rud-/floatstone) the karstification occurs on a larger

scale due to the primary and secondary porosity of these facies.

In addition, the above mentioned processes of dolomitization and de-dolomitization can also cause

considerably higher rock permeabilities. While the massive limestones are in general favorable for

karstification, the basin facies or thick to platy limestones show a karstification resistance due to their

significantly higher clay content (Wolfgramm et al. 2011), so that CO2-rich groundwater can only

dissolve small amounts of carbonate.

According to Lüschen et al. (2011) karstification in the region north of Munich is observed in seismic

sections. For the following statements it is assumed that karstification is at least in the north eastern

part of the Molasse Basin widespread throughout the aquifer and its intensity and characteristics

depend on facies zones. During the Purbeck a first karstification phase occurred, especially in the

northern part of the Molasse Basin. This part is characterized by the absence of Purbeck sediments, so

meteoric waters could enter the Malm carbonates easily and in depth. The waters migrated also to the

south, where the karstification intensity decreases (Birner et al. 2011). During the orogenesis of the

Alps the south German Massif was tilted. In the north the Jurassic rock formations are exposed and in

the south pushed under the Alps. Large-scale stress analyses show that in the Molasse Basin

transtensive to strike-slip structures are dominant. Due to the N-S aligned main stress axis and the

shape of the Molasse Basin also sinistral NE-SW striking transtension and transpression faults are

locally observed (Reinecker et al. 2010, Fig. 15).

As an example the main fault structure in the 3D seismic exploration of the Unterhaching region is

interpreted as a sinistral transtension regime with according sinistral transpression regime (Lüschen et

al. 2011). Attached to the major faults often Riedel shears are observed. These tectonic structures

cause a disaggregation of the formation in the E-W and NE-SW striking related major fault zones in

the central Molasse Basin. Due to their development in a transtensive regime adjacent faults are

typically formed at the deep block which can also be confirmed by seismic profiles and according

structural analyses carried out in the Molasse Basin (Wolfgramm et al. 2011). These faults zones were

also active for hydrothermal fluids during different diagenesis phases (Liedmann 1992). This caused

an intensive dissolution, extension and dolomitization process in the vicinity of the fault zones and

therefore leads to potentially higher hydraulic conductivities and permeabilities, respectively.

In terms of geothermal exploration more than 50 well bores in the Molasse Basin confirm that the

facies pattern is of comparable importance as tectonics in terms of the intensity of karstification and

thus also for the formation permeability (Schulz et al. 2012). Inferred from the spatial distribution of

total circulation losses of hydrocarbon and hydrothermal drillings in the karst and fracture aquifer of

the Malm, the karstification along major fault zones with adjacent fracture networks is due to their

improved hydraulic conductivity considerably increased (Böhm et al. 2013). The tectonical setting,

karstification and adjacent dolomitization are the key factors for a successful geothermal exploration

in the Molasse Basin, as they are controlling the flow rates of the Upper Jurassic carbonates.

5. Conclusions

The obtained data from the Upper Jurassic limestones of South Germany show that prognosis of

reservoir properties by applying facies models to the deeper subsurface can be implemented as an

additional exploration tool. The determination of geothermal reservoir properties serves in general to

distinguish between petrothermal and hydrothermal systems (Sass & Götz 2012) and can also be used

for optimized drilling and stimulation design purposes. Outcrop analogue studies offer effective

opportunities to gain data to be predicted via transfer models to greater depths and higher

temperatures, which lead to a better understanding of production capacities of geothermal reservoirs.

Furthermore, these studies provide a sufficient data base to determine thermo-physical reservoir

characteristics of the rock matrix of geothermal reservoir formations. Facies concepts are applied as

exploration tool producing conservative results. Adding information on secondary porosities,

karstification, dolomitization and stress field into a reservoir model will lead to realistic reservoir

capacities.

The studied rocks of the Upper Jurassic are not a homogenous formation of limestones. Even on a

small scale different facies zones and their interfingering, which can be differentiated in geometry,

structure, fabric and composition, can be identified. These differences affect the thermo-physical

properties of the rocks and show facies related trends. The hydraulic parameters vary in the order of 4

magnitudes within a stratigraphic unit or facies zone, but show in general a range of poor to very poor

hydraulic conductivity according to the classification of the DIN 18130. It can be inferred from the

outcrop studies that hydraulic active pathways are bound to fracture networks, faults and adjacent

karstification and/or dolomitized zones. The secondary reservoir permeability is strongly related to the

tectonic setting and history and additionally depending on the hydro chemical conditions of the

carbonate reservoir, which also is facies controlled, to maintain open flow paths. Based on the

investigation of the matrix parameters the sustainable heat transport into the utilized geothermal

reservoir can be assessed. Thus, the long-term capacities for different utilization scenarios can be

calculated more precisely. With the help of 3D seismic surveys the investigations on the lateral

extension and facies heterogeneity will give insight on the transmissibility of different target horizons.

The thermofacies characterization and prediction of geothermal reservoir parameters is therefore a

powerful tool for the exploration, extension, operation and quality management of planned and

existing geothermal reservoirs, respectively. The key to a reliable reservoir prognosis, reservoir

stimulation, and sustainable reservoir utilization for the Malm in the Molasse Basin is to integrate

statistically tested databases of tectonic, hydraulic and thermofacies models into 3D reservoir models.

The presented geothermal rock parameters for the different lithofacies can be used to improve

numerical simulations and 3D models of different project locations in the Molasse Basin. The database

will be available via the Fachinformationssystem Geophysik (FIS-GP) of the Leibniz Institute of

Applied Geophysics (LIAG).

6. Acknowledgements

We thank the Bayerisches Landesamt für Umwelt (LfU) for the support and access to drill cores from

shallow and deep research drillings for the investigation of physical rock parameters. Furthermore, we

acknowledge the contribution of numerous students, who performed measurements of geothermal rock

parameters in the framework of their theses at the Institute of Applied Geosciences of the Technische

Universität Darmstadt. We kindly acknowledge the helpful and constructive comments and remarks of

Sonja Philipp, University of Göttingen, and two anonymous reviewers which greatly improved the

manuscript.

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Figure captions

Fig. 1: Study area with location of studied outcrops and drillings.

Fig. 2: Cross section of the western Molasse Basin (modified, Clauser et al. 2002).

Fig. 3: Palaeogeography of the Upper Jurassic in central Europe with study area Swabian and

Franconian Alb (modified from Pawellek 2001, based on Meyer and Schmidt-Kaler, 1989, 1990).

Fig. 4: Comparison of the different lithostratigraphic units in the Franconian and Swabian Alb based

on reference sections after Geyer & Gwinner (1979) and Villinger & Fleck (1995).

Fig. 5: A stratigraphic trend of increasing thermal conductivity is detected within the Malm from

Malm α to Malm ζ.

Fig. 6: Thermofacies classification of Upper Jurassic limestones of southern Germany based on the

international Dunham classification and including the nomenclature introduced by Pawellek (2001);

arrows indicate increasing values, arrow in both directions means approximately same range of values.

Fig. 7: Schematic sketch of the depositional area of carbonates of the Purbeck and Malm and

corresponding thermo- and petrophysical matrix properties in correlation with 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 available.

Fig. 8: Grain density for different lithotypes and dominant rock types (Dunham classification).

Fig. 9: Porosities of Upper Jurassic stratigraphic units and hydraulic conductivity interpretation.

Fig. 10: Porosity/Permeability relation of different lithotypes (only mean values are displayed).

Fig. 11: Correlation of permeability and porosity. Overview of permeability and porosity relations of

global carbonate reservoirs (including statistical trends: P90 (90% of reservoirs have porosity greater

than this value), P50 (median), and P10) in comparison with matrix values of carbonates of the study

area (modified, based on Ehrenberg und Nadeau, 2005); blue arrow indicates permeability shift of

approximately two magnitudes considering fracture network, dolomitization and karstification.

Fig. 12: Temperature distribution at the top of the Malm formation in the south eastern part of the

Molasse Basin (modified, based on Schulz et al. 2007 (www.geotis.de))

Fig. 13: Near surface (outcrop) data in comparison with measurements from shallow (Solnhofen-

Maxberg and Oberdolling) and deep core drillings (Moosburg SC4).

Fig. 14: left: karstification along fault in thick-bedded and platy limestones (Solnhofen); right:

karstification on larger scale (karst cavities) in reefal limestone (Hülen).

Fig. 15: Present-day orientation of maximum horizontal stress (SH) determined from borehole

breakouts and drilling-induced fractures. Symbol size is proportional to quality of the data. A (good) –

D (poor) quality stress orientations (long axes of symbols) in 96 wells across the German Molasse

Basin demonstrate that the present-day SH in the German Molasse Basin is predominantly oriented N–

S (modified from Reinecker et al. 2010).

Tab. 1: Number of measurements and mean values for different lithofacies types.

Tab. 2: Facies types and associated thermo-physical properties at laboratory (measured values) and

reservoir temperatures (calculated values) according to different transfer models; matrix dominated:

Mud-/wackestone, grain-/component dominated: Rud-/floatstone, grain- and dolostone.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Table 1

Parameter Total number of

measurements

Number of measurments per facies- (lithotype)

[mean value]

Basin facies

(mud-/wackestone)

Massive reefal facies

(rud-/float-/grainstone)

Thermal

conductivity

862 426 [1,9 W/(m∙K)] 436 [2,6 W/(m∙K)]

Thermal

diffusivity

801 407 [1,05 ∙ 10-6 mm/s] 394 [1,31 ∙ 10-6 mm/s]

Specific heat

capacity

796 (calculated) 410 [775 J/(kg∙K)] 386 [733 J/(kg∙K)]

Density 236 138 [2,66 g/cm³] 98 [2,73 g/cm³]

Porosity 1126 552 [3.5%] 574 [6%]

Permeability 1014 569 [5 ∙ 10-17 m²] 445 [1 ∙ 10-16 m²]

Table 2

Parameter Facies type Value range at

20°C, dry

Value range for

saturated

conditions at

(20°C)

Value range at

(150°C),

saturated

Transfer model

Specific heat

capacity

[J/kgK]

Matrix dominated

613 - 1045 645 - 1081 806 - 1227 Vosteen &

Schellschmidt

(2003) Grain/component

dominated

547 - 1167 565 - 1184 711 - 1330

Thermal

conductivity

[W/(mK)]

Matrix dominated 1.35 - 2.62 1.60 - 2.79 1.80 - 2.46 Zoth & Hänel

(1988) Grain/component

dominated

1.72 - 4.87 1.85 - 4.94 1.92 - 3.53

Matrix dominated 1.35 - 2.62 1.60 - 2.79 1.83 - 2.51 Sass et al.

(1971)

Grain/component

dominated

1.72 - 4.87 1.85 - 4.94 1.92 - 3.92

Matrix dominated 1.35 - 2.62 1.60 - 2.79 1.41 - 2.25 Vosteen &

Schellschmidt

(2003), Clauser

et al. (2002)

Grain/component

dominated

1.72 - 4.87 1.85 - 4.94 1.56 - 3.71