Diagenetic effects on porosity–permeability relationships in red beds of the Lower Triassic Bunter Sandstone Formation in the North German Basin

Sedimentary Geology 321 (2015) 139–153

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Diagenetic effects on porosity–permeability relationships in red beds of the Lower Triassic Bunter Sandstone Formation in the North German Basin Mette Olivarius a,b,⁎, Rikke Weibel a, Morten L. Hjuler a, Lars Kristensen a, Anders Mathiesen a, Lars H. Nielsen a, Claus Kjøller a a b

Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus, Denmark

a r t i c l e

i n f o

Article history: Received 26 November 2014 Received in revised form 3 March 2015 Accepted 5 March 2015 Available online 17 March 2015 Keywords: Red bed diagenesis Reservoir quality Aeolian versus fluvial Depositional environments Porosity Permeability

a b s t r a c t Carbonate and anhydrite cement, clay clasts and inter-granular clay are the main components that reduce reservoir quality in the studied Bunter Sandstone Formation. The impacts of these parameters on porosity and permeability are determined by combining petrographic mineral quantification with conventional core analysis of samples from the Danish part of the North German Basin. The depositional environments are considered because they largely control the distribution of cements, clays and grain sizes. The lateral variability of depositional environments is defined by the position in the basin and the proximity to the source areas. The stratigraphic distribution of depositional environments is related both to local topography and to climate because high aridity promoted aeolian deposition. The Bunter Sandstone Formation has high porosity and permeability in most of the sandstone intervals in the northern North German Basin. The reservoir quality is good as long as the cements and clays are present as confined bodies that leave the remaining pore spaces available for flow. In contrast, inter-granular clay and pervasive cementation hinder virtually all flow through the sandstone. The ephemeral fluvial deposits have an average porosity and permeability of 20.3% and 810 mD, respectively, and the values are 24.6% and 807 mD for the aeolian sandstones, excluding the unconsolidated aeolian sands which presumably have higher porosity and permeability. The aeolian sandstones of the Volpriehausen Member have very good reservoir quality since they have a thickness of about 25 m, are laterally continuous, are largely clay-free and the cement occurs in small amounts. The sandstones of the Solling Member consist mainly of ephemeral fluvial deposits, which generally have good reservoir quality. However, some intervals have high contents of inter-granular clays or pervasive carbonate, anhydrite or halite cement and these components reduce the permeability significantly. The lateral distribution of the ephemeral fluvial sandstones is variable and therefore difficult to predict when planning a geothermal exploration well. Thus, the Volpriehausen Member is the preferred target. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Factors controlling porosity–permeability relationships in sandstones are readily studied in continental facies deposited under arid conditions. The diagenetic alterations are limited to a few major phases so the relative effects of the individual cement types and clay types on reservoir quality can be deciphered (e.g. Cowan, 1993; Meadows and Beach, 1993; Purvis and Okkerman, 1996). Mechanical compaction dominates at burial depths of less than 2 km (e.g. Paxton et al., 2002; Peltonen et al., 2009; Marcussen et al., 2010) where diagenesis is mainly ⁎ Corresponding author at: Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark. Tel.: +45 91333772. E-mail address: [email protected] (M. Olivarius).

http://dx.doi.org/10.1016/j.sedgeo.2015.03.003 0037-0738/© 2015 Elsevier B.V. All rights reserved.

controlled by stress. The maximum burial depth of the Bunter Sandstone Formation prior to structural inversion is 2.3 km in the northern North German Basin (Nielsen and Japsen, 1991; Japsen et al., 2007), thus representing an ideal case to examine what may cause variations in reservoir quality of sediments deposited during arid climatic conditions. Iron oxide coatings on framework grains are formed in most oxidizing depositional environments (e.g. Walker et al., 1981; Weibel, 1999; Weibel and Friis, 2004). They retard or inhibit quartz overgrowths during burial (Heald and Larese, 1974) so sandstones deposited during arid conditions generally retain good reservoir quality at a greater depth than normal. The Bunter Sandstone Formation in the northern North German Basin is also interesting in a geothermal context (Mathiesen et al., 2010; Mahler et al., 2013). It occurs at appropriate burial depths

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where the pore fluid is sufficiently warm for geothermal exploitation, and both the porosity and permeability are generally high. However, cementing minerals are present in many intervals in varying amounts (Fine, 1986; Weibel and Friis, 2004), so there is a need for clarifying how they affect reservoir quality. This is accomplished by combining sediment petrography with measurements of porosity and permeability. The relationship between specific types of cementation and stratigraphy, depositional environment and provenance will help predict reservoir quality in future exploration wells by resolving the local and regional differences. Improvement of pre-drilling prediction of the reservoir quality is also relevant when the CO2 storage potential of the formation is evaluated (Vangkilde-Pedersen et al., 2009), and the lateral correlation of the reservoir is crucial in this context. Thus, the aims of this study are to: (1) evaluate the individual effects on porosity and permeability of those detrital and authigenic phases that affect the reservoir quality; (2) determine how the reservoir quality is related to depositional environment, climate, grain size distribution and provenance; and (3) identify the best reservoir intervals of the Bunter Sandstone Formation in the northern North German Basin based on the vertical and lateral variability.

1992; Geluk and Röhling, 1997) cannot be applied in the marginal basin setting of the present study due to hiatuses. A tentative subdivision of the Bunter Sandstone Formation in the northern North German Basin was first made by Pedersen (1998) and later in more detail by Michelsen and Clausen (2002) based on wireline log interpretations of selected wells. Michelsen and Clausen (2002) divided the Bunter Sandstone Formation into the Volpriehausen, Detfurth, Hardegsen and Solling Members corresponding to the German formations in the Middle Buntsandstein. The Volpriehausen Member has a uniform thickness in the northern part of the North German Basin and the Solling Member is also present throughout the area, but the thickness varies (Olivarius, 2015). The Detfurth Member is missing in some parts of the northern North German Basin and the Hardegsen Member is only present in the southernmost part of the area presumably due to the erosion associated with the unconformity at the base of the Solling Member (Röhling, 1991). The Bunter Sandstone Formation is encountered at burial depths of 1.1–2.1 km onshore Denmark (Nielsen and Japsen, 1991) corresponding to maximum burial depths of 1.6–2.3 km prior to Mesozoic–Cenozoic uplift events (Japsen et al., 2007). The Bunter Sandstone Formation is more than 1000 m thick in the Horn and Glückstadt Grabens positioned centrally in the North German Basin (Fig. 1B) and has thicknesses of up to 500 m outside the grabens (Röhling, 1991; Geluk, 2005). The sand content decreases towards the basin center (Bertelsen, 1980; Bachmann et al., 2010). The formation wedges out towards the Ringkøbing–Fyn High in the northern part of the basin (Fig. 1B) resulting in thicknesses of maximum 300 m in the Danish onshore area (Michelsen and Clausen, 2002).

2. Geological setting Regional subsidence prevailed in the Central European Basin during the Early Triassic when the Bunter Sandstone Formation and its timeequivalent clastic successions were deposited (Ziegler et al., 2004; Scheck-Wenderoth and Lamarche, 2005; Bachmann et al., 2010). The formation succeeds the lowermost Triassic Bunter Shale Formation in the North German Basin (Fig. 1A) (Bertelsen, 1980). The Bunter Sandstones Formation is enveloped by evaporitic deposits of the Upper Permian Zechstein Group and the Middle Triassic Röt Formation. The Bunter Sandstone Formation was defined as the upper part of the Bacton Group by Rhys (1974) in the southern part of the North Sea Basin and was later recognized by Bertelsen (1980) in the Danish area. Deposition of these sands took place during most of the Olenekian and the earliest Anisian (Fig. 1A) (Kürschner and Herngreen, 2010). The stratigraphy of the Bunter Sandstone Formation is poorly constrained (McKie and Williams, 2009) due to the unfossiliferous nature of these continental deposits. The well-established cyclostratigraphy from the central North German Basin (Röhling, 1991; Aigner and Bachmann,

A System Series

* evaporitic ** evaporites

S Stage

N

Denmark

North German Basin

Norwegian-Danish Basin

RFH

3. Depositional environments The deposits of the Bunter Sandstone Formation are red beds and each member usually consists of sandstone at the bottom which grades upward into shale (Fig. 2). Warm and arid climatic conditions prevailed during deposition of the Volpriehausen Member; the aridity decreased during deposition of the subsequent members (Bourquin et al., 2011). The Volpriehausen Member represents sand that was primarily deposited south of the major fault that separates the platform and basin areas (Figs. 2, 3A). The sand was transported northwards by aeolian activity from elevated basement areas in the Variscan belt

B Wells

Sediment transport

Structural highs

Basin outline

HG Horn

Graben

GG Glückstadt

Variscan belt

Graben

HG

Hiatus

Gassum Fm

Denmark

R i n g k ø b i n g

Vinding Fm

Norian

Keuper Fm*

Arnum-1

Oddesund Fm*

Carnian

Olenekian Induan

Middle

Muschelkalk Fm* Röt Fm*

Falster Fm* Ørslev Fm*

N o r t h

Skagerrak Fm

Bunter Shale Fm Zechstein Group**

300 km

H i g h

Rødby-1

G e r m a n

Bunter Sandstone Fm

B a s i n

Germany

Changhsingian Wuchiapingian

F y n

GG

Tønder-4 Tønder-5 Tønder-3

Ladinian Anisian

-

Hønning-1

Tønder Fm*

Upper Lower

Permian

Triassic

Upper

Rhaetian

Gassum Fm

Helgoland

N 25 km

Fig. 1. A: Stratigraphic scheme of the Danish onshore Upper Permian and Triassic succession simplified after Vejbæk (1990), Clausen and Pedersen (1999), Michelsen and Clausen (2002), Nielsen (2003) and Kürschner and Herngreen (2010). B: Location map of the wells where the Bunter Sandstone Formation has been sampled in the North German Basin. The Ringkøbing– Fyn High (RFH) outline is from Vejbæk and Britze (1994) and Nielsen (2003). The Variscan belt outline is from Sánchez Martínez et al. (2012). The regional basin outline shows the approximate area in which the Bunter Sandstone Formation was deposited under lacustrine and occasional aeolian conditions during the Early Triassic (Péron et al., 2005). Sediment transport directions are indicated by arrows placed at the source areas that supplied sediment to the northern North German Basin (Olivarius, 2015).

M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

GRnor m 0 API 250 Dept h DT (m) 160 us/ft 60

1650

1651.05 1655.12 1655.39 1657.60 1661.93 1669.72 1671.90 1680.30

Dept h GRnor m (m) 0 API 250

1733.30 1737.21 1738.86 1739.81 1740.09

Dolomite

1851.33 1856.30 1859.06 1866.80 1868.42 1870.15 1870.40

1300

1850

1795.40 1795.50 1796.33 1798.24 1799.27

1350

Shale Salt

1794.82 1799.33 1804.63 1805.26 1808.16 1808.71 1809.40 1810.47

1800

1800

Sandstone

1198.66

1763.65

1750

Cored intervals

1151.62

1250

1800

1750

1750

1550

Bunter Shale Fm

1700

1700

1632.15 1632.15 1632.40 1633.53

1625.05 1643.05 1643.15 1649.47 1657.05 1658.45 1661.07 1667.60 1667.70 1672.34 1678.16 1678.18 1678.18 1681.71 1683.66 1688.28

Rødby-1

1150

Tønder- 5

GRnor m 0 API 250 Dept h DT (m) 160 us/ft 60

1700

Tønder- 4

GRnorm 0 API 250 Dept h DT (m) 160 us/ft 60

1700

1470.40

Tønder-3

1650

1440.90 1442.44 1442.50 1444.83

Sampling was conducted in the Arnum-1, Hønning-1, Rødby-1, Tønder-3, -4 and -5 wells (Fig. 1B). In the present study, a tentative stratigraphic member division of the Bunter Sandstone Formation into the Volpriehausen, Detfurth and Solling Members (Fig. 2) was interpreted by comparing log patterns from the sampled wells to loginterpretations by Michelsen and Clausen (2002). The mineralogy was quantified by point counting of 61 thin sections of which 45 are new and 16 are from Weibel and Friis (2004). The thin sections were prepared from sandstones in order to investigate the

1600

Dept h GRnorm (m) 0 API 250

1650

1450

Dept h GRnorm (m) 0 API 250

4. Samples and methods

1750

Hønning-1

1500

Bunter Sandstone Fm

Röt Fm

Arnum-1

floodplain graded into alluvial fans deposited along the Ringkøbing– Fyn High which formed a local source area for the ephemeral fluvial deposits in the Volpriehausen and Solling Members (Fig. 1B) (Olivarius, 2015). Fine-grained shoreline sandstones were deposited at the playa shoreface and beach and have been cored in the Solling Member. They were influenced by relatively weak wave action as is evident from the small-scale trough-formed cross-lamination produced by wave-ripple migration (Clemmensen, 1985). Occasional plane bed conditions are seen by horizontal lamination. The shoreline migrated laterally in response to the water level in the playa lake. The playa deposits are common in the Volpriehausen, Detfurth and Solling Members (Figs. 2, 3). They are composed of heteroliths and thin layers of very finegrained sandstone. The heteroliths show irregular and wavy bedding and consist of mudstone with sandy laminae and small sand patches (Clemmensen, 1985). Desiccation cracks show that they were subaerially exposed at times. The playa sediments were deposited in a low-energy environment with occasional storms depositing sand.

1200

(Fig. 1B) to the northern North German Basin (Olivarius, 2015). Sand flat deposits are present in the lowermost 10 m of the Volpriehausen Member in the Tønder area. They consist primarily of sand-rich heteroliths with irregular lamination (Clemmensen, 1985). The sand patches are very fine-grained and were deposited as aeolian ripples that adhered to the subaerial sand flat surface. The sand flat deposits are overlain by a 25 m thick succession of fine-grained aeolian sandstones that were deposited as sand sheets during sandstorms. They show alternating low-angle cross-bedding, high-angle large-scale cross-bedding and horizontal lamination produced by ripple and grainfall deposition. An aeolian origin is indicated by the fine grain size, excellent sorting, rounded and frosted grains, aeolian stratification types, little cementation and by the absence of clay, mica and fossils (Clemmensen, 1985). The aeolian sandstones are interbedded with ephemeral fluvial sandstones towards the top. Fine-grained floodplain deposits characterize the Volpriehausen Member in the platform area. Ephemeral fluvial deposits are represented by up to 30 m of dominantly fine-grained sandstones at the base of the Solling Member (Figs. 2, 3B). They show thin fining-upward beds, low- and high-angle cross-bedding, cross-lamination, mica-rich horizons, mud drapes, intra-formational clay clasts and have a unidirectional current pattern (Clemmensen, 1985; Olsen, 1987). The clay clasts originated from erosion of interchannel muddy overbank areas when the channels shifted their course under sheet flow conditions during flood events. Point-bar sequences locally form part of the fluvial deposits in the Solling Member, each sequence consisting of a lag conglomerate overlain by a thick sandstone layer with a mudstone layer on top. The point-bar sandstones contain low-angle longitudinal cross-bedding and small-scale cross-lamination. Point bars were formed in the distal region by lateral channel migration (Fig. 3B). In proximal regions, the

141

Tentative subdivision: Solling Member

Anhydrite

Detfurth Member

Marlstone

Volpriehausen Member

>10% Carbonate cement >10% Anhydrite cement >10% Clay clasts >10% Inter-granular clay >10% Ooids

Fig. 2. Lithological columns and wireline logs of the Bunter Sandstone Formation in the sampled wells showing sample depths; only the point-counted samples are included. Contents N10% of cements, clays and ooids are indicated for each sample. The stratigraphic subdivision into members is interpreted on the basis of the work by Michelsen and Clausen (2002). Well locations are shown in Fig. 1B.

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A) Volpriehausen Member (early Olenekian) H

Basement

h ig

Volpriehausen Member and a few from the Detfurth Member are included. The point bar and shoreline sediments are from the Solling Member. Playa sediments were sampled in the Volpriehausen, Detfurth and Solling Members.

Alluvial

4.1. Point counting by optical microscopy

rm fo at l P

Thin sections were made with blue epoxy impregnation to aid identification of open pore space. Half of each thin section was etched and stained with sodium cobaltinitrite for K-feldspar identification. The detrital and authigenic phases were quantified by point counting. The point counting technique involves automatic sliding of a thin section on a Pelcon stage which moves in steps with a length defined by the operator. 500 minerals were petrographically determined in each thin section and the porosity was also counted. The minerals were divided into 34 classes and the porosity was identified as primary or secondary.

Ephemeral fluvial fluvial

Overbank

N sin

Ba

Aeolian sand sheet

25 m

Sand flat

10 km

Playa

4.2. Scanning electron microscopy

B) Solling Member (late Olenekian - early Anisian) H

Basement

h ig Alluvial

rm fo at l P Overbank

N in as

Shoreline

4.3. Porosity and permeability

Ephemeral fluvial fluvial Point bar

B

Playa

25 m

Ephemeral phemeral fluvial

10 km

The SEM analyses were performed on carbon-coated thin sections and gold-coated rock chips glued on stubs. A Philips XL40 SEM equipped with a ThermoNoran energy dispersive X-ray spectrometry (EDX) detector was used for the analyses. Elemental analyses of the grains were made by EDX to give a qualitative identification of the minerals.

Playa

Fig. 3. Depositional environments of the Bunter Sandstone Formation in the northern North German Basin. A: The Volpriehausen Member was deposited during arid climatic conditions, and most of the sand was supplied from the south by aeolian activity. B: The Solling Member was deposited during semi-arid climatic conditions, and the sand was primarily supplied from the north by ephemeral rivers. The platform area is bounded by two major faults which separate it from the basin area and the Ringkøbing–Fyn High.

general petrography. A few of the sandstones grade into siltstone. Porosity was measured on samples corresponding to 50 of the point counted thin sections; permeability was measured on 41 of these samples, since some of the core material was not suitable for permeability analysis. Data from previous conventional core analyses are included (GEUS report files no. 8776, 9715, 9757, 10384, 10385, 10390, 10431, 10434, 27185) so that 375 measurements of both porosity and permeability are included in total. Twenty-nine measurements were discarded since the permeabilities were lower than the detection limit of 0.02 mD for the used equipment. The majority of the discarded measurements are from playa sediments. The 61 petrographically quantified thin sections represent six depositional environments including aeolian, ephemeral fluvial, point bar, sand flat, shoreline and playa. Depositional environments of the sediments sampled in the Tønder wells were interpreted by Clemmensen (1985) and are extrapolated to the Arnum-1, Hønning-1 and Rødby-1 wells. The sampled aeolian and sand flat deposits are from the Volpriehausen Member. Most of the ephemeral fluvial sediments are from the Solling Member, but also some samples from the

Porosity and permeability were measured by conventional core analysis (American Petroleum Institute, 1998). N2-gas was used to determine permeability. NaCl was removed by cleaning the samples in methanol prior to analysis, except for the most fragile samples as they would decompose by this method. The permeability was not measured for the samples that were extremely porous, had large fractures or were too small to fit the holder. 5. Results The sandstone intervals in the Bunter Sandstone Formation are dominantly pale red to reddish brown (Fig. 4A) passing into orange pink in cemented areas. The sandstones are generally fine-grained except for the very fine-grained sand flat and playa deposits (Table 2). Greenish gray reduction spots (Fig. 4B) and moderate orange-pink anhydrite nodules are abundant in some intervals. The sandstones are arkosic to subarkosic and the grains are overall sub-angular, but subrounded to well-rounded in the aeolian sandstones. The aeolian sediments are very well to well sorted, the fluvial and shoreline sediments are well sorted and the sand flat and playa deposits are moderately to very poorly sorted (Table 2). 5.1. Porosity–permeability relationships The relation between porosity and permeability in the Bunter Sandstone Formation is illustrated in four plots to show how stratigraphy, sampling locality, grain size and depositional environment influence reservoir quality (Fig. 5). The porosity and permeability correlate, and the majority of the sediments have intermediate to high porosities (N13%) and high permeabilities (N100 mD). The porosities and permeabilities of the sandstones in the Volpriehausen Member are generally high (c. N17%; c. N100 mD), but considerably lower in the Detfurth Member (c. 14–18%; c. 5–125 mD) and a wide range is found in the Solling Member (c. 2–34%; c. 0.03– 5000 mD) (Fig. 5A). The Detfurth Member onshore Denmark has only been cored in the most fine-grained intervals (Fig. 2), so the data are not considered representative of this member. Some of the data, primarily from the Tønder-3 well, plot in a cluster above the overall

M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

A

143

B

1 cm

2 mm

Fig. 4. Core photographs showing the typical red-bed appearance of the Bunter Sandstone Formation caused by red grain coatings. A: Pale reddish brown, very fine- to fine-grained sandstone from the Tønder-5 well at 1738 m depth. B: Reddish brown, silty claystone with reduction spots from the Tønder-4 well at 1628 m depth.

by a different porosity–permeability trend than the sandstones which have considerably larger variations in porosity. Permeabilities of b 2 mD correspond to low porosities in the sandstones (c. 2–8%) and to intermediate porosities in the claystones (c. 11–18%). The shoreline samples have intermediate porosities and low to intermediate permeabilities (c. 9–21%; c. 3–100 mD) (Fig. 5D, Table 1) whereas they are generally higher in the sand flat deposits (c. 13–28%; c. 3–1000 mD). The majority of the ephemeral fluvial sediments have high permeabilities of N100 mD. Poor reservoir quality corresponding to porosity b10% and permeability b1 mD is found in a group of

porosity–permeability trendline since they have intermediate porosities (c. 13–19%) and high permeabilities (c. 300–4000 mD) (Fig. 5B). The sandstones are divided into very fine-grained (≥ 63–125 μm), fine-grained (≥ 125–250 μm) and medium-grained (≥ 250–500 μm) in Fig. 5C. The very fine-grained sandstones plot primarily below the trendline of the sandstones because of lower permeabilities. The medium-grained sandstones plot predominantly above the trendline as a result of higher permeabilities. The claystones, siltstones and heteroliths have low to intermediate porosities and permeabilities (c. 6–24%; c. 0.02–200 mD) and they are characterized

100000

A

Stratigraphic members

estimate of loose sand

Permeability (mD)

10000

B

Wells

estimate of loose sand

1000 100 Arnum-1

10

Hønning-1

1

Tønder-3

0.1

Solling

Tønder-4

Detfurth

Tønder-5

Volpriehausen

Rødby-1

0.01 100000

C

Grain sizes

estimate of loose sand

Permeability (mD)

10000

D

Depositional environments

estimate of loose sand

1000 100 10 1 0.1 0.01 0

Sandstones: y = 0.0003x4.43 R² = 0.68

5

10

15

20

25

Porosity (%)

Medium sandstone

Aeolian

Fine sandstone

Ephemeral fluvial

Very fine sandstone

Point bar

Siltstone

Shoreline

Heterolith

Sand flat

Claystone

Playa

30

35

40 0

5

10

15

20

25

30

35

40

Porosity (%)

Fig. 5. Porosity–permeability plots for the Bunter Sandstone Formation with the trendline of the sandstones plotted for the purpose of comparison. The same dataset is plotted with four different color codes for the 375 analyzed samples. The detection limit of the permeability is 0.02 mD. Most aeolian samples were unconsolidated so the porosity and permeability could not be measured, but they are important since they have the best reservoir properties. Therefore, the porosity–permeability range of aeolian sand from Davis (1969) is shown as gray fields.

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M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

ephemeral fluvial and point-bar sediments from the Tønder-4 well (Fig. 5B, D). Only few of the point-bar sandstones have good reservoir quality (Fig. 5D, Table 1). The measured reservoir properties of the aeolian sandstones (Fig. 5D) are not representative since measurement of the loosest deposits was impossible. Therefore, a gray box in Fig. 5 shows the porosity and permeability intervals of 35–40% and 5000–50000 mD, respectively, estimated by Davis (1969) for loose aeolian sand. The aim is to ensure that the expected very good reservoir quality of the aeolian deposits is not overlooked. 5.2. Detrital mineralogy The modal composition of the Bunter Sandstone Formation is shown in Fig. 6 and Table 2 in relation to both stratigraphy and depositional environments. The full petrographic data table is available as electronic supplementary material. The number of samples used to characterize the point bar, sand flat, shoreline and playa are not statistically significant, but they were sampled in representative core intervals to give a best estimate of these depositional environments. Quartz and feldspar are the dominant detrital minerals in all members and in most depositional environments (Fig. 6, Table 2). The majority of quartz grains are monocrystalline and the feldspars include both K-feldspar and plagioclase. Clay clasts are most abundant in the sand flat sediments and least abundant in the aeolian sandstones. Detrital infiltration clay is dominant in the playa sediments and abundant in the sand flat sediments (Fig. 6B). Detrital clay is found in only three of the aeolian samples and is generally present in small amounts in the ephemeral fluvial, point bar and shoreline sediments. The infiltration clay is present as coatings on grain surfaces and occasionally as meniscus cement bridging the grains. Small amounts of rock fragments are present in all members and depositional environments (Fig. 6), and they are primarily of sedimentary and plutonic origin. Carbonate clasts are included in the category of sedimentary rock fragments, and they are found in small amounts in some samples but are more abundant in clay clast conglomerates. The amount of heavy minerals is small, especially in the Volpriehausen Member. Small amounts of altered grains are found in all members and depositional environments. All samples with ooid contents of N 10% belong to the Volpriehausen Member (Fig. 2) and occur mainly in aeolian sediments (Fig. 7A) and in some ephemeral fluvial deposits (Fig. 7B). Ooids are not present in the samples from the Arnum-1, Hønning-1 and Rødby-1 wells. The ooids consist of calcite; some ooids are completely recrystallized, but they are typically well-preserved apart from local pressure solution and burial deformation effects. 5.3. Authigenic mineralogy Quartz and feldspar overgrowths are few and thin (Fig. 6B). Carbonate cement is present in all members and depositional environments (Fig. 6) and includes non-ferroan calcite and dolomite. The amount varies considerably from zero in a single sample up to a maximum of 29%. The

average proportion of calcite versus dolomite is about 2:1. Calcite cement is mainly present as small irregular patches of pore-filling poikilotopic or sparry cement and sometimes as rhombs. Calcite cement is often rimming the ooids (Fig. 7A), and calcite has locally replaced some of the feldspar grains. Some ooids have been recrystallized into calcite crystals which have eliminated the concentric structure. Dolomite occurs as rhombs that are scattered through the sediments (Fig. 7C). Pervasive carbonate cementation is present in some core intervals; primarily in thin clayey sandstones within claystone successions. Anhydrite cement is present in amounts up to 38% in the Bunter Sandstone Formation. The content is very low in the aeolian sandstones and it is very variable in the ephemeral fluvial sediments. Almost three times more anhydrite cement is present in the Solling Member than in the Volpriehausen Member (Fig. 6A, Table 2). The anhydrite occurs as patchy pore-filling cement in sandstones (Fig. 7D) and as displacive nodules in claystones (Fig. 7E). However, it is also displacive in some sandstones. Anhydrite has in some places replaced framework grains, ooids and carbonate cement. Samples with N8% carbonate cement contain little or no anhydrite cement. An abundance of red coatings also corresponds to a low content of anhydrite cement. The coatings are present on grain surfaces (Fig. 7F) and produce the red color of the sediments. The coatings consist of goethite and hematite needles (Weibel and Friis, 2004) and they are present in most samples, up to a maximum of 8%. Red coatings on clay minerals have been point counted as detrital clay in the playa sediments since the minerals cannot be separated by optical microscopy. Authigenic clays include illite, mixed-layer illite/smectite, mixedlayer smectite/chlorite, chlorite and vanadium-rich illite (Weibel and Friis, 2004). They have a pore-filling morphology and are present in amounts of up to 10% in a few samples, but are otherwise rare. Other authigenic minerals include analcime, anatase and barite in decreasing order of abundance. Analcime and anatase are present in amounts of up to 5%, whereas barite is rare. Halite is regularly found and may be an artifact produced by drying of the pore fluids, but Laier and Nielsen (1989) found that it is an authigenic phase in some intervals. 5.4. Mineralogy in relation to porosity and permeability Contents of N10% carbonate, anhydrite, clay clasts and inter-granular clay are indicated in Fig. 2 for each sample. A cut-off value of 10% of each of these phases is used to show how porosity and permeability are affected by high versus low contents of cements and clays (Figs. 8, 9). Inter-granular clay refers to the total amount of infiltration and authigenic clays (Fig. 6). These clays have been merged because they affect the reservoir properties similarly (e.g. Colter and Ebbern, 1978; Schmid et al., 2004). All sediments with a porosity of b 19% and/or a permeability of b 250 mD have a content of N10% of one or several of the dominant cement and clay types (Fig. 8). The effects of each cement and clay type on porosity, permeability, grain size, sorting and inter-granular volume are listed in Fig. 10. Determination of the degree of sorting is based on Longiaru (1987). The calculation of inter-granular volume was defined by Houseknecht (1987) and modified by Ehrenberg (1989) as the sum of primary

Table 1 Porosity and permeability data of the Bunter Sandstone Formation. The numbers with an * are underestimated since many of the aeolian sediments in the Volpriehausen Member are unconsolidated. Thus the best reservoir rocks could not be analyzed. Stratigraphic member

Number of analyses Porosity, average (%) Porosity, mean (%) Porosity, range (%) Permeability, average (mD) Permeability, mean (mD) Permeability, range (mD)

Depositional environment

Volpriehausen

Solling

Aeolian

Ephemeral fluvial

Point bar

Sand flat

Shoreline

Playa

123 22.9* 23.6* 1.0–37.1* 721* 530* 0–7261*

187 18.9 19.1 1.0–33.5 719 325 0–6538

49 24.6* 25.5* 1.0–37.1* 807* 680* 1–2861*

195 20.3 21.8 1.0–34.4 810 457 0–6538

16 13.5 9.0 1.0–31.7 565 1 0–4532

28 20.8 21.3 1.0–29.6 1646 179 0–7261

10 13.9 11.8 1.0–25.2 55 30 1–217

21 18.0 17.5 1.0–26.0 69 33 0–369

M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

A

Stratigraphic member

Well

n

Solling

Arnum-1, Hønning-1, Tønder-3, Tønder-4, Tønder-5, Rødby-1

38

Volpriehausen

Tønder-3, Tønder-4, Tønder-5

23

145

Detrital mineral % 0

20

40

Detrital minerals

Authigenic mineral % 60

80

0

20

40

Quartz K-feldspar Plagioclase Mica Clay clasts Clay

Depositional environment

Well

n

Aeolian

Tønder-3, Tønder-4, Tønder-5

11

Ephemeral fluvial

Arnum-1, Hønning-1, Tønder-3, Tønder-4, Tønder-5, Rødby-1

33

Point bar

Tønder-4, Tønder-5

5

Sand flat

Tønder-3

4

Shoreline

Tønder-4

3

Playa

Arnum-1, Hønning-1, Tønder-4, Rødby-1

B

Detrital mineral % 0

20

40

Authigenic mineral % 60

80

0

20

Rock fragments 40

Heavy minerals Altered grains Ooids

Authigenic minerals Quartz Feldspar Carbonate Anhydrite Red coatings Clay

5

Others

Fig. 6. Detrital and authigenic mineral contents as an average for each stratigraphic member (A) and for each depositional environment (B). The number of point counted samples is listed as n.

porosity and authigenic phases. The sandstones that contain b 10% of each of carbonate cement, anhydrite cement, clay clasts and intergranular clay have high average porosities and permeabilities of 28.3% and 1822 mD, respectively (Fig. 10A). Carbonate cement generally occurs in small amounts (b14%) and does not have a major influence on the reservoir quality of the analyzed sediments (Fig. 9A, B). The carbonate cement lowers the permeability (Fig. 10B), but its relation to porosity is less clear. High carbonate content is often associated with a small grain size. The thin intervals with pervasive carbonate cementation are rather few and have not been analyzed.

Increasing content of anhydrite cement correlates with decreasing porosity and permeability (Fig. 10C). The negative correlation between anhydrite cement and porosity is most evident when discarding the results from three of the ephemeral fluvial samples with porosities of 23–24% (Fig. 9C). The anhydrite cement has replaced and displaced framework grains in these three sandstones. The lowest permeabilities correlate with inter-granular clay content rather than anhydrite content (Fig. 9D). The very low porosity and permeability of the ephemeral fluvial sandstones from the Solling Member in the Tønder-4 well (Fig. 5) are assumed to be caused by pervasive anhydrite cementation, but they have not been point counted.

Table 2 Results from point counting of thin sections. The content of each mineral is a percentage of the sum of detrital and authigenic minerals and primary and secondary porosities. The maximum burial depths prior to structural inversion are based on estimates by Japsen et al. (2007).

Number of samples Burial depth (m) Maximum burial depth (m) Detrital mineralogy Quartz K-feldspar Plagioclase Mica Clay clasts Clay Rock fragments Heavy minerals Altered grains Ooids Authigenic mineralogy Quartz Feldspar Carbonate Anhydrite Red coatings Clay Others Primary porosity (%) Secondary porosity (%) Inter-granular volume (%) Grain size (μm) Sorting

Stratigraphic member

Depositional environment

Volpriehausen

Solling

Aeolian

Ephemeral fluvial

Point bar

Sand flat

Shoreline

Playa

21 1764–1870 1964–2070 37.5 6.5 3.5 0.5 6.4 3.1 1.7 0.7 0.8 7.6 0.1 0.1 7.4 2.7 1.5 0.1 0.5 19.1 0.2 31.6 142 0.6

39 1152–1740 1652–1940 35.7 8.3 5.4 1.4 3.6 3.4 3.0 1.8 1.2 0.6 0.3 0.1 5.6 8.7 2.5 0.9 1.0 15.8 0.5 35.0 151 0.5

11 1795–1870 1995–2070 39.6 6.5 3.2 0.2 0.3 0.2 1.8 0.6 0.6 11.7 0.1 0.0 7.7 1.2 2.2 0.0 0.7 23.3 0.2 35.2 155 0.3

33 1152–1805 1652–2005 37.6 8.9 5.4 1.4 3.3 1.1 2.8 1.7 1.5 1.4 0.3 0.2 6.2 5.5 2.6 0.7 0.9 18.1 0.5 34.5 160 0.4

5 1684–1740 1884–1940 45.5 7.4 3.6 0.2 2.2 0.8 1.3 2.0 0.3 0.9 0.0 0.0 2.1 11.2 2.5 0.0 1.5 18.2 0.2 35.6 162 0.4

4 1808–1810 2008–2010 24.9 3.1 3.4 2.0 27.7 15.7 0.8 0.7 0.2 0.0 0.0 0.0 9.1 0.2 0.8 0.1 0.0 11.3 0.0 21.5 85 1.1

3 1643–1649 1843–1849 26.9 8.5 7.3 2.2 6.5 0.7 8.2 1.8 1.2 1.3 0.7 0.3 6.0 21.4 1.2 1.2 1.7 2.3 0.6 34.8 145 0.5

5 1199–1764 1699–1964 20.8 4.3 3.8 1.0 4.1 29.4 1.2 1.2 0.5 0.2 0.1 0.1 9.0 16.7 0.0 2.3 0.0 4.3 1.0 32.4 71 1.8

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M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

A

50 µm

B

100 µm

Ooid Ooid

C

50 µm

D

100 µm

Anhydrite Carbonate

E

200 µm

F

50 µm

Red coatings

Anhydrite

Fig. 7. Morphological examples of ooids and authigenic phases. A: Well-preserved ooids showing both concentric and radial fabric; pressure solution has occurred along grain contacts and calcite cement has formed at the rims. B: Broken ooids caused by fluvial reworking. C: Scattered dolomite rhombs of various sizes (pastel shades). D: Localized pore-filling patches of anhydrite (purple areas). E: Displacive growth of anhydrite in the clay-rich areas (large white mineral). F: Thick red coatings (reddish brown rims on grains). Thin section micrographs with crossed nicols (A, C, D) and direct light (B, E, F). Wells and depths: A, Tønder-5, 1856.30 m. B, Tønder-3, 1799.33 m. C, Tønder-5, 1870.15 m. D, Tønder-5, 1739.81 m. E, Tønder-4, 1662.96 m. F, Rødby-1, 1151.62 m.

A content of red coatings N2% corresponds to porosities of N19% and permeabilities of N250 mD (Fig. 9E, F). Increasing contents of clay clasts and inter-granular clay corresponds to decreasing porosity and permeability (Fig. 10D, E). A content of N10% clay clasts equals porosities of 12–26% and permeabilities of 3–2540 mD (Fig. 9), but the least permeable of these sandstones also have a high content of inter-granular clay. Inter-granular clay contents of N10% correspond to porosities of 5–25% and permeabilities of 1–31 mD. All samples with permeabilities of b40 mD have a content of clay clasts and/or inter-granular clay of N10%. There is a tendency for clay clasts and especially inter-granular clay to occur in the largest amounts in the most fine-grained sediments with the poorest sorting (Fig. 10). The point counted porosity is in general an underestimate compared to the porosity measured by core analysis (Fig. 11). In particular, the porosity of the fine-grained sediments is underestimated. The amount of secondary porosity is very small (Table 2), amounting to a maximum of 3%.

6. Discussion The grain size, sorting, clay abundance and amount of clay clasts are strongly related to depositional environment and they influence the porosity and permeability in the Bunter Sandstone Formation. The volumetrically important early diagenetic phases include carbonate cement, gypsum cement (transformed into anhydrite during burial) and red coatings, and they are also linked to the depositional environments which to some extent are stratigraphically constrained. Thus, it is attempted to distinguish the effect of these individual parameters. 6.1. Carbonate cement The porosity is not notably reduced by carbonate cement when it is present in as small amounts as in this study (Figs. 9A, 10B) where most samples contain 1–13% carbonate cement. Pressure solution of ooids (Fig. 7A, B) and replacement of altered grains provided the source of cement in most of the sediments with the highest amounts of

M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

100000 estimate of loose sand

Permeability (mD)

10000

14 23

1000

12

11

12

17 22

31 19

100

26 22

10 16

15 11

13 29

>10% Carbonate cement

1

38

>10% Anhydrite cement >10% Clay clasts

0.1 0

>10% Inter-granular clay

5

10

15

20

25

30

35

40

Porosity (%) Fig. 8. Porosity–permeability relationships of the petrographically quantified sediments. Trendlines are shown for samples containing N10% of carbonate and anhydrite, respectively. The numbers indicate their content of the respective cement types. Estimate of loose sand as in Fig. 4.

carbonate cement. Most of the altered grains were presumably feldspar grains originally. Furthermore, cementation occurred during early diagenesis and was therefore likely to have caused a stiffening of the sandstones, thus stabilizing the rock framework and reducing the influence of mechanical compaction (e.g. Marfil et al., 1996; Bjørlykke, 1999). Carbonate cement has reduced the permeability in the sediments (Figs. 9B, 10B), but flow has not been completely hindered since the cement occurs as scattered rhombs and patches (Figs. 7C, 10B). It was also observed by Colter and Ebbern (1978) that confined carbonate patches do not have a large effect on reservoir properties whereas pervasive carbonate cement occludes the pores efficiently. Large amounts of ooids are only present in the Volpriehausen Member in aeolian deposits (Fig. 2). The ooids have probably not been transported far as indicated by their well-preserved nature (Fig. 7A), contrary to the fluvially reworked ooids which have been crushed (Fig. 7B). Ooids are well known from the Bunter Shale and Bunter Sandstone Formations in the North German Basin. They occur both in in situ oolitic limestones, which are interpreted to have formed by migration of shoals along the lake margin, and as ooids that were mixed into offshore sand by reworking during storm events (Becker, 2005; Voigt et al., 2011). Ooids may form along the shoreline of saline lakes (e.g. Milroy and Wright, 2000; Ohlendorf et al., 2014). The ooids of the Volpriehausen Member were probably reworked either from such temporary deposits, at times when the lake was dry, or from oolitic deposits of the underlying Bunter Shale Formation. Transport and redeposition occurred by aeolian activity in a similar way as other components in the Bunter Sandstone Formation (Olivarius, 2015).

6.2. Anhydrite cement The nodules of anhydrite found in some of the claystone and sandstone intervals must have formed as evaporative gypsum cement shortly after deposition since they have displaced the surrounding sediment and are frequently found in association with desiccation cracks. The precipitation of nodular gypsum was caused by evaporation of pore fluids which percolated up through the sediment. The prismatic lath texture of anhydrite (Fig. 7E) shows that the nodules probably originally consisted of gypsum which dehydrated and thus transformed into anhydrite during burial (Clemmensen, 1985; Fine, 1986; Olsen, 1987; Laier and Nielsen, 1989). Besides transformation of gypsum into anhydrite, some of the patchy pore-filling anhydrite cement in the

147

sandstones formed during late diagenesis as shown by their textural relationship with other authigenic minerals. Anhydrite cement generally reduces porosity significantly (Fig. 10C). However, there are some exceptions where sediments have a high porosity despite a high content of anhydrite cement (Figs. 8, 9C); such cases result from anhydrite replacement of framework grains in combination with nodular anhydrite growth that displaces the adjacent framework grains. Anhydrite cement also reduces permeability (Fig. 10C), but the cement does not prevent flow completely since it occurs in confined patchy areas. Sulfate cement is dominant in the aeolian sandstones in the southern North German Basin (Beyer et al., 2014), and these results are opposite of the present study. This may be due to the coarser grain size in the southern part of the basin which promotes sulfate precipitation (Pape et al., 2005; Beyer et al., 2014). The finer grain size in the northern part of the basin is caused by the much longer transport distance of these aeolian sediments (Olivarius, 2015). The anhydrite abundance (Fig. 6) varies according to stratigraphy since the lowest amounts of anhydrite are present in the aeolian and sand flat sediments which belong to the Volpriehausen Member (Fig. 6A). It is unlikely that the fluids migrating upwards from the Zechstein evaporites during burial would have produced more anhydrite cement in the Solling Member than in the Volpriehausen Member (Fig. 6A), but an increase in the salinity of the brines in the Solling Member might have occurred when the overlying evaporitic Röt Formation was deposited (Fig. 1A). 6.3. Clay clasts Clay clasts may reduce porosity and permeability (Fig. 10D), but the two samples with clay clasts N 20% are not representative since both samples contain large amounts of inter-granular clay. The average permeability of about 550 mD in the sediments containing 5–20% clay clasts shows that the flow moves fairly easily around the clay clasts. This is because they are generally large and act as oversize detrital grains, as opposed to small inter-granular flow-obstructing constituents. Their fairly even distribution within the sediments, instead of being concentrated in layers, also means that the clay clasts do not block the flow. The fairly high permeability shows that disintegration of clay clasts has not occurred since even minor amounts of fines migrating into the surrounding sandstone would block or narrow some of the pore throats and thereby reduce the permeability significantly in the affected areas (e.g. Colter and Ebbern, 1978). 6.4. Inter-granular clay Large amounts of infiltration clay are generally associated with finegrained sediments and are therefore mostly found in the playa and sand flat depositional environments (Fig. 6B), resulting in rather low permeabilities and intermediate porosities (Fig. 5D). The infiltration clay settles between the framework grains at the time of deposition and thereby closes some of the pore throats. The occasional presence of meniscus cement indicates clay filtration in the vadose zone. Authigenic clays have formed sporadically and are unrelated to grain size. The porosity is reduced when inter-granular clay is present and the permeability is substantially reduced (Fig. 10E). This shows that intergranular clay has a large control on the reservoir quality and especially on the permeability since virtually no flow through the pore system is possible when inter-granular clay is present in amounts N 10%. Smaller amounts presumably also decrease the permeability considerably because the clay closes many pore throats. The porosity is fairly large relative to the very low permeability showing that many pores are still present even though they are largely sealed for flow. The low inter-granular volume of sediments containing large amounts of inter-granular clay (Fig. 10E) is caused by an underestimation

148

M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

30

A

B

Carbonate cement (%)

25 >10% carbonate cement >10% anhydrite cement

20

>10% clay clasts >10% inter-granular clay

15

10

5

0 40

C

D

E

F

Anhydrite cement (%)

35 30 25 20 15 10 5 0 8 7

Red coatings (%)

6 5 4 3 2 1 0 0

5

10

15

20

25

30

Porosity (%)

35 1

10

100

1000

10000

Permeability (mD)

Fig. 9. Relationship between the reservoir properties and the major authigenic phases. Contents N10% of cements and clays are indicated.

of porosity during point counting (Fig. 11) due to the presence of microporosity within the clays. The difference between core-analysis porosity and point-count porosity has also previously been attributed to microporosity (Dutton and Loucks, 2010). 6.5. Red coatings Samples characterized by thick red coatings typically have high porosities and permeabilities (Fig. 9E, F). Red coatings form under oxidizing conditions during early diagenesis due to precipitation of iron oxides/hydroxides. Their formation is initiated by precipitation of goethite needles on grain surfaces and the coatings are transformed into hematite during burial (Weibel, 1999; Weibel and

Grobety, 1999). Red coatings may to some extent have prohibited quartz and feldspar overgrowth, but the overgrowths would probably not have been pervasive anyway since the sediments are not deeply buried. Dissolution of heavy minerals has locally sourced the red coatings which are thickest in the pore spaces surrounding the dissolving grains. Large amounts of red coatings are present in the majority of the weakly cemented sandstones (Fig. 9E, F). Non-oxidized beds are often related to a fine grain size, whereas a coarser grain size results in higher permeability and the higher flow rates might then enhance the supply of Fe for goethite precipitation. Thus, the highly permeable sandstones obtain high contents of red coatings which help preserve the high permeability.

M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

A

100 µm

Cement + clay Porosity (%)

B

100 µm

C

C C

C

Permeability (mD)

Grain size Sorting (µm)

IGV

Number of

(%)

samples

<5 of each

29.1

2731

178

0.4

31.1

10

<10 in total

28.0

2375

174

0.4

31.3

14

<10 of each

28.3

1822

154

0.4

32.5

22

<20 in total

28.0

1742

153

0.4

33.7

27

<20 of each

26.3

1557

156

0.4

33.1

37

Carbonate

Porosity

Permeability

IGV

Number of

cement (%)

(%)

(mD)

Grain size Sorting (µm)

(%)

samples

0-5

23.6

1594

153

0.5

35.9

21

>5-10

23.3

1022

153

0.6

29.4

18

>10-20

27.0

779

122

0.5

33.9

9

>20

22.1

272

123

1.3

37.4

2

Anhydrite

Porosity

Permeability

Number of

(%)

(mD)

Grain size Sorting (µm)

IGV

cement (%)

(%)

samples

C C

(%)

149

C

C

D

E

100 µm

500 µm

100 µm

0-5

26.6

1377

148

0.6

31.0

34

>5-10

25.7

1233

130

0.3

35.1

4

>10-20

17.4

1008

164

0.6

34.3

>20

15.2

324

133

0.6

42.4

7

Clay clasts

Porosity

Permeability

Number of

(%)

(mD)

Grain size Sorting (µm)

IGV

(%)

(%)

samples

5

0-5

25.1

1562

150

0.5

34.7

35

>5-10

23.8

577

131

0.4

36.4

7

>10-20

19.9

544

162

0.6

28.4

6

>20

18.2

3

85

1.5

11.5

2

Inter-granular

Porosity

Permeability

Number of

(%)

(mD)

Grain size Sorting (µm)

IGV

clay (%)

(%)

samples

0-1

25.7

1566

165

0.4

34.3

34

>1-10

23.3

687

141

0.5

37.7

7

>10-20

21.4

18

90

0.8

27.0

4

>20

15.9

7

67

1.8

24.9

5

Fig. 10. Overview of the cement types and clay types that have the largest impact on the porosity and permeability. The average values of porosity, permeability, grain size and sorting are listed. Thin section micrographs with direct light. IGV is the inter-granular volume. B: Carbonate cement is designated by a ‘C’. Wells and sample depths: A, Tønder-4, 1667.60 m. B, Arnum-1, 1440.98 m. C, Tønder-5, 1738.86 m. D, Tønder-3, 1810.47 m. E, Hønning-1, 1633.53 m.

6.6. Halite cement Halite cement is present in some intervals where it has precipitated as a relatively late phase (Weibel and Friis, 2004) as no other authigenic phases have overgrown it. This could mean that much of the halite precipitated from the saline pore fluids after core recovery. Fine (1986) found examples of halite as an artifact from drilling mud. However, the presence of authigenic halite has been documented by Laier and Nielsen (1989) in the Tønder wells where it mainly occurs in the Solling Member. Therefore, the porosity and permeability may be overestimated in the cleaned samples from the Solling Member, but perhaps only locally as the reservoir properties of cleaned and uncleaned samples do not generally diverge much; they are only drastically reduced in the uncleaned samples from certain intervals which presumably contain halite cement.

The very low reservoir properties of some fluvial sandstones from the Solling Member sampled in the Tønder-4 well (Fig. 5) are not related to halite cement since the samples include both cleaned and uncleaned core plugs. The most fragile core plugs could not be cleaned as they would otherwise decompose so any possible halite has not been removed. The porosity and permeability might in these cases be underestimated if the halite was formed after core recovery. 6.7. Timing of authigenic phases Gypsum precipitates shortly after deposition and the conversion from gypsum to anhydrite is generally a fairly early event during burial (Murray, 1964; Hardie, 1967). However, it depends on several factors such as the thermal conductivity of the overlying deposits (Jowett et al., 1993). The precipitation of calcite and dolomite occurs in general

150

M. Olivarius et al. / Sedimentary Geology 321 (2015) 139–153

40

Point counted porosity %

y=x

30

y = 0.90x - 5.27 R² = 0.58

20

10

0 0

10

20

30

40

Measured porosity % Fig. 11. Core-analysis porosity compared to point-count porosity. In general, point counting results in an underestimate of porosity.

also early after deposition (Wood and Boles, 1991; Hendry et al., 1996; Girard et al., 2002) and they form readily from concentrated groundwater in an arid environment. Thick red coatings are most abundant in sandstones with low amounts of carbonate cement and especially of gypsum/anhydrite cement, and this is in accordance with the very early precipitation of these pore-filling cements. The increase in the salinity of pore fluids during evaporation results in a fixed sequence of mineral precipitations including calcite, dolomite and finally gypsum (Schmid et al., 2006). This could explain why carbonate cement is present in virtually all sandstones, why calcite is more abundant than dolomite and why anhydrite cement occurs in only some of the sandstones. Hence, gypsum could only precipitate if the salinity of the pore fluid became sufficiently concentrated with sulfate which has possibly only happened in some of the sediments. This may be the reason why abundant carbonate cement is never found in combination with a large amount of anhydrite cement. Alternatively, it could be the result of local variations in the sulfatesaturation of the groundwater. Late diagenetic quartz cement is of low abundance and has little influence on the porosity and permeability. 6.8. Reservoir properties in relation to depositional environments Aeolian sandstones provide the highest initial porosity and permeability due to their high degree of sorting and low degree of packing (Beard and Weyl, 1973). The high detrital quartz content in combination with red coatings that retard overgrowths and the low contents of inter-granular clay and anhydrite cement (Fig. 6B) have resulted in aeolian sediments remaining unconsolidated during burial. The best reservoir quality in the southern part of the basin is also present in aeolian sandstones followed by fluvial, sand flat and finally lacustrine sandstones (Beyer et al., 2014). The porosities and permeabilities are lower than in the present study due to deeper burial prior to structural inversion, but otherwise are the results comparable. Fluvial redeposition of aeolian sediments was a common process in the North German Basin when the Bunter Sandstone Formation was deposited (Mader, 1982; Clemmensen, 1985; Uličný, 2004). The rather large amounts of ooids in some of the ephemeral fluvial sediments in the Volpriehausen Member combined with low mica content and high quartz content show that the sediments are partly reworked from aeolian deposits. Therefore, the ephemeral fluvial deposits in the Volpriehausen Member are high quality reservoir rocks since they

have inherited some of the characteristics of their aeolian sediment source. The abundant clay clasts and patchy anhydrite cement in the medium-grained, ephemeral fluvial sandstones in the Solling Member in the Tønder-3 well have probably reduced porosity more than permeability (Fig. 5). Grain size is related to depositional environment and provenance and it influences permeability since the very fine-grained sandstones overall have lower permeability than the trendline for all sandstones whereas the medium-grained sandstones have permeabilities above average (Fig. 5C). The rather low porosity and permeability of the shoreline depositional environment (Fig. 5D) are presumably associated with the high anhydrite content (Fig. 6B). The sediments in the sand flat depositional environment have a steeper porosity–permeability trend than the general trendline of the examined dataset (Fig. 5D). The rather high porosity in the sand flat sediments may be caused by a small content of anhydrite cement in combination with high clay content (Fig. 6B). However, the low point-count porosity shows that much of the porosity measured by core analysis is microporosity within the clay (Table 2) and explains why it is associated with low permeabilities. The porosity of the playa claystones (Fig. 5C, D) is a little lower than the average trend for claystones in general (Mondol et al., 2007) since the claystone intervals in the Bunter Sandstone Formation contain rather large amounts of quartz and feldspar. High porosities in claystones at shallow burial depths are normal, because of the high initial porosity which is up to twice as high as in the sandstones, whereas the permeability is low in claystones (e.g. Yang and Aplin, 1998; Kwon et al., 2001; Mondol et al., 2008). Playa siltstones and sandstones usually contain large amounts of inter-granular clay (Fig. 6B) similar to the playa claystones. Thus, in spite of their fairly high and uniform porosities, the sandstone intervals in the playa successions are considered poor reservoir rocks due to the thin bedding, the high content of clay and the low permeabilities caused by a dominant very fine grain size.

6.9. Reservoir properties in relation to burial depth The center of the North German Basin has subsided continuously except for a mid-Cretaceous uplift event, so the Bunter Sandstone Formation is present at depths of N3 km (Senglaub et al., 2005; Bachmann et al., 2010). The remaining mid-Jurassic to Cenozoic uplift events mainly affected the marginal basin areas which are less deeply buried, so the porosities and permeabilities are significantly lower in the basin center than in the marginal areas (Senglaub et al., 2005; Japsen et al., 2007; Wolfgramm et al., 2008). Late Jurassic uplift caused local leaching of anhydrite in the southwestern part of the North German Basin resulting in good reservoir quality in a limited area (Gdula, 1983; Bruijn, 1996). High porosities and permeabilities comparable to those in the study area are present further southeast along the northern margin of the North German Basin (Wolfgramm et al., 2008). The extensive red coatings of the studied sandstones are assumed to inhibit or retard quartz diagenesis at increased burial. However, reduced porosity and permeability due to dissolution at grain-to-grain contacts and quartz precipitations are observed in the Bunter Sandstone Formation in other parts of the North German Basin (Wolfgramm et al., 2008; Beyer et al., 2014) and from the Iberian Range (Marfil et al., 1996). Local divergence from the general trend could be due to dehydration of gypsum in the underlying Zechstein deposits. This process generates overpressure and produce fluids that precipitate anhydrite if they are released into the overlying Bunter Sandstone Formation (Nollet et al., 2005). Overpressure is considered to retard diagenetic alteration by limiting inter-granular pressure solution and thereby eliminating an important silica source for quartz cementation (e.g. Ramm, 1992; Osborne and Swarbrick, 1999).

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6.10. Reservoir quality An overall correlation exists between porosity and permeability in the sandstone intervals of the Bunter Sandstone Formation (Fig. 5C) and the scatter along the trendline is caused by the varying amounts of cements and clays (Figs. 8, 10). The deviations from the trendline are related to depositional environment, climate and provenance since these parameters define the contents of carbonate cement, anhydrite cement, clay clasts and inter-granular clay and the grain size. The porosity is linked primarily to the dominant cement or clay type (Fig. 10B–E). The permeability is controlled by how the distribution of this cement or clay type affects the flow conditions, because some of the cement and clay types cause isolation of pore spaces. The Volpriehausen Member has excellent reservoir quality since the sandstone intervals consist of largely clay-free, weakly cemented aeolian deposits (Fig. 6A), except for certain intervals where carbonate cement has reduced the permeability, but the reservoir quality remains good (Fig. 5A, C). The aeolian deposits of the Volpriehausen Member have a wide lateral and fairly constant vertical continuity (Fig. 3A) (Bachmann et al., 2010; Olivarius, 2015). Therefore, the Volpriehausen Member constitutes a suitable reservoir for geothermal exploitation in the northern North German Basin, also because it is situated at depths where the temperature of the formation water is sufficient for geothermal energy production (55–60 °C; Balling et al., 1981). The best reservoir quality is also present in the aeolian intervals of the time-equivalent Sherwood Sandstone in the Irish Sea (Cowan, 1993; Meadows and Beach, 1993) and the Buntsandstein in the southern North German Basin (Beyer et al., 2014). On the contrary, the Buntsandstein in the western North German Basin revealed deterioration of reservoir permeability due to preferential groundwater flow and associated cementation in the aeolian intervals (Purvis and Okkerman, 1996). The excellent reservoir properties in the Volpriehausen Member are related to several factors: Infiltration clay was not deposited by the aeolian processes; cementation by gypsum/anhydrite was limited; and the long transport distance from the southern provenance resulted in a mature mineral assemblage. The ephemeral fluvial deposits also have good reservoir quality in the Volpriehausen Member, since they mostly consist of reworked aeolian deposits. However, the reworking has lowered the porosity and permeability to some extent due to reduced sorting (Table 2) and improved packing caused by broken ooids (Fig. 7B). The porosity and permeability of the Solling Member are high in most sandstone intervals (Fig. 5A, C). The content of anhydrite cement is generally high (Fig. 6A) meaning that both the porosity and permeability are reduced (Fig. 10B, C). However, the reservoir quality is still fairly good in the anhydrite cemented sandstones since it was initially excellent, excluding the local pervasive cements that have caused drastically reduced reservoir properties in some intervals of the Tønder-4 well (Fig. 5B). The presence of cementing halite (Laier and Nielsen, 1989), the abundant clay in some intervals and the relatively unpredictable lateral and vertical continuity of the sandstones due to the dominance of ephemeral fluvial deposits (Fig. 3B) (Clemmensen, 1985; Olivarius, 2015) have a negative influence on reservoir quality in the Solling Member. Thus, the Volpriehausen Member represents the most suitable reservoir for geothermal exploitation in the Bunter Sandstone Formation, but the Solling Member is presumably also a valuable reservoir in some areas. 7. Conclusions The mineralogical differences that exist between the members of the Bunter Sandstone Formation are related to the detrital composition, the climatic conditions and the depositional environments. Carbonate cement is virtually always present, but in small amounts so its effect on porosity is minor. Anhydrite cement is present in larger amounts so it lowers the porosity more, except where it has displaced

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and replaced framework grains. Large amounts of anhydrite cement are only present in samples containing little carbonate cement and thin red coatings. Red coatings are associated with the best reservoir intervals. Both carbonate and anhydrite cement reduce permeability, but not severely as long as they are present in confined pore-filling patches or nodules. The clay clasts function as grains, but they reduce porosity and permeability since they are large and easily deformable. Flow through the sandstones is effectively obstructed in the intervals where inter-granular clay or pervasive carbonate, anhydrite or halite cement are present, a situation that primarily defines the Solling Member. Pervasive cementation lowers the porosity to b7% whereas fairly high porosities are present in the sandstones with a high content of inter-granular clay due to its flow-ineffective microporosity. The inter-granular clay is primarily present as infiltration clay on grain surfaces, but may also occur as authigenic clay that has grown into the pore spaces. The Volpriehausen Member has excellent reservoir quality in the sandstone intervals since they consist dominantly of laterally extensive aeolian deposits that are well-sorted, almost clay-free and contain low amounts of anhydrite cement. Thus, the vast majority of the aeolian sandstones have porosities of N 17% and permeabilities of N100 mD. Most of the ephemeral fluvial sandstones in the Volpriehausen Member are reworked aeolian deposits and the high porosities and permeabilities have been preserved to some extent. The reservoir quality of the sandstones in the Solling Member is generally good, but less favorable than in Volpriehausen Member. This is because the Solling Member comprises mainly ephemeral fluvial sandstones that contain more cement and clay than the reworked aeolian sandstones in the Volpriehausen Member. The depositional mechanism and the local provenance area are responsible for the poorer sorting and higher content of clays and rock fragments in the Solling Member than in the Volpriehausen Member. Thus, the depositional environment and the provenance are the main controlling factors of the reservoir properties. Acknowledgments The Danish Council for Strategic Research is thanked for the financial support. Lisbeth L. Nielsen, Marga Jørgensen, Hans J. Lorentzen, Nanna Rosing-Schow and Nikolaj F. Petersen are thanked for plugging the samples and performing the conventional core analyses at the GEUS Core Analysis Laboratory. Jon Ineson is thanked for the linguistic polishing of the manuscript. Editor Brian Jones, reviewer Reinhard Gaupp and an anonymous reviewer are thanked for the valuable advice on how to improve the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.sedgeo.2015.03.003. These data include Google map of the most important areas described in this article. References Aigner, T., Bachmann, G.H., 1992. Sequence-stratigraphic framework of the German Triassic. Sedimentary Geology 80, 115–135. American Petroleum Institute, 1998. API RP 40. Recommended Practices for Core Analysis. Second edition. American Petroleum Institute, Washington DC. Bachmann, G.H., Geluk, M.C., Warrington, G., Becker-Roman, A., Beutler, G., Hagdorn, H., Hounslow, M.W., Nitsch, E., Röhling, H.-G., Simon, T., Szulc, A., 2010. Triassic. In: Doornenbal, H., Stevenson, A. (Eds.), Petroleum Geological Atlas of the Southern Permian Basin Area. European Association of Geoscientists and Engineers Publications, pp. 149–173. Balling, N., Kristensen, J.I., Breiner, N., Poulsen, K.D., Rasmussen, R., Saxov, S., 1981. Geothermal measurements and subsurface temperature modelling in Denmark. GeoSkrifter 16 (172 pp.). Beard, D.C., Weyl, P.K., 1973. Influence of texture on porosity and permeability of unconsolidated sand. The American Association of Petroleum Geologists Bulletin 57, 349–369.

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