sandstone ratio control on carbonate cementation and reservoir quality in Upper Permian Rotliegend sandstones, offshore the Netherlands

Journal Pre-proof Mudstone/sandstone ratio control on carbonate cementation and reservoir quality in Upper Permian Rotliegend sandstones, offshore the Netherlands Johannes M. Miocic, Jean-Pierre Girard, Robert Schöner, Reinhard Gaupp PII:

S0264-8172(20)30076-3

DOI:

https://doi.org/10.1016/j.marpetgeo.2020.104293

Reference:

JMPG 104293

To appear in:

Marine and Petroleum Geology

Received Date: 1 August 2019 Revised Date:

9 February 2020

Accepted Date: 10 February 2020

Please cite this article as: Miocic, J.M., Girard, J.-P., Schöner, R., Gaupp, R., Mudstone/sandstone ratio control on carbonate cementation and reservoir quality in Upper Permian Rotliegend sandstones, offshore the Netherlands, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/ j.marpetgeo.2020.104293. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit Author Statement

Johannes Miocic: Writing Original Draft, Review & Editing, Visualisation, Formal analysis Jean-Pierre Girard: Investigation, Supervision, Formal analysis, Writing – Review & Editing, Resources, Project administration Robert Schöner: Supervision, Conceptualisation, Writing – Review & Editing Reinhard Gaupp: Project administration, funding acquisition, Supervision, Writing – Review & Editing, Conceptualization

1

Mudstone/sandstone ratio control on carbonate cementation and reservoir

2

quality in Upper Permian Rotliegend sandstones, offshore the Netherlands

3 4

Johannes M. Miocic 1,2*, Jean-Pierre Girard3, Robert Schöner1,4, Reinhard Gaupp1

5 6

1

7

07749 Jena, Germany

8

2

9

Freiburg, Albertstr. 23b, 79104 Freiburg, Germany

Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Burgweg 11,

Institut für Geo- und Umweltnaturwissenschaften, Albert-Ludwigs-Universität

10

3

11

4

12

Germany

13

*corresponding author: [email protected]

Total E&P, Scientific & Technical Center , Pau, France Landesamt für Bergbau, Energie und Geologie, Stilleweg 2, 30655 Hannover,

14 15

Abstract

16

The eolian-fluvial sandstones of the Upper Permian Rotliegend formation, which

17

were deposited in the Southern Permian Basin, are today deeply buried (~3-4 km)

18

and constitute important gas reservoirs in the Netherlands and the southern North

19

Sea. The reservoir properties of the sandstones have been documented to be

20

strongly affected by diagenesis, but the primary diagenetic factors impairing reservoir

21

quality and their cause remain variably interpreted in the literature. Here, we present

22

the results of a detailed investigation on the diagenetic processes controlling

23

reservoir quality in the Lower Slochteren formation in the L and K blocks offshore the

24

Netherlands, where fluvial and aeolian sandstones intercalate with playa lake muds

25

in a delta setting. Quantitative analysis of the diagenetic mineral phases occurring in

1

26

all main depositional facies (eolian, fluvial, playa-lake) was carried out on more than

27

200 samples from 21 wells, with the authigenic mineral composition of an additional

28

500 samples being evaluated qualitatively. The integration of petrographical

29

observations

30

distribution/abundance is not dominantly driven by depositional facies, nor are

31

reservoir properties. Early pore-filling dolomite cement can be as high as 40% and

32

represents the main control on reservoir properties. Detailed analysis of its spatial

33

distribution shows it to be distinctly related to mudstone proximity and

34

mudstone/sandstone (M/S) ratio. Sandstones occurring as thin beds in mudstone-

35

rich depositional sequences (high M/S ratio) typically exhibit strong pervasive

36

carbonate cement regardless of sedimentary facies. In contrast, sandstones forming

37

thick beds in mudstone-poor sequences (low M/S ratio) are commonly free/low in

38

dolomite cement. Our results demonstrate, for the first time, that reservoir quality in

39

the Rotliegend sandstones in the delta setting of the Netherlands is primarily

40

controlled by early dolomite cement. The latter is most developed in areas with high

41

(>70%) vertical M/S ratio, where it may be a devastating factor for reservoir quality.

42

Best reservoir sandstones should be expected where the depositional stacking

43

pattern is poor in shaly deposits (playa-lake and distal sheet-flood sediments), such

44

as in the southern/ south-eastern part of the studied area.

with

log

data

and

core

descriptions

reveals

that

cement

45 46 47

Keywords: Reservoir quality, diagenesis, Rotliegend, Permian, Southern Permian

48

Basin, carbonate cements, mudstones, sandstones

49 50

2

51

1. Introduction

52

Upper Permian (Rotliegend) eolian and fluvial sandstones that were deposited at the

53

southern margin of the Southern Permian Basin (SPB) constitute important

54

petroleum reservoirs in the Netherlands, in northern Germany and in the southern

55

North Sea. The reservoir properties of these moderately deeply buried (~3-4 km)

56

sandstones are strongly affected by diagenetic processes (Glennie et al., 1978;

57

McNeil et al., 1995; Molenaar and Felder, 2019; Purvis, 1989). It has been proposed

58

that early diagenesis is mainly related to depositional facies, while the controls on

59

burial diagenesis may be complex and are far more difficult to unravel, especially in

60

the deeply buried parts of the basin (Amthor and Okkerman, 1998; Gaupp and

61

Okkerman, 2011). Influx of external fluids derived from lithologies that are

62

compositionally different from the Rotliegend sandstones may play an important role

63

during burial diagenesis. Organic rich fluids originating from the underlying

64

Carboniferous have been identified to cause a number of diagenetic reactions in the

65

Rotliegend, including bleaching, feldspar dissolution and kaolinite/illite growth

66

(Gaupp et al., 1993; Goodchild and Whitaker, 1986; Schöner and Gaupp, 2005;

67

Ziegler, 2006). Fluids originating from the overlying Zechstein evaporites have been

68

invoked as possibly responsible for much of the carbonate and sulphate burial

69

cements present in the Rotliegend sandstones (McNeil et al., 1998; Purvis, 1992;

70

Sullivan et al., 1994). Mudstones of the central Rotliegend playa-lake (Silverpit

71

Formation) are considered to have expelled Mg-rich fluids that caused chlorite

72

growth in lake-margin sandstones (Gaupp et al., 1993). Mudstones also commonly

73

occur interbedded with sandstones in the fluvial and mixed fluvial/eolian settings of

74

the Rotliegend, and a possible cause to significant diagenetic reactions may be

75

diffusive mass transfer driven by chemical gradient between adjacent shale and

3

76

sandstone as documented by (Boles and Franks, 1979; Ma et al., 2019; Thyne,

77

2001; Xi et al., 2019). In a recent study, Molenaar and Felder (2019) proposed that

78

the extensive dolomite cementation observed in the Rotliegend sandstones may be

79

controlled by the presence of previously overlooked dispersed detrital carbonate

80

grains. Overall, the quantitative impact of diagenetic controls on the reservoir quality

81

of the Rotliegend sandstones remains poorly understood.

82

In the present study we investigate a sandstone-mudstone series from the Lower

83

Slochteren formation of the Rotliegend sandstones offshore the Netherlands,

84

sampled from cores from petroleum exploration/production wells (Total and GDF

85

Suez/Engie). Based on quantitative petrographic and geochemical investigations,

86

coupled with petrophysical, well log and sedimentological data, we document the

87

diagenetic sequence and demonstrate that early carbonate cementation is the

88

diagenetic event most impacting reservoir quality in the Rotliegend sandstones. We

89

further discuss and provide a quantitative assessment of the controlling influence of

90

the mudstone/sandstone ratio on the development of carbonate cementation

91

processes in the study area. Our model of factors controlling carbonate cementation

92

and reservoir quality in the Rotliegend is significantly different from the one

93

suggested by Molenaar and Felder (2019).

94 95

2. Regional setting

96

This study investigates 21 wells that are located on the Cleaver Bank High in the

97

Southern North Sea, offshore the Netherlands (Fig. 1), where the Rotliegend is

98

presently buried to depths between 3600 m and 4100 m. It unconformably overlies

99

Upper Carboniferous shallow marine to continental sediments and is overlain by Late

100

Permian marine evaporites (Zechstein) (Figs. 2 & 3). During the Rotliegend, the

4

101

study area was located at the south-western margin of the Southern Permian Basin,

102

where fluvial fairways from the south transported clastic material towards the basin

103

center. The study area is located in the fluvial delta of the western fluvial axis where

104

sand was transported into the Silverpit Lake. Eolian deposition took place in between

105

the braided fluvial systems and, depending on the groundwater-table, dry or wet

106

dune deposits and inter-dune deposits developed. Wet sand flat deposits and

107

mudflats accumulated closer to the shore line of the central playa-lake, which

108

accommodated claystones and evaporites (Geluk, 2007; Mijnlieff and Geluk, 2011;

109

Minervini et al., 2011).

5

110 111

Figure 1: Map showing distribution of eolian, fluvial, and playa-lake facies within the Southern

112

Permian Basin during the Rotliegend deposition (after Mijnlieff and Geluk, 2011). The study area,

113

indicated by the black box, is located in the southern end of the playa lake, where eolian and distal

114

fluvial facies intercalate with playa deposits. White dots indicate the location of the studied wells.

115

The study area is located close to the northern sand limit where the interaction of

116

expansion and retreat of the playa-lake / mudflat created a complex intercalation of

117

lacustrine, eolian and fluvial environments (Gaupp et al., 2000; Legler et al., 2005;

118

Mijnlieff and Geluk, 2011). Towards the basin center the intercalation of mudstones

119

increases and sandstones pinch out. A complete description of the depositional

120

environment and the sedimentary facies types can be found in Gast (1991) and 6

121

Fryberger et al. (2011). In this study a simplified classification (Fig. 2) comprising five

122

major sedimentary facies was used following Lafont et al. (2000). Facies 1 (F1)

123

corresponds to playa mudflat and lake shales, facies 2 (F2) to distal fluvial sand

124

sheet flood (terminal fan) and starved lake margin sands, facies 3 (F3) to proximal

125

fluvial sands, facies 4 (F4) to damp eolian sands, and facies 5 (F5) to dry eolian

126

sands. The subcroping Carboniferous rocks are of Westphalian B to Stephanian age

127

and represent a more and more continental setting. While the Westphalian B

128

contains mainly fluvial sediments with dominant floodplains and swamps with

129

extensive peat development, the Westphalian C is dominated by fluvial channel

130

deposits which progressively become more red-bed dominated in the Westphalian D

131

and the (Kombrink et al., 2010).

132 133

Figure 2: Block diagram illustrating the depositional facies (F1 to F5) of the Lower Slochteren

134

Sandstone Formation used in this study. After Lafont (2000) and Geluk (2007).

135

A general burial history applicable to the study area is shown in figure 3 (after Girard

136

et al. (2008)). It illustrates three main phases: (1) a progressive subsidence from the 7

137

time of deposition through Jurassic times, (2) a significant uplift (~1500 m) starting in

138

the Late Jurassic and continuing during the early Cretaceous and (3) a more rapid

139

subsidence during late Cretaceous and Cenozoic times. In all wells studied, present-

140

day burial depth represents maximum burial depth, and are very consistent

141

throughout the study area, ranging from 3.5 to 4.0 km. The majority of samples come

142

from the gas-bearing leg of the reservoir, a few are from the underlying aquifer.

143 144

Figure 3: General burial history of the study area. Temperature estimates are based on unpublished

145

vitrinite reflectance data.

146 147

3. Methods

8

148

Results are based on petrographic observations, well-log, sedimentological and

149

petrophysical data of 21 wells on the Dutch Cleaver Bank High area (Fig. 1). In total

150

700 thin-sections were examined qualitatively. The results of quantitative

151

petrographic investigations for porosity, detrital mineralogy and authigenic minerals

152

by point-counting (300 points) 208 thin-sections were extracted from unpublished

153

internal reports (Felder, 2009; Girard et al., 2008). Additional specific petrographic

154

investigations (see below) were conducted on a selection of samples from a

155

particular well regarded as the reference well of the study. It shows all the significant

156

diagenetic transformations documented in the Rotliegend sandstones of the study

157

area. Fourteen thin-sections from the reference well were studied for cement

158

generations and textural relationships by electron microprobe (EMP) and scanning-

159

electron-microscopy

160

cathodoluminescence (CL) mode. Additional CL measurements were carried out

161

using a hot cathode CL microscope at the University of Freiberg (Neuser et al.,

162

1995). Carbonate cement compositions were quantitatively analyzed using the

163

wavelength dispersive X-ray spectroscopy (WDX) mode of the EMP. Gold-coated

164

rock chips were examined under (SEM) in order to investigate leached grains and

165

textural relationships.

166

Nine sandstone and three mudstone samples from the reference well were chosen

167

for analysis of their oxygen and carbon stable isotope composition. Preferred

168

sandstone samples contained high carbonate contents (7-30 vol.-%) and at the best

169

only one dominant type of carbonate cement. However, as bulk rock samples were

170

analysed, some mixing of the different carbonate cement generations could not be

171

avoided. Based on EMP mineral maps the amount of each type of carbonate cement

172

(siderite, Fe-dolomite, ankerite) was estimated. Bulk rock powders were analyzed for

(SEM)

using

back-scatter-electron

9

(BSE)

and

173

O and C isotopic composition of the bulk carbonate by reacting the powder with

174

100 % phosphoric acid at 70°C using a Gasbench II c onnected to a ThermoFinnigan

175

Five Plus masspectrometer. The values are reported in per mil relative to V-PDB by

176

assigning a δ13C value of +1,95‰ and a δ18O value of -2,20‰ to the standard

177

NBS19, and corrected using the phosphoric acid fractionation factors by Rosenbaum

178

and Sheppard (1986). Analytical reproducibility derived from replicate analysis of

179

laboratory standards is better than ±0,06‰ (1σ).

180

Porosity and permeability data of 2000 core plugs from the 21 studied wells

181

measured by standard techniques (porosity by helium injection; permeability with

182

nitrogen as a fluid) were provided by the operators (unpublished data from Total E&P

183

and GdF-Suez E&P).

184

For facies analysis the simplified definitions by Lafont et al. (2000, see above) were

185

used. In each well, vertical distribution of lithologies was established by attributing a

186

“mudstone” or “sandstone” lithology at a cm-scale, based on available internal core

187

descriptions. Mudstone-sandstone ratios (M/S ratio) were then calculated using two

188

sizes of sliding-windows (2 m and 4 m windows; see Fig. 3) for the depth intervals of

189

interest. The M/S ratios are given as fractions, where 1 represents only mudstone

190

beds within the sliding-window and 0 only sandstone beds. The M/S ratios can be

191

illustrated similar to wireline logs.

10

192 193

Figure 4: Image illustrating the sliding window concept. The thickness distribution of sandstones and

194

mudstones in a window of 2 m (or 4 m) is used to calculate mudstone/sandstone (M/S) ratios. High

195

M/S ratios indicate high amounts of mudstone within the sampling window while low M/S ratios

196

represent a high volume of sandstone. Two examples, A and B, with different stratigraphic

197

successions are shown together with their M/S ratios for the 2m and 4m sliding window.

198

4. Results

199

4.1 Sedimentology of the Lower Slochteren and reservoir properties

200

In most of the studied wells all five depositional facies occur, with the Lower

201

Slochteren formation having an overall varying thickness ranging from less than 10

202

to more than 40 m. Independently of the depositional facies the sandstones are often

203

structureless, with only few sections being laminated or cross bedded. Individual 11

204

sandstone beds rarely have thicknesses exceeding 2 m with the majority of beds

205

showing thicknesses of around 1 m or less. Thick sandstone beds tend to be

206

associated with proximal fluvial (F3) or dry eolian (F5) depositional settings. Stacked

207

sandstone beds of more than 10 m thickness are uncommon, with interbedded

208

mudstones (F1) being present or frequent in all studied wells.

209

A traditional porosity-permeability plot is shown in figure 5 for the samples studied

210

illustrating that porosity-permeability values follow one single trend, are highly

211

variable (porosity from 1 to 20%, permeability from 0.01 to close to 1000 mD) and

212

largely overlap among the different depositional facies. All sandy facies (i.e. F2, F3,

213

F4 and F5) can show good or poor reservoir quality, with F5 (dry eolian sand)

214

showing the highest porosity-permeability values (porosity >16%, permeability

215

>100mD). Not surprisingly, the F1 facies tends to exhibit poor reservoir quality

216

(porosity <10%, permeability <1mD) due to its shaly nature. These observations

217

indicate that depositional facies is not the prime control on reservoir quality in the

218

studied samples.

219

12

220 221

Figure 5: Porosity-permeability plot of all studied samples, illustrating that all samples follow a single

222

trend and that values are highly variable. Note that all sandy facies (F2-F5) can exhibit good and poor

223

reservoir quality. Data plotted includes both the qualitatively and quantitatively analysed thin-sections.

224

All depositional facies show a wide range of overlapping M/S ratios, with eolian

225

samples (F4 and F5) having lower M/S ratios than fluvial (F2 and F3) sandstones

226

(Tab. 1, Fig. 6). Based on data displayed in Fig. 6, the depositional facies seem to

227

show some relation to the porosity of the studied sandstones, as the average

228

porosity increases from ∼6% in distal fluvial sandstones (F2) to ∼10% in proximal

229

fluvial (F3) and wet eolian (F4) sandstones, and up to ∼14% in dry eolian

230

sandstones (F5). However, as indicated above, there is a very significant overlap of

231

porosity ranges among the different sand (F2 to F5) facies (Figs. 5 and 6) which can

232

have poor or good reservoir quality. There is some suggestion that the M/S ratio may 13

233

be playing a role in this variation, as high M/S are limited to low porosity values and

234

only low M/S ratios extend to high porosity (Fig. 6). This will be further illustrated and

235

discussed in the rest of this article.

236 237

The studied samples are very fine to medium grained sandstones. Grain size ranges

238

mainly between 60 to 300 µm, with a net predominance of fine-grained sandstones

239

(120-250 µm). Grain size is not discriminant of depositional facies in the studied

240

sands. Generally, dry eolian sands (F5) tend to be more fine- to medium-grained

241

while finer sands are often associated with a distal fluvial setting (F2), but significant

242

overlaps in grain-size ranges exist between the different sedimentary facies. In terms

243

of sorting, distal (F2) and proximal (F3) fluvial sandstones are highly variable,

244

ranging from poor to well sorted, while wet (F4) and dry (F5) eolian sandstones tend

245

to be moderately well to well sorted. However, there is no distinct relationship

246

between grain size and sorting, regardless the depositional environment. Average

247

detrital clay content decreases from ∼6% in distal fluvial sandstones (F2) to ∼3% in

248

distal fluvial (F3) and wet eolian (F4) sandstones and ∼2 % in dry eolian sandstones

249

(F5) (see Tab. 1).

250

14

251 252

Figure 6: Plot of porosity vs M/S ratio illustrating how the wide range of porosity values found for each

253

depositional facies can be explained by the vertical distribution of mudstones. Data plotted includes

254

both the qualitatively and quantitatively analysed thin-sections.

255 256

4.2 Detrital composition of the Lower Slochteren sandstones

257

The sandstones are dominated by mono- and polycrystalline quartz (40-70 vol.-%).

258

Unstable rock fragments (2-21 vol.-%) comprise, with decreasing frequency, felsic

259

volcanic rock fragments, fragments of sandstones and siltstones, of metapelites and

260

metapsammites and of mudstones. Feldspars, both plagioclase and K-feldspar, are

261

very rare (generally <1 vol.-%) and typically show alteration and replacement by

262

kaolinite and carbonate. Detailed point-counting of alteration and replacement

263

features in samples of the reference well indicate that the original feldspar content 15

264

was significantly higher (3-12 vol.-%). Preserved carbonate clasts/grains of

265

unambiguous detrital origin are basically absent in the studied samples (only a few

266

specimens found in the ∼700 thin sections examined), in contrast to Molenaar and

267

Felder (2019). The studied samples classify, according to their present day

268

composition, as quartzarenite and sublitharenite and subordinate litharenite, and

269

detrital composition is similar for all investigated facies and throughout the

270

stratigraphy as illustrated in the QFL sandstone classification diagram of McBride

271

(1963) (Fig. 7). Grain contacts are typically point-to-point contacts and more rarely

272

long-to-long contacts. Concavo-convex or sutured contacts were not encountered.

273 274

Figure 7: QFL diagram after McBride (1963) illustrating the detrital composition of the Lower

275

Slochteren sandstone in the studied samples. Most sandstones are quartzarenites and

16

276

sublitharenites. The reconstructed depositional composition of the reference well highlights that the

277

original amount of feldspar was higher, shown with crosses are only samples from the reference well.

278

4.3 Diagenetic mineralogy in the Lower Slochteren sandstones

279

The main diagenetic (authigenic) minerals observed in the studied Rotliegend

280

sandstones include carbonates, quartz, kaolinite/dickite and iron oxide (hematite)

281

coatings. Anhydrite cement can be abundant locally, and rare sporadic occurrences

282

of fibrous illite, pyrite and barite are documented. The diagenetic sequence derived

283

from petrographical observations is shown in figure 8, volumetric ranges of the

284

diagenetic phases are listed in table 1. The range of the main diagenetic phases for

285

each depositional facies is illustrated in figure 9.

286 287

Figure 8: Comprehensive sequence of diagenetic processes and products observed in the Rotliegend

288

sandstones in the study area.

17

289

Table 1: Summary data table showing the minimum, maximum and average abundance of diagenetic mineral phases, detrital components, sedimentological

290

and reservoir quality parameters per depositional facies of the Lower Slochteren sandstone samples studied quantitatively. F2 Min

F3 Average

Max

n

Min

F4 Average

Max

n

Min

F5 Average

Max

n

Min

Average

Max

n

Diagenetic phases Dolomite (%)

0.0

6.1

30.7

40

0.3

4.9

16.7

36

0.3

4.4

9.7

6

0.0

3.5

17.0

51

Fe-Dol./Ankerite (%)

0.0

9.4

34.7

56

0.7

8.9

34.0

54

0.3

5.0

13.0

7

0.0

4.4

39.0

67

Siderite (%)

0.0

1.1

4.0

46

0.0

2.2

9.0

47

0.3

1.7

5.7

6

0.0

2.0

10.0

59

Total Carbonates (%)

1.3

17.1

39.0

57

0.3

12.5

35.0

61

1.0

8.0

14.0

7

0.7

8.2

39.0

78

Quartz (%)

0.7

6.4

15.0

55

0.3

4.8

10.0

54

1.0

7.1

17.7

7

1.7

7.7

17.0

71

Kaolinite (%)

0.0

0.9

6.5

54

0.0

1.1

6.3

54

0.0

0.5

1.6

7

0.0

1.1

10.6

72

Dickite (%)

0.0

2.2

13.2

57

0.0

2.5

15.0

54

1.6

2.8

5.9

7

0.0

3.0

8.4

72

Hematite coatings (%)

0.0

1.8

6.7

50

0.0

1.2

7.7

52

0.0

1.1

2.0

7

0.0

1.1

4.3

72

Anhydrite (%)

0.0

0.5

4.7

29

0.0

2.2

10.7

19

0.3

3.8

10.7

3

0.0

1.8

21.0

29

Detrital components Quartz (% of QFL)

67.2

85.5

97.6

47

78.5

86.9

96.7

47

77.9

86.6

98.0

6

70.7

87.9

97.0

68

Feldspar (% of QFL)

0.0

0.2

1.7

47

0.0

0.3

1.7

47

0.0

0.3

1.4

6

0.0

0.3

1.9

68

Lithoclasts (% of QFL)

2.4

14.2

32.8

47

2.8

12.8

20.5

47

1.6

13.1

22.1

6

1.7

11.8

29.3

68

Detrital clay (%)

0.0

5.5

32.8

57

0.0

3.2

16.7

61

1.0

3.3

7.0

9

0.0

2.2

15.7

78

Sedimentological parameters

18

Mean grain size (µm)

52

133

237

29

72

176

302

47

69

140

204

6

56

177

377

58

Sorting (phi std. dev)

0.45

0.65

1.03

29

0.42

0.66

1.03

47

0.57

0.67

0.81

6

0.45

0.64

0.88

58

M/S 2m

0.0

0.4

1.0

57

0.0

0.3

1.0

61

0.0

0.3

1.0

9

0.0

0.3

1.0

78

M/S 4m

0.1

0.5

1.0

57

0.0

0.4

1.0

61

0.0

0.5

1.0

9

0.0

0.4

1.0

78

Reservoir Quality Permeability (mD)

0.0

0.5

9.9

47

0.0

4.6

52.1

57

0.0

9.1

50.6

8

0.0

72.2

1189

72

Porosity (%)

0.8

5.7

12.6

48

0.5

8.5

16.5

58

3.3

9.6

14.7

8

0.0

12.0

20.1

74

Inter. Porosity (%)

0,3

1,7

5,7

29

0,3

2,5

8,3

47

1,0

3,6

7,3

6

0,3

4,0

9,0

58

Secondary Porosity (%)

0,0

1,0

4,2

29

0,0

2,3

11,4

47

0,7

2,1

4,3

6

0,3

3,3

10,0

58

19

291

Hematite-illite-Fe-Ti-oxide coatings with varying thickness cover the surface of

292

detrital grains in most samples (0.5-8 vol.%). These coatings are the earliest

293

authigenic product.

294

Quartz cement is present in all samples, in variable abundance (1 to 18 vol.-%), with

295

most samples and well averages not exceeding 10 vol.-%. The quartz cement forms

296

euhedral, syntaxial overgrowths around detrital grains, on top of the hematite-clay

297

coatings, and shows variable thickness and continuity around grains. Quartz

298

cementation is not facies specific and does not correlate with burial depth (within the

299

limited range, i.e. ∼500m, of burial depth investigated) nor with the sandstone-

300

mudstone distribution. Similarly, there is no distinct relationship between quartz

301

cement abundance and grain size, sorting, detrital clay content, grain composition or

302

other diagenetic phases. Quantitative data suggest that the amount of quartz cement

303

has been largely influenced by the differential development of early carbonate (Fe-

304

dolomite) cementation (more quartz cement in samples exhibiting less early

305

carbonate cement, see below).

306

Kaolinite and dickite are common in most studied samples (0-12 vol.-%) and

307

represent two generations of authigenic clays that can be distinguished texturally.

308

The early generation of vermicular, poorly crystallized kaolinite (generally <10 µm) is

309

commonly associated with Fe-oxide-stained microcrystalline illite/chlorite and

310

bitumen and partly replaces feldspar and mica. It predates the quartz cement and is

311

locally engulfed by Fe-dolomite and ankerite. The second generation is formed by

312

well-crystallized blocky crystals (10-25 µm). It shows no Fe-oxide or bitumen staining

313

and typically forms pore-filling aggregates close to or within leached grains. It

314

postdates Fe-dolomite and ankerite but is engulfed by quartz cement. Based on the

315

crystal habit and characteristic XRD-pattern (Bailey, 1980) the second generation

20

316

was unambiguously identified as dickite in three samples of the reference well. Both

317

kaolinite and dickite exhibit visible intercrystal porosity that may account for up to

318

40 % of the volume in the kaolinite aggregates (Nadeau and Hurst, 1991; Sardini et

319

al., 2009). In many cases the grain-replacive kaolinite/dickite preserves the original

320

outline of the detrital (feldspar) grain and the outline is hardly deformed by

321

mechanical compaction, indicating that the replacement occurred at some significant

322

depth (i.e. >2 km or so).

323

Authigenic sulphate cement is present in very variable abundance (0-20 vol.-%),

324

but mainly in two wells in which it can constitute the dominant cement in some

325

samples . Anhydrite occurs as pore-filling poikilotopic cement and disseminated

326

small crystals. It also occurs as fracture filling extending in the pore space adjacent

327

(<1 cm) to fractures. It postdates Fe-dolomite/ankerite, kaolinite/dickite and quartz

328

cements. Barite is present in all wells in low abundance (0-8 vol.-%) and occurs as

329

euhedral pore-filling crystals, or as anhedral poikilotopic crystals locally in some

330

samples. Barite postdates Fe-dolomite and ankerite, quartz and kaolinite/dickite.

331

Aside from kaolinite/dickite, authigenic clays also occur as poorly developed (0-

332

2 vol.- %) illite and Fe-chlorite, which grow into open pore-space or replace kaolinite.

333

Authigenic carbonates represent the predominant pore-filling cements in most of the

334

studied samples. Ferroan dolomite, ankerite and siderite can be distinguished. The

335

term ankerite is used as defined by Deer et al. (1992) for a carbonate with

336

Mg/(Fe,Mn) ratio of ≤4 (in mol.-%).

337

Fe-dolomite and ankerite (1-40 vol.-%) are closely linked spatially and texturally

338

within all samples and wells, ankerite forming rims overgrown, in optical and

339

chemical continuity, on top of Fe-dolomite rhombic cores. Fe-dolomite and ankerite

340

constitute a single carbonate cementation event, that will be termed Fe-

21

341

dolomite/ankerite in the following. Fe-dolomite/ankerite occurs as poikilotopic

342

cements (Fig. 10e) and zoned subhedral to euhedral crystals (Fe-dolomite core,

343

ankerite rim) (Fig. 10c & d). Fe-dolomite/ankerite cement commonly engulfs kaolinite

344

booklets and replaces grains of unknown nature (see discussion below). Samples

345

with a high Fe-dolomite/ankerite content commonly exhibit a floating-grain open

346

texture, i.e. a very high Intergranular volume (IGV up to 35-40%) suggesting early

347

cementation (Fig. 10e & f). Textural relationships indicate that these carbonates

348

predate quartz cement (no quartz overgrowth under the Fe-dolomite/ankerite

349

cement). In addition, samples with a high Fe-dolomite/ankerite content contain much

350

less quartz cement than samples with a low Fe-dolomite/ankerite content. These

351

carbonates are usually non-luminescent (due to high Fe content, (Richter et al.,

352

2003). Only in a few instances do Fe/dolomite/ankerite show dark-orange to dark-red

353

CL-colours. Occasionally carbonate cements with strong dark-orange to dark-red

354

CL-colours exhibit dull-luminescence ghosts of entirely replaced grains (Fig. 10g &

355

h). This replacement process is mainly evidenced from CL examination, but in the

356

absence of relicts, the nature of the original grains is unknown. They may be former

357

detrital carbonate (dolomite?) grains, as proposed by Molenaar and Felder (2019).

358

However, as indicated above and contrary to Molenaar and Felder (2019) basically

359

no preserved grains of detrital carbonates have been observed in the samples

360

studied. If it is hypothesized that the replaced grains observed within the Fe-

361

dolomite/ankerite cement under CL (fig. 10g&h) were formerly detrital carbonate

362

grains they would represent no more than 1% BSV. This was estimated by point-

363

counting all oversized intergranular patches of early Fe-dolomite/ankerite cement in

364

six representative samples (with % Fe-dolomite/ankerite ranging between 12 and

22

365

31%) and assuming that each oversized dolomite-cemented pore contained one

366

detrital carbonate grain.

367

Siderite cement (0-10 vol.-%) is also present in many samples in minor amounts. It

368

forms mainly coarse, blocky crystals and occurs as patchy micronodules, with a

369

maximum diameter of a few millimeters (Fig. 10a). Siderite commonly replaces

370

detrital grains (including quartz) located within the nodule. It postdates quartz

371

cementation, kaolinite and dickite formation and Fe-dolomite/ankerite precipitation.

372

Siderite is more common in samples with low or no content of Fe-dolomite/ankerite

373

than in samples that are strongly cemented by earlier carbonates.

374

375 376

Figure 9: Boxplots illustrating the frequency of diagenetic phases in the different depositional facies,

377

based on point-count data. Left: Main diagenetic phases. Right: The differentiated carbonate

378

cements. Note that for facies F1 only one sample has been point-counted (n=1).

23

379 380

Figure 10: Photomicrographs of authigenic carbonates. A) Blocky siderite (S) cement as patchy

381

micronodule. Thin-section photograph, normally polarised light. B) Small rhombic carbonate crystals

382

(arrows) occur in same samples as siderite (S). Thin-section photograph, normally polarised light. C)

383

Rhombic carbonate crystals show a zonation, often with a dolomitic core (Do) and an ankeritic rim

384

(Ank). Barite (Ba) also occurs and fills the space between the carbonate rhombs. Arrows indicate

24

385

remains of replaced grains. BSE image. D) Siderite (Sid) nodule growing around dolomite/ankerite

386

rhombs (black arrows) and kaolinite (red arrow) that most likely replaced a feldspar-rich grain. BSE

387

image. E) Floating grain texture of a strongly carbonate cemented (dolomite/ankerite) sample.

388

Oversized pores may be due to grain replacement although grain ghost outlines (usually underlined

389

by Fe-iron coats) were never found. Thin-section photograph, cross-polarised light. F) Poikilotopic

390

carbonate cement replacing a detrital grain, arrows indicate the remains of a hematite-illite-Ti-oxide

391

coating. Thin-section photograph, cross-polarised light. G&H) Thin-section photograph (G) and hot CL

392

image of poikilitic carbonate cement, showing clear zoning and replacing of detrital grains (arrows).

393

4.4 Stable isotope composition of carbonate cements

394

Twelve samples containing only one dominant generation of authigenic carbonate

395

were selected for determination of δ13C and δ18O composition in the reference well.

396

However, all samples represent some degree of mixture between Fe-dolomite,

397

ankerite and/or siderite (Tab. 2). Bulk rock isotopic analyses of the selected

398

carbonate cemented samples range from -4.8 to -2.3 ‰ in δ13C values and from -

399

10.5 to -8.0 ‰ in δ18O values (Tab. 2), and show a trend of increasingly negative

400

values with increasing proportion of Fe-carbonates which must reflect partly physical

401

mixing of different carbonates. Fe-dolomite-dominated samples show the least

402

negative δ13C (~-2.5 ‰) and δ18O (~-8.5 ‰) values whereas siderite-dominated

403

samples exhibit the most negative isotopic signal (δ13C: -2.9 to -4.8 ‰ and δ18O: -9.0

404

to -10.5 ‰). Ankerite-dominated samples show intermediate isotopic composition

405

(δ13C: -2.9 to -2.8 ‰ and δ18O: -8.7 to -9 ‰). The proportion of each type of

406

carbonate in the analysed samples was visually estimated on BSE-images in order

407

to calculate the approximate isotopic composition of end-member dolomite, ankerite

408

and siderite (Fig. 11, Tab. 2). Calculated end member carbonate cements have

409

pretty close approximate isotopic composition as follows. Fe-dolomite: δ13C∼-2 ‰;

410

δ18O∼-7 ‰; Ankerite: δ13C∼-3 ‰; δ18O∼-10 ‰; Siderite: δ13C∼-5 ‰; δ18O∼-12 ‰

25

411

(see Tab. 2). The calcite matrix occurring in typical F1 shales was also analysed for

412

comparison in three samples. These yielded rather consistent bulk isotopic

413

compositions averaging δ13C∼-5 ‰ and δ18O∼+3 ‰. This isotopic composition is

414

very different from that of the carbonate cements (Fe-dolomite, ankerite and siderite)

415

found in the sandstone samples (Fig. 11).

416

An attempt was made to perform laser ablation U-Pb dating of the Fe-

417

dolomite/ankerite cement on a selection of a few samples at the CEREGE U-Th-Pb

418

geochronology laboratory, University of Aix-Marseille, (c.f. Godeau et al., 2018).

419

However, U and Pb signals revealed insufficient (in ppb range) for U-Pb age

420

determination.

421 422

Table 2: Stable isotope composition of Rotliegend samples. End-members were calculated assuming

423

a binary mixture of dolomite and ankerite. Sample

Dominating

Dolomite % of

Ankerite % of

Siderite

Carbonate

total carbonates

total carbonates

total carbonates

PDB]

Cement

(estimated)

(estimated)

(estimated)

-2.92

-8.95

Ankerite

35

65

0

A2

-2.85

-8.73

Ankerite

45

55

A3

-2.52

-7.97

Dolomite

65

30

5

A4

-3.88

-10.31

Siderite

5.00

35.00

60.00

A5

-4.79

-10.52

Siderite

5

35

60

A6

-2.88

-9.08

Siderite

5

45

50

A7

-3.33

-10.15

Siderite

**

**

** (>50%)

A8

-2.53

-9.00

Dolomite

50

35

15

A9

-2.62

-8.84

Dolomite

55

45

0

A10

-4.68

2.35

Calcite

-

-

-

A11

-4.45

4.57

Calcite

-

-

-

A12

-5.69

2.47

Calcite

-

-

-

δ13C

δ18O

[‰ - V-

[‰-

PDB] A1

V-

26

%

of

Endmembers E1

-1.9

-7.3

Dolomite

Calculated Endmember

E2

-3.5

-10.0

Ankerite

Calculated Endmember

E3

-5.1

-11.7

Siderite

Calculated Endmember

E4

-4.9

3.1

Calcite

Average

424 425

27

426 427

Figure 11: Stable isotope signals of carbonate cements of Rotliegend sandstones and mudstones.

428

Dol. = Dolomite, Ank. = Ankerite, Sid. = Siderite, Calc=Calcite, dom=dominated, end-mem, =

429

calculated end-member. Indicated range of pristine marine Permian limestones (Veizer et al., 1999).

430

4.5 Definition of classes and spatial distribution of carbonate cements

431

The texture, mineralogy, and intensity of authigenic carbonate cementation allows

432

the definition of six distinct types of carbonate-cemented samples, which are

433

described in table 3 as 6 different classes (C1 to C6, Fig. 12).

28

434

Table 3: The six classes of carbonate cementation occuring in the studied samples. Carbonate

435

content generally increases from C1 to C6, as does the Fe-dolomite/ankerite content. Reservoir

436

quality decreases from C1 to C6. Carbonate

Dominant Texture

class

carbonate

C1

Siderite

Range

Avg

%carb.

%carb

3-10%

6%

1-18%

6%

Siderite

5-17%

10%

Rare siderite

5-20%

12%

Very rare siderite

10-21%

15%

None

12-39%

24%

Other carbonates

Patchy nodules, Rare Fe-dolomite poikilotopic crystals Fe-dolomiteC2

Siderite

Idem C1 ankerite rhombs

Fe-dolomite-

Disseminated pore-filling

ankerite

zoned rhombs

C3

Aggregates pore-filling Fe-dolomiteC4

zoned rhombs. Rare ankerite grain replacement Fe-dolomite-

Large zoned pore-filling

ankerite

poikilotopic crystals

C5

Porefilling poikilotopic Fe-dolomite-

crystals, engulfing

ankerite

kaolinite crystals. Some

C6

grain replacement.

437

29

438 439

Figure 12: Boxplot illustrating the distribution of the different carbonate cements in the point-counted

440

thin-sections (n=208) with regards to the six carbonate cementation classes.

441

The total abundance of carbonate cement generally increases from C1 to C6. The

442

C6 class is the one in which carbonate cementation is most intense and pervasive,

443

and most damaging to reservoir quality (Figs. 12 & 13). Samples of the C1-C2

444

classes, characterized by minor late stage siderite and minor to no dolomite/ankerite,

445

exhibit the best reservoir porosity (Fig. 13).

30

446 447

Figure 13: Abundance of carbonate cements vs He-Porosity. Porosity declines strongly with

448

increasing carbonate content. Total amount of carbonate cement increases from C1 to C6. Envelopes

449

for C1-C2 and C6 classes highlight the influence of carbonate cementation on porosity.

450 451

All six defined carbonate cementation classes are encountered in each depositional

452

facies. However, some particular associations between depositional facies and

453

diagenetic facies (carbonate type and texture) are worth mentioning (Fig. 14). The

454

cement-poor C1-C2 classes tend to most common in the F5 dry eolian facies, the C3

455

class is most common in the F4 wet eolian facies, the C4 class is most common in

456

the F3 proximal fluvial facies, and the cement-rich C6 class is most common in the

457

F2 distal fluvial facies (Fig. 14). This suggests some weak relationship between

458

intensity of carbonate cementation and depositional facies, according to which 31

459

highly-developed carbonate cementation would be more common when moving

460

towards the Silverpit lake (depositional facies 1) in the paleogeographical setting

461

(see Fig. 2).

462 463

Figure 14: Stacked barplot showing the frequency of the described carbonate classes with regards to

464

the depositional environment. Carbonate classes associated with good reservoir quality (C1-C3)

465

increase in frequency from F2 (distal fluvial) to F5 (dry eolian) sandstones while the frequency of

466

carbonate classes associated with poor reservoir quality (C4-C6) decrease in frequency. Note that

467

this apparent correlation between facies and carbonate cementation class is also a function of the

468

vertical mudstone-sandstone strata pattern (see Fig. 13).

469

While depositional environment may have some influence on the distribution of

470

carbonate cements in the studied samples, the proximity of mudstone (mainly F1

471

facies) beds appears to be the dominant controlling factor. This is illustrated in figure

32

472

15, showing the distribution of the different carbonate cementation classes as a

473

function of the mudstone/sandstone ratio. Strongly Fe-dolomite/ankerite cemented

474

samples (C5 & C6 classes) are most commonly found close to mudstones, i.e. in

475

intervals with high M/S ratios (>0.3). In contrast, in sandstone packages located

476

further away from mudstones, i.e. in intervals with low M/S ratio (< 0.2), the Fe-

477

dolomite/ankerite cementation is much less developed. The most weakly cemented

478

samples of the C1 class (late Siderite only) being essentially found in thick (eolian)

479

sandstone packages with very low M/S ratios (<0.1).

480

Overall there is a clear correlation between the amount of early dolomite/ankerite

481

cement, i.e. the carbonate cement class, and the M/S ratio (Fig. 15). In addition, the

482

amount of Fe-dolomite/ankerite cement, as quantified by carbonate classes, also

483

correlates to reservoir quality (Fig. 13): Samples of C2 to C3 classes have a fair

484

reservoir quality due to the small amount and disseminated distribution of rhombic

485

crystals. The increasingly poikilotopic habit of Fe-dolomite/ankerite in C4 to C6

486

classes leads to significant impairment of reservoir quality because both pore space

487

and pore-throats are significantly cemented, samples of C6 class having basically no

488

porosity left. Thus the reservoir quality of the Permian sandstones is largely

489

controlled by carbonate cementation and decreases from C1-C2 to C6 class.

490

Because of its patchy distribution and low volumetric abundance, the impact of

491

siderite cementation (C1) on reservoir properties is minor.

33

492 493

Figure 15: Boxplots of carbonate cementation classes (C1 to C6) against M/S ratios. A) Sliding

494

window size of 2 m, B) sliding window size of 4 m. Strong carbonate cementation is predominantly

495

found in sandstone samples that are in an interval with a high (> 0.3) M/S ratio, while samples lacking

496

significant carbonate cementation are found in sequences with low (<0.2) M/S ratios.

497

4.6 Mudstones

498

Mudstones are present in variable amounts in all studied wells. They are mainly

499

composed of red beds in thicknesses of several meters but also as thin (<30 cm)

500

shaly interlayers between sandstone beds. The mudstones consist mainly of quartz,

501

mica, clay-minerals and hematite. They are commonly laminated (sub-cm scale),

502

with flaser-bedding occurring locally, and may include sub-cm silty laminae.

503

Dominant clay-minerals are illite and kaolinite. Calcite has been identified by XRD,

504

and is observed in silty laminae as microsparitic matrix/cement. It may also be

505

present as a micritic phase in the clayey matrix but not readily visible in thin-section.

34

506

It is interesting to point out that thin (<1 m) sand beds found in mudstones packages

507

are systematically carbonate cemented.

508 509

4.7 Controls on Porosity and permeability

510

He-porosity values measured on core-plugs range from 0 to over 20 % (Figs. 13 &

511

16). The porosity values derived from point-counting range from 0 to 15 % for the

512

same samples (Tab. 1). The discrepancy between the two values relates to

513

microporosity that cannot be resolved by optical microscopy, and is associated to

514

lithic grains, altered feldspar grains and pore-filling/replacive kaolinite/dickite. The

515

porosity is mainly intergranular primary porosity, but some samples extensively

516

affected by feldspar leaching exhibit secondary intragranular dissolution porosity up

517

to 10 %. Horizontal permeability values are highly variable, ranging from <0.1 mD to

518

>100 mD.

519

Figure 15a illustrates that all sedimentary facies cover the entire range of porosity

520

and permeability values and that all wells and facies plot on the same porosity-

521

permeability trend. While poor porosity-permeability values can be found in all facies,

522

the very highest porosity-permeability values (PHI>18% and k>100mD) are mainly

523

found in dry eolian (F5) sands (Fig. 16a, Girard et al., 2008). Figures 16c & d, and

524

Fig. 13 show that samples with a high carbonate content display poor reservoir

525

quality and vice versa. Reservoir quality correlates loosely with the M/S ratio (Fig.

526

16b): Porosities >10 % and permeabilities >1 mD occur almost solely in samples

527

with a M/S ratio <0.5.

528 529

35

530 531

Figure 16: Figure illustrating porosity-permeability vs (A) depositional facies, (B) M/S ratio, (C)

532

carbonate cementation class, and (D) carbonate cement content of the studied Lower Slochteren

533

sandstone samples.

534 535

Influence of subcrop on diagenetic cements

536

The nature of subcropping rocks (i.e. organic-rich sediments such as coals) can

537

have a significant influence on diagenetic processes in the overlying sediments

538

(Gaupp et al., 1993). In order to evaluate this, the abundance of the different pore-

539

filling cements has been investigated in relation to the nature of subcrop (Fig. 17). It

540

can be seen that the subcrop has no influence on the abundance of quartz and

541

kaolinite cement as the data shows significant overlap for all types of subcrop.

542

Carbonate cementation and type of carbonate cement also show no significant 36

543

difference for different types of subcrop, suggesting that there is no significant

544

relationship between the nature of subcrop and the intensity/type of carbonate

545

cementation.

546 547

Figure 17: Boxplots illustrating that subcrop type does not seem to have a strong influence on the

548

main diagenetic phases. Note that there are only 17 samples from wells with a Stephanian subcrop,

549

compared to more than 50 samples for both Westphalian B and Westphalian UC.

550

5. Discussion

551

5.1 Non-carbonate cements

552

Most of the diagenetic processes observed in the studied samples have been

553

reported previously in other areas of the SPB. The origin of early diagenetic

554

vermicular kaolinite and hematite-illite coatings is discussed in detail elsewhere

555

(Amthor and Okkerman, 1998; Busch et al., 2020; Gaupp et al., 1993; Glennie et al., 37

556

1978; Platt, 1994; Ziegler, 2006). They are related to continental semi-arid to arid

557

shabka/playa environments and formed close to the surface under the influence of

558

meteoric water. Late diagenetic quartz and blocky kaolinite/dickite are also

559

commonly found in Rotliegend sandstones (Gaupp and Okkerman, 2011; Platt,

560

1991; Schöner and Gaupp, 2005) and interpreted as resulting from burial diagenesis.

561

Dickite seems to be associated with influx of acidic fluids originating from the

562

maturation of stratigraphically or tectonically proximal Carboniferous coal measures

563

(Gaupp et al., 1993; Platt, 1993; Ziegler, 2006). Transformation of early diagenetic

564

clay minerals during burial may also form kaolinite (Waldmann and Gaupp, 2016).

565

Late diagenetic sulphate minerals in the SPB are generally interpreted as formed

566

from fluids originating in the overlying Zechstein salt, as a result of brine circulation

567

likely induced at the time of basin inversion and promoting introduction of Zechstein

568

waters into the underlying Rotliegend reservoirs (McNeil et al., 1995; Purvis, 1992;

569

Pye and Krinsley, 1986; Sullivan et al., 1990). Alternatively, the late diagenetic

570

sulphates could result from recrystallization of early diagenetic precursors similar to

571

the model proposed for carbonate cementation by Molenaar & Felder (2019),

572

although there is no evidence of such a process in the studied samples. Overall,

573

leaving aside the carbonate cementation which is the focus of our study, the other

574

diagenetic transformations observed in our samples are identical and in line with the

575

observations/interpretations of prior studies in various areas of the SPB and will not

576

be discussed any further in the following.

577 578

5.2 Carbonate cements

579

Early and late diagenetic carbonate cements are common in Rotliegend deposits of

580

the Southern North Sea (McNeil et al., 1998; Ziegler, 1993). While early diagenetic

38

581

carbonate cements include calcite, dolomite and magnesite (Glennie et al., 1978;

582

Purvis, 1989; Sullivan et al., 1990), burial cements are commonly composed of Fe-

583

dolomite, ankerite and siderite (Leveille et al., 1997; Purvis, 1992).

584

The Fe-dolomite/ankerite cement in the studied wells postdates the formation of

585

hematite-clay coatings and vermicular kaolinite, and texturally predates the main

586

quartz precipitation, which is thought to have started to form around 70-90°C

587

(Worden & Morad, 2000) which coincides with the latter part of the first burial phase

588

(Fig. 3, Glennie et al., 1978). Thus the “early” carbonates formed during shallow to

589

intermediate burial, prior to the Late Jurassic uplift. Several interpretations of the

590

origin of early to intermediate burial (Fe-) dolomite in the Rotliegend sandstones of

591

the Southern North Sea have been proposed. Glennie et al. (1978) suggested that

592

dolomite can form by dolomitisation of an earlier calcite generation. However, in the

593

studied samples there is no petrographical evidence for a calcite precursor. Purvis

594

(1989) and Sullivan et al. (1990) have shown that the influx of marine Zechstein

595

waters during the late Permian Zechstein transgression can lead to carbonate (and

596

sulfate /chloride) cementations in the Rotliegend. Similarly, Vincent et al. (2018)

597

interpreted non-ferroan dolomite as precipitate from Zechstein fluids at temperatures

598

of ~100°C and suggest that Fe-rich carbonates in Ro tliegend sandstones offshore

599

the Netherlands are the result of Carboniferous fluids migrating upwards. The latter

600

two models of carbonate cementation are unlikely to apply to the Lower Slochteren

601

sandstones in this study because the spatial distribution of authigenic carbonates is

602

neither linked to the proximity of Zechstein deposits nor to the subcropping

603

Carboniferous. More recently, the distribution of dolomite cements in the Rotliegend

604

has been linked to variable and dispersed content of detrital carbonate clasts/grains,

605

which acted as nuclei and source for authigenic carbonates (Molenaar and Felder,

39

606

2019). The authors advocate that their model excludes the need for an external

607

supply of solutes by large-scale flow of diagenetic fluids, however, they do not

608

provide any quantification of the amount of detrital carbonate grains nor any mass

609

balance evaluation. In the samples studied here, hardly any preserved detrital

610

carbonate clast/grains were identified in thin sections and the spatial distribution of

611

carbonate cements is distinctly primarily linked to the proximity of mudstones as

612

demonstrated above. We do not see any reason why, if any detrital carbonate grains

613

were present in the initial deposits, they would have been limited to sand layers, of

614

all sedimentary facies, located near or in between the mudstone intervals. In

615

addition, since the detrital carbonates putatively present in the carbonate cemented

616

intervals would have been replaced by diagenetic dolomite themselves they cannot

617

represent the source of solutes for the abundant surrounding dolomite cement.

618

Consequently, we do not favour the model proposed by Molenaar and Felder (2019).

619 620

5.3 Controls on carbonate cement distribution

621

The eodiagenetic dolomite cements of Rotliegend sandstones in other studies have

622

been interpreted as the result of precipitation from evaporation of meteoric

623

groundwaters in the arid playa environment (Amthor and Okkerman, 1998; Gaupp

624

and Okkerman, 2011; Goodchild and Whitaker, 1986; Platt, 1994; Pye and Krinsley,

625

1986). Increasing ionic enrichment of groundwater flowing from the alluvial deposits

626

towards the center of the playa due to very high evaporation rates as well as CO2

627

loss would lead to the development of rims of early cements, with carbonates

628

precipitating in the proximal alluvial and fluvial deposits and sulphates and salts in

629

the distal deposits closer to the playa lake (Drong, 1979). This cementation model,

630

however, cannot explain the distribution of carbonate cements in the studied wells, in

40

631

particular as dolomite cements are found in all depositional facies and no indications

632

for early diagenetic sulphates or salts are found. Additionally, the authigenic

633

carbonates post-date eodiagenetic kaolinite phases, indicating that they are not

634

related to sub-syndeposition evaporation processes.

635

Nonetheless, the close relationship between mudstone proximity and carbonate

636

cementation observed in the studied samples indicates that there is some influence

637

of the depositional environment on the carbonate cement distribution. As the

638

mudstones are interpreted to be the result of clay deposition during ephemeral

639

flooding events (Molenaar and Felder, 2019) they represent times of high

640

groundwater tables. Evaporation of the ground water following the flooding events

641

could result in highly enriched pore fluids with regards to carbonates in and around

642

the mudstones. These fluids could favour development of carbonate cementation in

643

sand layers above and below such mudstones as they were later expelled from the

644

mudstones as a result of compaction.

645

Many prior works have documented carbonate cementation in sandstones to be

646

related to sandstone-shale (mudstones) transition in marine and continental settings,

647

with stronger cementation occurring close to mudstones (Carvalho et al., 1995;

648

Dutton, 2008; Lai et al., 2017; Ma et al., 2019; Morad et al., 2010; Xi et al., 2019;

649

Yuan et al., 2015). Several authors have developed geochemical models that show

650

that significant mass exchange/transfer between mudstones and sandstones may

651

occur during diagenesis as a result of diffusion or advection (compaction fluids)

652

(Land et al., 1997; Milliken et al., 1994; Thyne, 2001; Wintsch and Kvale, 1994).

653

Most of these models rely on diagenetic mineral reactions that occur within

654

mudstones, such as smectite-to-illite transformation, which can introduce silica and

655

metal ions into sandstones, at burial depths where temperatures exceed 70-80°C. In

41

656

our study, the most heavily dolomite cemented samples show very high intergranular

657

volume, with IVG values up to 35-40%, clearly indicating that the degree of

658

compaction at the time of carbonate cement precipitation was very low. This

659

suggests that the early dolomite cementation observed in our samples is unlikely to

660

be related to burial-related clay transformation in shales. Interestingly, smectite-to-

661

illite transformation has also been overserved at temperatures of less than 40°C, and

662

thus this transformation could be an early burial source of Mg for dolomite cements

663

in the close-by sandstones, in particular as significant mass transfer may also occur

664

at shallow depth/low temperature as differential compaction processes during early

665

burial would force fluids out of mudstones into surrounding sandstones (Beard and

666

Weyl, 1973; Mondol et al., 2007). The Fe incorporated into the dolomite cements

667

could either be sourced from such compactional mudstone fluids, or be related to the

668

reduction of iron from tangential hematite rich clay coatings within the sandstones

669

(Molenaar and Felder, 2018). Petrographic evidence, i.e. the elevated intergranular

670

volume (IGV) values of 35-40%, indicates that early dolomite cement started to

671

develop at shallow depth, prior to any significant compaction. The stable isotope

672

data

673

respectively (Fig. 11, Tab. 2) which are compatible with moderate formation

674

temperatures around 40-55°C (assuming meteoric wate r with δ18O ~ -5.5 ‰ SMOW

675

(based on a reconstructed paleo-latitude of the studied site at Permian time of about

676

20°N) and using the dolomite-water fractionation by Horita, 2014). Therefore,

677

considering the close spatial association of the carbonate cementation intensity with

678

the proximity of mudstones, we believe that mass transfer from mudstones to the

679

adjacent sandstones occurring during early-moderate compaction and controlled

680

diagenetic dolomite-ankerite precipitation. The pore fluids of the mudstones were

yielded δ18O values of

-7.3 ‰ and -10.0 ‰ for dolomite and ankerite

42

681

likely enriched in dissolved Ca and Mg due to deposition in an evaporitic playa

682

environment (Gaupp et al., 2000; Morad et al., 2000), and introduction of these fluids

683

into the adjacent sandstones during compaction would have led to precipitation of

684

pore-filling carbonate cements. The solutes could also come from the dissolution of

685

minor amounts of syndepositional matrix carbonates (mainly calcite) occurring locally

686

in the Silverpit mudstones. Reducing conditions, which could have mobilized Fe3+ to

687

Fe2+ within the Rotliegend for the introduction into precipitating carbonates, could

688

also have been influenced by the influx of maturation fluids from the underlying coal-

689

bearing Carboniferous. However, there is no clear correlation between subcropping

690

Carboniferous lithologies and Fe-carbonate cement content within the Rotliegend

691

(Fig. 17).

692 693

5.4 Controls on reservoir quality

694

The reservoir quality of the Lower Slochteren sandstones in the northern K blocks

695

offshore the Netherlands is primarily controlled by both the initial depositional

696

environment (facies) and carbonate cementation, however these two factors are not

697

directly linked.

698

The primary sediment composition and grain size, and thus the initial pore size

699

distribution are governed by the depositional environment. As a consequence, dry

700

eolian sands (F5) are expected to have better initial reservoir quality, in relation to a

701

low content of fines and a better sorting, while distal fluvial sheet flood sands and

702

lake margin sands (F2) would have a significantly poorer initial reservoir quality, in

703

relation with some fine/clay content and poorer sorting.

704

The influence of carbonate cementation and mudstone-sandstone strata patterns on

705

reservoir quality is more specifically illustrated by two typical wells shown in figure

43

706

18. In well A for instance, proximal fluvial sands (F3) occur within thick (>10 m) sand

707

packages (3939-3953 m) and as thin (<2 m) beds interbedded within mudstones

708

(3915-3928 m). The thin beds show a strong carbonate cementation (12-22%) and

709

low porosities (~5 %), while the thick sand layers have a low carbonate content (4-

710

8%) and high porosities (>10 %). Similar observations are made in well B (Fig. 18).

711

Secondary controls on reservoir quality are compaction processes and the formation

712

of burial diagenetic minerals (kaolinite, quartz, siderite and sulphates) which further

713

reduce porosity and permeability contrast amongst the different facies. Strongly early

714

carbonate cemented samples were not as severely affected by the later diagenetic

715

processes due to the low residual porosity remaining after early carbonate

716

cementation (Fig. 19).

717

The proposed model of reservoir quality control and evolution is illustrated as a very

718

simplified sketch in figure 19 showing the evolution of porosity in the different

719

depositional facies as a result of burial and cementation. At deposition, aoelian

720

sands would exhibit better porosity than fluvial deposits for reason exposed above

721

(Fig. 19A). During shallow/moderate burial, compactional fluids are expelled from the

722

mudstones and drive carbonate cementation in the adjacent sandstones, producing

723

a large range in porosity values between sand beds located close to or in between

724

thick mudstone packages (which become heavily cemented) and sands located far

725

from mudstones (which maintained high porosity) (Fig. 19B). Dry eolian (F5) sands

726

would maintain a higher porosity than other sands (e.g. F3) at similar M/S ratios

727

during early diagenesis as they started with a higher initial porosity. Late diagenetic

728

processes would then reduce further the variation in reservoir quality amongst all

729

facies (Fig. 19C) in the sands that still had some residual porosity.

730

44

731 732

Figure 18: Figure illustrating the relationship between carbonate cementation and mudstone-

733

sandstone ratio (M/S) for two typical wells. M/S ratios (solid and dotted lines) are indicated on the left

734

side of the stratigraphic columns, together with He-porosity from plugs (blue bars). On the right side of

735

the stratigraphic column the carbonate cementation is illustrated as total carbonate content in %

736

(black solid bars) and as carbonate classes C1 to C6 (grey solid bars). Note that not every thin-

737

section has been point-counted. Both wells show high carbonate cementation in intervals of high M/S

738

ratio or located in proximity of thick shale packages (see intervals 3910-3935 and 3952-3958 in well

739

A, 3865-3890 in well B). In contrast, carbonate cementation is low in intervals of low M/S ratio or far

740

from thick shale packages (see 3940-3953 in well A, 3890-3905 in well B).

45

741 742

Figure 19: Left: Series of porosity vs M/S ratio plots illustrating qualitatively the controls on reservoir

743

quality in the studied Rotliegend sandstones. (A) Initial reservoir quality is controlled by depositional

744

facies, with dry eolian sands (F5) having the highest porosity (porosity estimates based on Beard and

745

Weyl, 1973. (B) During shallow burial the location within the sandstone-mudstone strata sequence

746

controls the intensity of carbonate cementation and thus reservoir quality. (C) During subsequent

747

burial reservoir quality decreases further due to late diagenetic minerals and compaction affecting

748

residual porosity. Right: Qualitative sketches of carbonate cementation type development with

749

regards to stage of burial (shallow vs deep) and the M/S ratio.

750 751

5.5 Predicting reservoir quality

752

The two main factors driving reservoir quality in the studied Lower Slochteren

753

sandstones are the depositional environment which controls initial porosity, and the

754

distance to mudstones (mainly F1 shales) and sedimentary strata pattern (M/S ratio)

755

which controls the intensity of early dolomite cementation (Fig. 20). These two later 46

756

factors are directly related to the depositional sedimentary model, and the

757

vertical/lateral evolution of sedimentary deposits. Consequently, the ability to predict

758

the risk of intense carbonate cementation and hence the Lower Slochteren reservoir

759

quality largely relies on the ability to develop a reliable depositional model which

760

anticipates the lateral and vertical facies distribution. The location of the study area

761

within the SPB, close to the northern limit of sand deposition where expansion and

762

retreat of the playa lake created a complex alternation of facies, makes the

763

prediction of depositional environments particularly challenging. Even thin, sub-

764

seismic resolution (<2 m thickness) mudstone beds can impact the reservoir quality

765

of adjacent sandstones significantly if the strata pattern is favorable for mudstones

766

(i.e. if sand beds are thin as well, and the mudstone beds numerous). Certainly,

767

more favorable facies are to be looked for to the South of the study area, away from

768

the playa lake southern border, where one would expect thicker sand packages (i.e.

769

greater Net To Gross values).

770

771 772

Figure 20: Plot illustrating the influence of the M/S ratio on porosity (left) and total carbonate cement

773

content (right) as well as on carbonate cementation classes. The risk of high carbonate cementation

47

774

in the studied Rotliegend sandstones becomes very significant at M/S ratios greater than 0.5

775

(practically all samples in intervals with M/S ratio > 0.5 belong to the C6 carbonate cementation class,

776

and have porosity values lower than 7%).

777

The distribution of fluvial fairways, dune fields and mudflats within the basin was

778

controlled by pre-Variscan and Variscan structural elements which formed steps in

779

the paleotopography, with differential erosion and active faulting adding additional

780

topographic relief elements (Mijnlieff and Geluk, 2011). Thus, a good knowledge of

781

the subcroping Carboniferous rocks, in particular their distribution and weathering

782

resistivity, as well as relation to syn-sedimentary faults, are important elements for

783

generating a reliable depositional model. In addition, detailed facies correlations

784

between wells are critical to evaluate lateral variation in the stratinomic patterns of

785

mudstones and sandstones. The use of a chemostratigraphy approach (Schuurman,

786

1998) to support correlations may reveal practical in order to achieve a higher level

787

of detail and confidence. The inclusion of a sequence stratigraphy concept with

788

wetting and drying cycles could also improve the predictive capability of a

789

cementation model. Predicting the reservoir quality, and hence the distribution of

790

carbonate cementation, in the studied sandstones calls upon a greater refinement

791

and reliability of the depositional sedimentary model, at least in wells of intermediate

792

maximum burial (<4km). The abundance of drilled wells and core footage available in

793

the area of interest can certainly help achieve this goal.

794 795

6. Conclusions

796

The reservoir quality of the deeply (~3-4 km) buried Permian Rotliegend sandstones

797

offshore the Netherlands is primarily controlled by the development of diagenetic

798

carbonate cementation (up to 35-40% in volume), which overcomes the initial

48

799

distribution of reservoir properties related to the original depositional facies. The

800

carbonate cementation impairing reservoir quality is dominantly pore-filling

801

intergranular Fe-dolomite/ankerite formed early in the diagenetic sequence at

802

shallow to moderate depth (<2,5 km) and low temperature (most likely <60°C) during

803

the first phase of burial at early to intermediate compaction prior to Late Jurassic

804

uplift. This Fe-dolomite/ankerite cement is observed in all depositional facies and is

805

particularly developed in the proximity of mudstone packages deposited in a playa

806

and mudflat environment or as part of distal fluvial sheet floods. Sandstones

807

occurring as thin beds within a sequence of thick mudstones (intervals of high

808

mudstone/sandstone ratio) typically exhibit strong pervasive carbonate cementation

809

regardless of the depositional facies. In contrast, sandstones occurring as thick beds

810

within a sequence with no or little mudstones layers (intervals of low

811

mudstone/sandstone ratio) are typically exempt of or poor in diagenetic carbonate

812

cement.

813

Based on the results of this study, we propose that compactional fluids expelled from

814

the mudstones as a result of mechanical compaction at shallow/moderate burial

815

were introduced in the adjacent sandstones. These mudstones interstitial fluids are

816

likely to have been enriched pore-waters (due to arid evaporative conditions) and

817

would have provided the required solutes to form the carbonate cement. Late

818

diagenetic cements (including quartz, kaolinite/dickite, anhydrite) further reduced

819

reservoir quality in the sandstone intervals away from mudstones that were not

820

completely plugged by the early Fe-dolomite/ankerite cementation.

821

The ability to predict reservoir quality in the Lower Slochteren sandstones of the

822

Rotliegend interval occurring offshore the Netherlands therefore requires a good

823

understanding of the 3D spatial distribution of depositional facies and in particular a

49

824

good understanding of the location and thickness of mudstone-rich sequences.

825

Elaboration of a detailed reliable sedimentary model is therefore a crucial element to

826

further exploration in the Permian Rotliegend sandstones at the southern margin of

827

the Silverpit lake.

828 829 830

Acknowledgments

831

The authors would like to thank Total E&P for permission to publish this work. The

832

paper greatly benefited from detailed and constructive reviews by M. Felder

833

(PanTerra Geoconsultants, The Netherlands), L. Net (Repsol, Spain). We

834

acknowledge the support during the editorial process.

835 836

References

837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858

Amthor, J.E., Okkerman, J., 1998. Influence of early diagenesis on reservoir quality of Rotliegende sandstones, northern Netherlands. AAPG Bull 82, 2246–2265. Bailey, S.W., 1980. Structure of layer silicates, in: Brindley, G.W., Brown, G. (Eds.), Crystal Structures of Clay Minerals and Their X-Ray Identification, Mineralogical Society Monographs. London, pp. 1–123. Beard, D.C., Weyl, P.K., 1973. Influence of texture on porosity and permeability of unconsolidated sand. AAPG Bull 57, 349–369. Boles, J.R., Franks, S.G., 1979. Clay diagenesis in Wilcox sandstones of Southwest Texas; implications of smectite diagenesis on sandstone cementation. J. Sediment. Res. 49, 55–70. https://doi.org/10.1306/212F76BC-2B24-11D7-8648000102C1865D Busch, B., Hilgers, C., Adelmann, D., 2020. Reservoir quality controls on Rotliegend fluvioaeolian wells in Germany and the Netherlands, Southern Permian Basin – Impact of grain coatings and cements. Mar. Pet. Geol. 112, 104075. https://doi.org/10.1016/j.marpetgeo.2019.104075 Carvalho, M.V.F., De Ros, L.F., Gomes, N.S., 1995. Carbonate cementation patterns and diagenetic reservoir facies in the Campos Basin cretaceous turbidites, offshore eastern Brazil 12, 741–758. Deer, W.A., Howie, R.A., Zussman, J., 1992. An Introduction to the Rock-Forming Minerals, 2nd ed. Longman, Harlow, Essex, England. Drong, H.J., 1979. Diagenetische Veränderungen in den Rotliegend Sandsteinen im NWDeutschen Becken. Geol. Rundsch. 68, 1172–1183. https://doi.org/10.1007/BF02274693 50

859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904

Dutton, S.P., 2008. Calcite cement in Permian deep-water sandstones, Delaware Basin, west Texas: Origin, distribution, and effect on reservoir properties 92, 765–787. Felder, M., 2009. Spatial and stratigraphic diagenesis trends in the Lower Slochteren sandstones of the K2a-Field, Dutch offshore. Gaz de France internal report. Fryberger, S.G., Knight, R., Hern, C., Moscariello, A., Kabel, S., 2011. Rotliegend facies, sedimentary provinces, and stratigraphy, Southern Permain Basin UK and Holland: A review with new obeservations, in: Gaupp, R., Grötsch, J. (Eds.), The Permian Rotliegend of The Netherlands, SEPM Special Publication. pp. 51–88. Gast, R.E., 1991. The Perennial Rotliegend Saline Lake in NW Germany. Geol. Jahrb. A 119, 25–59. Gaupp, R., Gast, R.E., Forster, C., 2000. Late Permian playa lake deposits of the Southern Permian Basin (Central Europe), in: Gierlowski-Kordesch, E.H., Kelts, K.R. (Eds.), Lake Basins through Space and Time, AAPG Studies in Geology. pp. 75–86. Gaupp, R., Matter, A., Platt, J., Ramseyer, K., Walzebuck, J.P., 1993. Diagenesis and Fluid Evolution of Deeply Buried Permian (Rotliegende) Gas Reservoirs, Northwest Germany. AAPG Bull 77, 1111–1128. Gaupp, R., Okkerman, J.A., 2011. Diagensis and Reservoir Quality of Rotliegend Sandstones in the Northern Netherlands - A Review, in: Grötsch, J., Gaupp, R. (Eds.), The Permian Rotliegend of the Netherlands, SEPM Special Publication. SEPM Society for Sedimentary Geology, pp. 193–226. Geluk, M.C., 2007. Permian, in: Wong, T.E., Batjes, D.A.J., de Jager, J. (Eds.), Geology of the Netherlands. Royal Netherlands Academy of Arts and Sciences, pp. 63–83. Girard, J.-P., Kluska, J.-M., Walgenwitz, F., 2008. BLock K6, The Netherlands offshore: Diagenesis of the Lower Slochteren Sandstones and impact on reserovir properties. Total internal report DGEP/GSR/TG/ISS/CLAS R08-095. Glennie, K.W., Mudd, G., Nagtegaal, P.J.C., 1978. Depositional environment and diagenesis of Permian Rotliegendes sandstones in Leman Bank and Sole Pit areas of the UK southern North Sea. J. Geol. Soc. Lond. 135, 25–34. Godeau, N., Deschamps, P., Guihou, A., Leonide, P., Tendil, A., Gerdes, A., Hamelin, B., Girard, J.-P., 2018. U-Pb dating of calcite cement and diagenetic history in microporous carbonate reservoirs: Case of the Urgonian Limestone, France. Geology 46, 247–250. https://doi.org/10.1130/G39905.1 Goodchild, M.W., Whitaker, J.H.M., 1986. A petrographic study of the Rotliegendes Sandstone reservoir (Lower Permian) in the Rough Gas Field. Clay Miner. 21, 459– 477. Horita, J., 2014. Oxygen and carbon isotope fractionation in the system dolomite–water– CO2 to elevated temperatures. Geochim. Cosmochim. Acta 129, 111–124. https://doi.org/10.1016/j.gca.2013.12.027 Kombrink, H., Besly, B.M., Collinson, J.D., Den Hartog Jager, D.G., Drozdzewski, G., Dusar, M., Hoth, P., Pagnier, H.J.M., Stemmerik, L., Waksmundzka, M.I., Wrede, V., 2010. Carboniferous, in: Doornenbal, J.C., Stevenson, A.G. (Eds.), Petroleum Geological Atlas of the Southern Permian Basin Area. EAGE Publications b.v., Houten, pp. 81–99. Lafont, F., Euvrard, B., Roumagnac, A., 2000. The Netherlands central offshore - update on the terminology and facies definition in the Lower Slochteren. Total E&P, Pau. Lai, J., Wang, G., Chen, J., Wang, S., Zhou, Z., Fan, X., 2017. Origin and Distribution of Carbonate Cement in Tight Sandstones: The Upper Triassic Yanchang Formation

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Chang 8 Oil Layer in West Ordos Basin, China. Geofluids 2017, 13. https://doi.org/10.1155/2017/8681753 Land, L.S., Mack, L.E., Milliken, K.L., Leo Lynch, F., 1997. Burial diagenesis of argillaceous sediment, south Texas Gulf of Mexico sedimentary basin: A reexamination. Geol. Soc. Am. Bull. 109, 2–15. https://doi.org/10.1130/0016-7606(1997)1092.3.CO;2 Legler, B., Gebhardt, U., Schneider, J.W., 2005. Late Permian non-marine–marine transitional profiles in the central Southern Permian Basin, northern Germany. Int. J. Earth Sci. Geol. Rundsch. 94, 851–862. Leveille, G.P., Primmer, T.J., Dudley, G., Ellis, D., Allinson, G.J., 1997. Diagenetic controls on reservoir quality in Permian Rotliegendes sandstones, Jupiter Fields area, southern North Sea, in: Ziegler, K., Turner, P., Daines, S.R. (Eds.), Petroleum Geology of the Southern North Sea: Future and Potential, Geological Society Special Publications. London, pp. 105–122. Ma, B., Cao, Y., Eriksson, K.A., Wang, Y., 2019. Carbonate cementation patterns, potential mass transfer, and implications for reservoir heterogeneity in Eocene tight-oil sandstones, Dongying depression, Bohai Bay Basin, China: Evidence from petrology, geochemistry, and numerical modeling. AAPG Bull. 103, 3035–3067. https://doi.org/10.1306/04101917330 McBride, E.F., 1963. A classification of common sandstones. J. Sediment. Petrol. 33, 664– 669. McNeil, B., Shaw, H.F., Rankin, A.H., 1998. The timing of cementation in the Rotliegend sandstones of the southern North Sea: a petrological and fluid inclusion study of cements. J. Pet. Geol. 21, 311–327. https://doi.org/10.1111/j.17475457.1998.tb00784.x McNeil, B., Shaw, H.F., Rankin, A.H., 1995. Diagenesis of the Rotliegend Sandstones in the VFields, southern North sea: a fluid inclusion study. Geol. Soc. Lond. Spec. Publ. 86, 125–139. https://doi.org/10.1144/GSL.SP.1995.086.01.10 Mijnlieff, H., Geluk, M., 2011. Palaeotopography-governed sediment distribution—a new predictive model for the Permian Upper Rotliegend in the Dutch sector of the Southern Permian Basin, in: Grötsch, J., Gaupp, R. (Eds.), The Rotliegend of the Netherlands. SEPM Special Publication. Milliken, K.L., Mack, G.H., Land, L.S., 1994. Element mobility in sandstones during burial: Whole-rock chemical and isotopic data, Frio Formation, south Texas. J. Sediment. Res. A64, 788–796. Minervini, M., Rossi, M., Mellere, D., 2011. Cyclicity and facies relationships at the interaction between aeolian, fluvial, and playa depositional environments in the Upper Rotliegend: regional correlation across UK (Sole Pit Basin), the Netherlands, and Germany, in: Grötsch, J., Gaupp, R. (Eds.), The Rotliegend of the Netherlands. SEPM Special Publication. pp. 119–146. Molenaar, N., Felder, M., 2019. Origin and distribution of dolomite in Permian Rotliegend siliciclastic sandstones (Dutch Southern Permian Basin). J. Sediment. Res. 89, 1055– 1073. https://doi.org/10.2110/jsr.2019.58 Molenaar, N., Felder, M., 2018. Clay Cutans and the Origin of Illite Rim Cement: An Example from the Siliciclastic Rotliegend Sandstone in the Dutch Southern Permian Basin. J. Sediment. Res. 88, 641–658. https://doi.org/10.2110/jsr.2018.33 Mondol, N.H., Bjørlykke, K., Jahren, J., Høeg, K., 2007. Experimental mechanical compaction of clay mineral aggregates—Changes in physical properties of mudstones during 52

952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997

burial. Mar. Pet. Geol. 24, 289–311. https://doi.org/10.1016/j.marpetgeo.2007.03.006 Morad, S., Al-Ramadan, K., Ketzer, J.M., Ros, L.F.D., 2010. The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy. AAPG Bull. 94, 1267–1309. https://doi.org/10.1306/04211009178 Morad, S., Ketzer, J.M., Ros, L.F.D., 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology 47, 95–120. https://doi.org/10.1046/j.1365-3091.2000.00007.x Nadeau, P.H., Hurst, A., 1991. Application of back-scattered electron microscopy to the quantification of clay mineral microporosity in sandstones 61, 921–925. Neuser, R.D., Bruhn, F., Götze, J., Habermann, D., Richter, D.K., 1995. Kathodolumineszenz: Methodik und Anwendung. Zeitblatt Für Geolgie Paleontol. 1995, 287–306. Platt, J., 1994. Geochemical evolution of pore waters in the Rotliegend (Early Permian) of northern Germany. Mar. Pet. Geol. 11, 66–78. Platt, J., 1993. Controls on clay mineral distribution and chemistry in the early Permian Rotliegend of Germany. Clay Miner. 28, 393–416. Platt, J., 1991. The Diagenesis of Early Permian Rotliegend deposits from northwest Germany (Diss.). Univ. Bern. Purvis, K., 1992. Lower permian rotliegend sandstones, southern north sea: a case study of sandstone diagenesis in evaporite-associated sequences. Sediment. Geol. 77, 155– 171. https://doi.org/10.1016/0037-0738(92)90123-9 Purvis, K., 1989. Zoned authigenic magnesites in the Rotliegend Lower Permian, southern North Sea. Sediment. Geol. 65, 307–318. Pye, K., Krinsley, D.H., 1986. Diagenetic carbonate and evaporite minerals in Rotliegend aeolian sandstones of the southern North Sea: Their nature and relationship to secondary porosity development. Clay Miner. 21, 443–457. Richter, D.K., Götte, T., Götze, J., Neuser, R.D., 2003. Progress in application of cathodoluminescence (CL) in sedimentary petrology. Mineral. Petrol. 79, 127–166. Rosenbaum, J., Sheppard, S.M., 1986. An isotopic study of siderites, dolomites and ankerites at high temperatures. Geochim. Cosmochim. Acta 50, 1147–1150. Sardini, P., El Albani, A., Pret, D., Gaboreau, S., Siitari-Kauppi, M., Beaufort, D., 2009. Mapping and Quantifying the Clay Aggregate Microporosity in Medium- to CoarseGrained Sandstones Using the 14C-PMMA Method. J. Sediment. Res. 79, 584–592. https://doi.org/10.2110/jsr.2009.063 Schöner, R., Gaupp, R., 2005. Contrasting red bed diagenesis: The southern and northern margin of the Central European Basin. Int. J. Earth Sci. Geol. Rundsch. 94, 897–916. Sullivan, M.D., Haszeldine, R.S., Boyce, A.J., Rogers, G., Fallick, A.E., 1994. Late anhydrite cements mark basin inversion: isotopic and formation water evidence, Rotliegend Sandstone, North Sea. Mar. Pet. Geol. 11, 46–54. https://doi.org/10.1016/02648172(94)90008-6 Sullivan, M.D., Haszeldine, R.S., Fallick, A.E., 1990. Linear coupling of carbon and strontium isotopes in Rotliegend Sandstone, North Sea: Evidence for cross-formational fluid flow. Geology 18, 1215–1218. Thyne, G., 2001. A model for diagenetic mass transfer between adjacent sandstone and shale. Mar. Pet. Geol. 18, 743–755. https://doi.org/10.1016/s0264-8172(01)00025-3

53

998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025

Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.G., Strauss, H., 1999. 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater. Chem. Geol. 161, 59–88. https://doi.org/10.1016/s0009-2541(99)00081-9 Vincent, B., Waters, J., Witkowski, F., Daniau, G., Oxtoby, N., Crowley, S., Ellam, R., 2018. Diagenesis of Rotliegend sandstone reservoirs (offshore Netherlands): The origin and impact of dolomite cements. Sediment. Geol. 373, 272–291. https://doi.org/10.1016/j.sedgeo.2018.06.012 Waldmann, S., Gaupp, R., 2016. Grain-rimming kaolinite in Permian Rotliegend reservoir rocks. Sediment. Geol. 335, 17–33. https://doi.org/10.1016/j.sedgeo.2016.01.016 Wintsch, R.P., Kvale, C.M., 1994. Differential mobility of elements in burial diagenesis of siliciclastic rocks 64, 349–361. Xi, K., Cao, Y., Liu, K., Wu, S., Yuan, G., Zhu, R., Zhao, Y., Hellevang, H., 2019. Geochemical constraints on the origins of calcite cements and their impacts on reservoir heterogeneities: A case study on tight oil sandstones of the Upper Triassic Yanchang Formation, southwestern Ordos Basin, China. AAPG Bull. 103, 2447–2485. https://doi.org/10.1306/01301918093 Yuan, G., Gluyas, J., Cao, Y., Oxtoby, N.H., Jia, Z., Wang, Y., Xi, K., Li, X., 2015. Diagenesis and reservoir quality evolution of the Eocene sandstones in the northern Dongying Sag, Bohai Bay Basin, East China. Mar. Pet. Geol. 62, 77–89. https://doi.org/10.1016/j.marpetgeo.2015.01.006 Ziegler, K., 2006. Clay minerals of the Permian Rotliegend Group in the North Sea and adjacent areas. Clay Miner. 41, 355–393. https://doi.org/10.1180/0009855064110200 Ziegler, K., 1993. Diagenetic and geochemical history of the Rotliegend of the Southern North Sea (UK Sector): A comparative study (Diss.). Univ. of Reading.

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Highlights: 

Diagenetic carbonate cementation impairs reservoir quality in Rotliegend sandstones



Carbonate cementation is clearly linked to spatial distance from mudstones



Compactional fluids from mudstones provide solutes to form cements in sandstones



Mudstone/sandstone ratio can be used to predict intensity of carbonate cementation

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: