Organic petrological characterization of Westphalian coals from The Netherlands: correlation between Tmax, vitrinite reflectance and hydrogen index

Org. Geochem. Vol. 20, No. 6, pp. 659-675, 1993 Printed in Great Britain. All rights reserved

0146-6380/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd

Organic petrological characterization of Westphalian coals from The Netherlands: correlation between Tmax, vitrinite reflectance and hydrogen index HARRY VELD,1'2WILLEMJ. J. FERMONT1 and LEO F. JEGERSl ~Geological Survey, P.O. Box 126, 6400 AC Heerlen and 2Laboratory of Palaeobotany and Palynology, University of Utrecht, Heidelberglaan 2, 3584 CS Utrecht, The Netherlands Abstraet--Maceral analyses, Tmax, hydrogen index, and vitrinite reflectance measurements have been carried out on a large set of Westphalian coal samples from cored wells situated in The Netherlands. Generalized maceral group analyses indicate substantial differences between the sedimentary regimes in the vicinity of the Variscan deformation belt and more distal areas. The hydrogen index of the coals from these different sedimentation areas is also significantly different. The apparent variation of Tmax-vitrinite reflectance correlations are discussed in relation to maceral group composition and hydrocarbon generation during maturation. The correlation between both maturity parameters is compared with literature data. The differences between our maturity data and published data are significant. These differences are quantified and illustrated by geological reconstructions of the South Limburg area (The Netherlands). It is demonstrated that the use of generalized correlation schemes for the purpose of geological modeling may lead to a misinterpretation of the geological history. Key words--maceral analysis, Rock-Eval, vitrinite reflectance, Tmax, hydrogen index, burial history, The Netherlands

INTRODUCTION A wide variety of both physical and chemical maturity parameters is applied to classification schemes in the coal industry, to oil/gas source rock analyses, and to geological reconstructions. Despite the difficulties in measuring vitrinite reflectance in sediments other than coal, reflectance data have become a standard for the degree of organic metamorphism. The evolution pattern of vitrinite reflectance is intrinsically related to the thermal history of organic matter. Measured vitrinite reflectance data are generally regarded as reliable thermal indicators for the estimation of palaeotemperatures in geohistory modeling. Therefore, other observed maturity data are often converted into vitrinite reflectance equivalents by means of empirical calibration curves. Another commonly measured indicator of maturity is the Rock-Eval pyrolysis parameter Tmax. Like vitrinite reflectance this parameter can be applied to a relatively wide maturity range (Espitali6 et al., 1977). As both parameters Tmax and vitrinite reflectance are temperature dependent, a fair correlation between these two parameters can be expected. Indeed, the correlation between Tmax and vitrinite reflectance values of coals is well-established (Teichmiiller and Durand, 1983; Durand and Paratte, 1983; Espitalir, 1986). These studies have shown a tight covariation of Tmax and vitrinite reflectance although in detail the correlation curve is not regular. The progressive changes in both Tmax and vitfinite reflectance values are essentially controlled by kinetic parameters which determine the geothermally

induced maturation processes of organic matter (Espitalir, 1986; Larter, 1989; Burnham and Sweeney, 1989; Ungerer, 1990). However, there are other factors that have specific influences on both maturity indicators. Vitrinite reflectance is measured on one coal constituent (telocollinite). This parameter is considered to be independent of the bulk-composition of the organic material. Tmax values are to a large extent determined by the type and the amount of organic material, and its averaged chemical composition (Espitalir, 1986). It has been suggested that the environmental conditions at the time of peat formation and the early diagenetic transformation of vitrinite precursors to a certain extent do control the ultimate chemical/physical properties of vitrinite as well (Hutton and Cook, 1980; Price and Barker, 1985; Fermont, 1988; Veld and Fermont, 1990; Mastalerz, 1991). Variations in Tmax values may also be related to the presence of mineral matter or oxidation phenomena of the organic material (Espitalir, 1986). A factor which is generally not considered in the chemical kinetic models related to vitrinite reflectance is that vitrinite maturation is not only a chemical process but also reflects a change in the structure of the vitrinite macromolecule, not necessarily accompanied by significant chemical changes (Carr and Williamson, 1990). These factors may have a significant influence on the ultimate values of Tmax and/or vitrinite reflectance. Therefore, it is not surprising that even for well-defined kerogen types, there is not one unique correlation between both parameters when different sedimentary basins are considered. The current paper 659

660

HARRYVELDet

presents the results of vitrinite reflectance and Tmax measurements carried out on a large set of coal samples from different wells situated in The Netherlands. These are compared to data sets derived from other sedimentary basins. The hydrogen index, representing the amount of released hydrocarbons during a Rock-Eval pyrolysis, is another important parameter related to type and maturity of the organic matter. This parameter is also discussed in relation to maturity and geographic oigin of the samples. The application of the maturity parameters Tmax and vitrinite reflectance in geological reconstructions will be considered. MATERIALS AND METHODS

Coal samples have been collected from several cored wells. These wells are situated in two areas, the South Limburg area in the southern part, and the

al.

Achterhoek/Twenthe area in the eastern part of The Netherlands (Fig. 1). From the South Limburg area samples from seven wells have been studied: Kemperkoul-1 (KPK-I), Limbricht-I (LBR-1), Jabeek (XLV), Oirsbeek (LI), Geleen (XIV), Wiggelraderhof (XL), and Douvergenhout (XIX). Samples from three wells in the Achterhoek/Twenthe area have been studied: Joppe- 1 (JPE- 1), Hengevelde1 (HGV-1), and Ruurlo-1 (RLO-1). The wells from both areas comprise coal-bearing Upper Carboniferous (Westphalian) strata. The ages range from Middle Westphalian A to Late Westphalian C. A summary of the depth intervals and the stratigraphy of each individual well is given in Table 1 and Fig. 2. As reference levels three different maine horizons have been chosen. The Aegir marine band is commonly considered as the Westphalian B/C boundary. The Domina marine band represents the Early-Late Westphalian B boundary. The Cathaina marine

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JPE-1

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GERMANY THE NETHERLANDS

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Fig. 1. Locality map with the studied wells in the Achterhoek/Twenthe area, and in the South Limburg area, The Netherlands.

661

Organic petrological characterization of Westphalian coals Table 1. Summaryof the depth intervalsand Westphalianstratigraphy of the 10investigatedwells.Aegir= WestphalianB/C boundary, Domina=Early/Late Westphalian B boundary, Catharina= Westphalian A/B boundary Marine Band Well KPK-I XLV LBR-I HGV-1 JPE-I XL LI XIX XIV RLO-I

Top Carb. Total depth Aegir Domina Catharina (rn) (m) (m) (m) (m) 489.00 789.27 827.70 850.60 915.40 348.00 348.30 349.52 606.80 792.30

1665.00 1355.00 1074.00 1500.00 1493.95 1001.55 565.74 1382.60 1043.66 1503.20

1227.67 1345.27 -1086.55 1130.97 570.00 460.30 ----

1598.03 --1360.80 1422.00 960.00 -689.52 881.80 813.74

-------1124.52 -1184.95

band represents the Westphalian A/B boundary. Correlations are based on the presence of these marine marker bands and palynological/palaeobotanical evidence. In the correlation diagram of Fig. 2 the lateral

thickness changes of the Westphalian strata are neglectext. More detailed information of the wells is given elsewhere (Fermont, 1986; Fermont, 1988; Van de Laar and Fermont, 1989). Due to the stratigraphic overlap of several wells there is a dominance of coal samples from Upper Westphalian B and Lower Westphalian C sediments. Coal petrographic analyses (vitrinite reflectance measurements and maceral analyses) and sample preparations have been carried out following standard procedures (ICCP, 1963, 1971). Mean random vitrinite reflectances of telocollinite from 403 samples based on 50 measurements per sample have been calculated. The quantitative maceral analyses have been performed on 130 samples according to the Stopes-Heerlen system of maceral classification (ICCP, 1963, 1971, 1975) by applying the point counting method. For the Rock-Eval analysis representative splits of the same samples as for the vitrinite reflectance

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HARRY VELD et al.

662

measurements have been prepared following the procedures outlined by Espitali6 et al. (1977). A Rock-Eval procedure includes the heating of the sample at a heating rate of 25°C/min. The pyrolysis products released between 300 and 600°C are recorded by the FID as the $2 signal (milligrams of hydrocarbons/gram of rock). The temperature at which the maximum amount of hydrocarbons is generated (culmination of the $2

peak) is recorded as Tmax (°C). The total organic carbon content (TOC) was determined by the Rock-Eval instrument equipped with a carbon module. The hydrogen index (HI, in milligrams of HC/gram TOC) is determined as the ratio of $2 per TOC. Every tenth sample a standard (IFP 55000) and a blank have been analyzed in order to monitor and readjust a possible drift of the equipment.

Table 2. Vitrinite reflectance (%Rm), Tmax (°C) and HI (mg HC/g TOC) data Seam

Depth

%Rm

Tmax

HI

a

b C

d e

I

Ib Ic Id II lib IIc lid IIg

iij III IIIa lllc llId IIIe IV IVa V Va Ve VI Via VIb VId VII VIIa VIII VIIIb VIIIc VIIId IX X XI XIa XlI XIIa XIII XIIla XIV XV XVI XVII XVIII XIX XX XXb XXI XXIa XXIb XXIc XXII XXIIb XXIIc XXIId XXIIe XXIIf XXIII

492.10 493.15 499.60 507.61 521.70 531.54 534.12 542.88 554.64 566.30 585.33 586.32 589.99 603.70 610.79 623.50 624.72 656.16 679.81 680.88 681.55 683.31 693.01 695.08 716.83 732.15 739.88 748.99 759.96 770.77 772.19 785.91 794.48 804.23 822.64 828.01 836.52 863.92 865.42 869.76 872.79 906.11 919.62 936.64 948.22 969.27 980.30 994.56 1005.58 1055.00 1064.52 1079.28 1082.71 1101.62 1108.11 1118.26 1125,12 1134,04 1140,82 1145.95 1155.77 1173.66

0.67 0.68 0.67 0.67 0.70 0.69 0.68 0.69 0.70 0.71 0.72 0,73 0.73 0,72 0.74 0,73 0.73 0,74 0,76 0.77 0.79 0.77 0.77 0.76 0.79 0.79 0.78 0.76 0.73 0.79 0.79 0.80 0.79 0.81 0.80 0.78 0.80 0.83 0.83 0.80 0.81 0.84 0.85 0.84 0.87 0.85 0.87 0.82 0.85 0.90 0.94 0.92 0.92 0.89 0.88 0.88 0.87 0.86 0.90 0.94 0.92 0.94

Seam

Depth

%Rm

Tmax

HI

KPK-I, Kemperkoul (Cont'd)

KPK-I, Kemperkoul (n = 93) 428 431 429 430 430 430 430 431 432 433 431 433 432 432 433 432 431 433 433 433 434 433 433 432 437 434 434 433 432 434 436 434 436 435 435 435 435 435 434 436 435 437 438 437 438 438 438 438 438 442 441 442 441 443 442 441 443 444 444 445 443 444

156 116 155 159 151 144 118 126 151 173 130 127 140 179 172 161 183 155 185 190 196 208 167 181 103 136 183 141 189 205 133 195 145 204 207 203 178 184 224 206 189 188 175 257 208 218 195 225 225 214 230 221 227 176 189 233 201 254 230 192 225 204

XXIV XXV XXVa XXVd XXVI XXVII XXVIII XXVIIIa XXVIIIb XXVIIIe XXVIIIf XXVIIIg XXVI~ XXVIIIm XXVIIIn XXVlllo XXIX XXX XXXI XXXIc XXXIe XXXIg XXXIh XXXII XXXIIb XXXIII XXXIV XXXIVa XXXV XXXVa XXXVb

1193.17 1227.67 1230.86 1248.00 1254.48 1262.12 1266.32 1273.21 1282.05 1296.16 1316.86 1319.06 1342.21 1382.57 1391.39 1392.70 1402.45 1437.15 1449.92 1483.67 1495.91 1503.92 1520.42 1527.67 1546.70 1550.38 1571.67 1598.03 1616.92 1644.42 1663.33

798.55 811.45 853.05 853.75 859.55 882.80 899.35 899.85 901.15 923.55 942.25 943.25 944.70 963.90 985.60 986.00 1065.15 1065.60 1083.80 1143.80 1164.90 1165.25 1168.95 1177.60 1188.25 1209.85 1268.80 1346.25

Depth

%Rm

Tmax

HI

428 426 428 428 428 433 431 434 431 431 432 433 434 430 432 432 434 433 434 433 431 435 434

223 179 122 154 173 169 221 199 205 140 207 223 175 145 160 146 191 159 191 190 178 225 186

424 424 427 431 427 424 428 424 428 428 430 428 428 427 429 427 427 429 425 430 429 429 430 431 431 430 430 431 429 432 432 431 431 432 433 432 435

216 238 187 157 243 298 249 270 204 207 168 215 222 260 199 274 285 213 262 219 280 301 290 208 228 264 220 205 217 230 228 256 261 257 226 233 212

LBR-I, Limbricht (n = 23)

0.95 0.82 0.85 0.89 0.88 0.92 0.90 0.95 0.93 0.92 0.97 0.97 0.96 0.98 0.99 1.00 1.01 1.01 1.05 1.05 1.08 1.08 1.10 1.07 1.14 1.16 1.16 1.16 1.25 1.26 1.22

447 449 447 448 449 451 450 448 447 452 453 451 453 454 455 457 456 458 458 461 461 462 463 461 462 465 467 464 469 468 468

223 230 206 211 187 194 199 187 245 315 173 184 171 152 186 203 201 176 176 178 171 146 193 196 175 179 172 173 175 164 131

0.79 0.80 0.80 0.79 0.79 0.79 0.79 0.79 0.80 0.81 0.80 0.81 0.79 0.79 0.81 0.83 0.87 0.87 0.87 0.90 0.91 0.91 0.93 0.93 0.91 0.91 0.95 0.88

433 433 432 434 431 432 433 433 432 433 433 433 433 434 434 434 434 434 435 438 440 438 438 434 436 438 439 439

80 142 151 187 169 148 134 188 152 155 90 123 96 135 139 111 161 161 167 106 160 139 117

XLV, Jabeek (n = 28) a b d e f h j k 1 m n o q r s t v w x aa ab I Ia Ic Id Ie Im II

Seam

156 122 141 181

I Ia Ib Ic Id If II III llIa IIIc IV V Va Vb Vd Vf VI Vlb VII VIII IX X XI

844.49 850.24 858.24 869.74 875.11 882.55 901.52 918.08 923.83 933.23 939.46 945.30 947.12 963.61 968.66 976.34 1003.60 1006.05 1006.97 1028.45 1043.75 1063.83 1065.50

0.74 0.71 0.74 0.73 0.75 0.76 0.77 0.79 0.76 0.77 0.79 0.79 0.79 0.80 0.80 0.80 0.79 0.79 0.80 0.83 0.83 0.84 0.84

HGV-I, Hengevelde (n = 48) a b I II IIa lib IIc lid III IV V Vb Vc Vd VI Via VIb VIe VId VII VIIa Vllb Vllc Vlld Vile Vllf Vllg Vllh VIii VIII IX IXa IXb IXc IXd IXe IXf

901.78 903.10 933.40 938.56 941.35 957.89 962.13 967.58 991.72 1015.74 1026.93 1034.85 1086.55 1104.59 1105.74 1113.92 1120.13 1120.67 1129.27 1154.85 1177.66 1178.22 1179.86 1180.26 1180.59 1185.19 1197.23 1199.66 1200.06 1214.01 1227.66 1242.52 1253.60 1259.89 1266.75 1275.68 1275.88

0.57 0.57 0.63 0.71 0.60 0.56 0.62 0.55 0.66 0.64 0.68 0.67 0.67 0.61 0.72 0.67 0.63 0.72 0.71 0.77 0.68 0.61 0.67 0.79 0.79 0.72 0.76 0.80 0.75 0.79 0.78 0.75 0.76 0.82 0.79 0.82 0.85

O r g a n i c p e t r o l o g i c a l c h a r a c t e r i z a t i o n o f W e s t p h a l i a n coals

663

Table 2-----continued Seam

Depth

%Rm

Tmax

HI

HGV-I, Hengevelde (Cont'd) IXg IXh X Xa Xb Xc XI XIa Xlc XId Xle

1276.96 1282.26 1313.31 1334.04 1373.50 1383.99 1412.73 1417.74 1462.76 1464.60 1473.08

970.26 972.70 978.46 979.98 980.50 980.96 981.30 1016.30 1017.26 1026.10 1028.03 1032.50 1033.94 1042.82 1043.94 1073.72 1084.00 1091.32 1091.67 1094.82 1108.07 1109.40 1130.98 1131.85 I 156.04 1163.93 1172.52 1175.35 1175.99 1189.07 1192.02 1198. I I 1207.24 1227.70 1238.85 1241.30 1246.80 1249.53 1255.14 1259.16 1269.20 1282.22 1287.15 1300.91 1317.20 1345.41 1347.21 1360.02 1372.57 1379.38 1404.34 1422.00 1455.06 1465.62 1471.89 1484.12

0.84 0.82 0.86 0.87 0.69 0.84 0.92 0.78 0.76 0.89 0.92

434 435 435 435 435 437 437 436 438 439 438

176 235 233 240 263 248 226 263 273 253 239

0.86 1.07 0.95 1.02 1.02 1.02 1.01 0.98 1.00 0.99 1.02 1.05 1.05 1.08 0.99 1.02 1.11 1.02 1.09 1.13 1.06 1.04 1.07 1.11 1. I 1 1.19 1.16 1.13 1.12 1.09 1.12 1.10 1.24 1.02 1.44 1.21 1.22 1.15 1.28 1.23 1.17 1.26 1.21 1.22 1.34 1.26 1.38 1.42 1.41 1.33 1.43 1.40 1.49 1.45 1.47 1.56

450 452 454 454 451 453 455 453 454 454 454 455 456 455 453 455 457 466 457 457 455 454 457 455 458 461 460 463 458 461 454 456 463 452 466 463 462 464 466 461 462 464 463 465 468 471 471 471 470 471 475 475 476 474 474 476

195 182 171 198 171 197 227 190 190 212 185 211 168 183 217 181 181 213 201 179 176 230 202 180 205 180 190 184 176 153 200 206 179 223 181 161 172 142 182 172 167 159 172 133 148 145 139 132 137 134 126 127 119 106 118 133

a b

140 123 83 210 184 140

d g k I II III IV V

XL, Wiggelraderhof (n = 48) d f g h k I

377.35 399.85 401.10 404.40 421.55 446.10

0.86 0.88 0.89 0.89 0.90 0.90

Depth

%Rm

Tmax

HI

XL, Wiggelraderhof (Cont'd)

JPE-I, Joppe (n = 56) a I Ia Ib Ic Id Ie If Ig Ih Ii Ij Ik I1 Im In II lib IIc III Illa Illb Illc Illd Ille IV IVa IVb IVc IVd IVe IVf V Va VI Via Vlb VIc VII VIIa Vllb VIIc VIId VIIe VIIf VIII VIIIa VIIIb VIlIc VIIId VIIIe VIIIf VIIIg VIIIh VIIIi IX

Seam

435 435 437 433 433 436

lI IIa lib IV V VI VIb VIc VId Vie VIf VIg VIk Vim VIn VII VIIa VIIb VIIc VIId VIIf VIIg VIIk VIIm VIIn VIIo VIIr VIIs VIIt VIIv VIII VIIIa VIIIb VIIIc IX X Xa XI XII Xlla XIlb XIII

465.55 459.50 465.65 506.60 530.35 570.60 574.55 575.45 578.30 580.15 589.05 589.65 593.20 602.55 607.30 614.35 622.05 622.95 656.55 669.75 691.15 704.10 722.00 726.85 727.50 738.15 758.40 765.25 771.55 799.65 842.00 854.85 855.45 871.05 882.25 894.50 905.05 914.00 936.55 939.85 962.25 989.35

c d e f g h I II IIa IIb IId IIe IIf IIg lib IIi IIk IIl IIm IIn

351.30 353.35 362.45 379.90 390.85 391.30 404.75 410.00 427.15 461.20 473.45 475.70 488.80 489.90 494.50 495.65 509.55 517.40 517.40 532.65 533.35 534.00

0.89 0.91 0.94 0.93 0.93 0.82 0.94 0.94 0.94 0.90 0.96 0.93 0.93 0.97 0.95 0.95 0.96 0.97 0.97 0.99 0.99 0.95 I.O0 0.97 0.97 1.01 1.03 1.00 1.00 1.00 1.05 1.04 1.07 1.04 1.08 1.07 1.06 1.05 1.10 1.11 1.14 1.18

434 437 436 440 439 441 440 441 440 440 441 438 440 442 442 441 441 459 444 442 446 443 447 445 445 448 451 449 452 451 461 454 456 455 456 454 455 455 457 454 459 464

187 91 160 116 177 184 155 79 187 208 150 153 181 147 147 150 184 156 177 188 134 178 133 205 189 184 171 157 92 189 162 176 161 167 155 182 164 157 178 171 165 165

0.90 0.89 0.89 0.88 0.91 0.91 0.92 0.92 0.93 0.82 0.91 0.95 0.94 0.94 0.91 0.93 0.92 0.93 0.91 0.94 0.93 0.93

434 434 433 434 436 436 436 437 439 435 434 435 436 435 434 434 439 438 440 442 442 443

187 166 188 177 150 166 178 146 182 183 154 I 17 158 162 189 145 187 161 163 187 192 214

439 441 440 442 445 445 451 450 450 451

133 125 153 175 188 I 12 169 136 145 159

XIX, Douvergenhout (n = 27) 375.35 382.70 438.50 476.60 532.20 568.70 609.75 631.00 641.15 658.45

0.91 0.91 0.94 0.95 0.96 0.99 1.06 1.03 1.04 1.07

Depth

%Rm

Tmax

HI

1.13 1.11 1.13 1.19 1.27 1.24 1.21 i.28 !.30 1.29 1.48 1.53 1.49 1.49 1.62 1.63 1.63

454 457 461 461 466 466 467 469 450 472 477 478 479 482 490 487 491

146 134 169 143 122 119 98 132 178 94 90 135 94 99 98 54 62

0.94 0.91 0.88 0.91 0.92 0.94 0.97 0.96 0.94 0.93 0.95 0.97 0.97 0.95 1.01 0.94 0.94 1.01 1.04

433 433 433 435 433 433 435 437 434 437 438 439 436 437 440 441 441 444 443

185 127 121 95 191 142 142 154 161 183 160 177 149 181 180 132 148 191 164

449 449 450 450 450 450 451 452 454 454 451 453 455 450 454 461 457 458 462 463 463 467 462 471 462 466 467 468 465 468 468 464 468 472 471 470 471 476 479

220 187 193 190 198 184 191 204 156 181 228 195 172 246 190 183 186 190 160 147 178 171 163 147 184 147 129 180 154 145 135 164 154 148 129 144 132 121 129

XIX, Dotwergenhout (Cont'd)

LL Oirsbeek (n = 22) ~.

Seam VI VII VIII IX X XI XIII XIV XV XVI XVIII XIX XX XXI XXIa XXII XXIII

714.30 738.55 782.70 83%00 873.80 888.85 902.25 953.00 995.95 1036.35 1145.35 1175.00 1206.20 1257.75 1278.85 1309.65 1329.40

XIV, Geleen (n = 19) I la Ib Ic le II lla lid lie lllb llIc V Vb VII VIIIa IX IXa X Xa

663~19 672.22 677.15 677.78 695.97 727.68 734.48 751.62 759.15 811.75 817.85 831.97 859.73 929.41 968.53 985.37 1005.38 1028.87 1040.12

RLO-I, Ruurlo (n = 39) a b c I Ia Ic Id le Ig II IIa llc lid lie IIf IIg IIh III IIIa IIIb IIIc IV IVa V Va Vb Vc VI Via VIb VIc VId Vie VII Vllb VIIc VIId VIIe VIIf

813.80 826.83 832.44 852.07 860.23 870. I I 888.73 912.91 954.71 997.00 999.57 1042.82 1050.46 1055.95 1066.88 1112.68 1116.14 1185.60 1188.52 1192.18 1194.31 1212.35 1213.89 1235.53 1238.87 1260.62 1262.43 1270.15 1287.31 1293.14 1301.67 1309.83 1310.72 1347.59 1372.38 1378.10 1435.81 1454.80 1478.82

0.96 0.89 1.02 1.02 0.99 1.05 1.01 1.02 1.11 1.05 0.97 1.07 1.05 1.02 1.04 1.18 l.lO 1.15 1.09 1.23 1.26 1.31 1.21 1.29 1.16 1.27 1.26 1.33 1.33 1.21 1.32 1.30 1.35 1.37 1.40 1.21 1.41 1.47 1.45

HARRYVELD et al.

664

Table 3. Summary of the measured maturity data for the investigated wells %Rm (oil) Tmax (°C)

RESULTS

Vitrinite reflectance The vitrinite reflectance data are listed in Table 2. A summary o f the vitrinite reflectance ranges and average vitrinite reflectance/depth gradients o f all 10 wells is given in Table 3. The overall reflectance values range from 0.56 to 0 . 9 2 % R m in well H G V - I and 0.91 to 1 . 6 8 % R m in well XIX. Figure 3 shows the vertical reflectance profiles of five selected wells, representing the full range of vitrinite reflectance values. There is a significant variation in the gradient values, which is not related to present depth and/or stratigraphy. F o r instance, the wells H G V - I and JPE-I have an almost identical stratigraphic range (Late Westphalian B-Early Westphalian C), yet the reflectance values of the latter are much higher. Moreover, the average gradient of well JPE-I is significantly higher as compared to well HGV-1. In

Well KPK-I XLV LBR-I XL LI XIX XIV HGV-I JPE- 1 RLO-I

Gradient (/km) 0.40 0.35 0.45 0.50 0.16 0.75 0.24 0.61 1.05 0.78

Range 0.6%1.26 0.79-0.95 0.71~).84 0.82-1.19 0.82~).95 0,91-1.68 0,88-1.04 0,55~.92 0.86-1.56 0.89-1.47

Range 428-469 431-440 426-440 433-464 433-443 439-491 433 444 424-439 450-476 449-479

Gradient (/km) 31.6 11.0 27.1 40.8 60.2 51.4 23.9 22.8 44.8 43.6

contrast, the reflectance values of well HGV-1 and RLO-1, which wells are of different ages, represent one well-defined trend (Fig. 3). The reflectance values in each well show a distinct non-linear increase with

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Organic petrological characterization of Westphalian coals

665

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increasing depth, although there are large differences between the vitrinite reflectance values of different wells at comparable present day depths. Furthermore, there is a remarkable difference between the vitrinite reflectance patterns in the wells of the Achterhoek/Twenthe area compared to those in South Limburg. The largest variation in reflectance values is observed in well HGV-1. The differences between subsequent samples along the depth profile of this well may range up to values of 0.15%Rm or more. Similar differences in vitrinite reflectance values are observed in well JPE-1 and, to a lesser degree in well RLO-I. Such large differences are not observed in the South Limburg area. Here, however, the individual trends in vitrinite reflectance show that significant lower vitrinite reflectance values occur in the interval immediately below the Aegir marine band

(Westphalian B/C boundary). Reduced vitrinite reflectance values are also observed in this interval of other wells. Tmax

The Tmax values are presented in Table 2. A summary of the trends is given in Table 3. The Tmax values against the depth of five wells are presented in Fig. 4. These are the same wells as depicted in Fig. 3. The overall Tmax values range from 424 to 439°C in well HGV-1 and from 439 to 491°C in well XIX. Although the individual fluctuations in the measured Tmax values may be different from the vitrinite reflectance fluctuations, the general pattern of increase of Tmax with increasing depth is rather similar to the pattern observed for vitrinite reflectance values against depth. Wells with the highest

HARRY VELD et al.

666

Table 4. Summary of the means and ranges of the maceral group frequencies in the samples of each well %Lipdnite

%Vitrinite

%Inertinite

Well

Min.

Mean

Max.

Min.

Mean

Max.

Min.

Mean

Max.

N

KPK-I XLV LBR-I XL LI XIX XIV

1.5 9.0 10.8 2.0 10.0 1.0 7.0

12.0 15.0 14.9 9.4 14.0 5.2 11.3

18.3 21.0 20.5 18.0 18.0 14.0 17.5

57.1 75.0 62.0 70.0 69.0 73.0 66.5

72.9 79.0 72.2 79.6 76.5 82.9 77.2

85.8 83.0 83.0 91.0 84.0 92.0 82.0

6.1 4.0 6.0 5.0 6.0 4.0 6.0

15.1 6.0 12.9 11.0 9.5 11.9 11.5

25.0 8.0 20.0 15.0 13.0 23.5 16.0

45 2 11 11 2 22 6

HGV-I JPE-I RLO-I

7.0 2.0 4.2

14.4 7.9 9.8

25.5 15.4 17.2

53.5 67.5 63.5

67.7 70.8 71.4

79.4 74.5 13.1

7.5 13.1 18.8

17.9 21.3 24.5

23.4 25.0

16 8 7

S. LIMBURG ACHTERH./TWENTHE

11.7 10.7

77.2 70.0

reflectance values also have the highest Tmax values. Tmax shows also a much wider variation with increasing depth in the Achterhoek ITwenthe area rela-

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Organic petrological characterization of Westphalian coals

667

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Hydrogen index The hydrogen index (HI) values of the samples is given in Table 2. The HI against depth of the five selected wells is displayed in Fig. 5. The HI values show a wide scatter between 54 (well XIX) and 315 (well KPK-I). Individual wells also show a wide variation of HI values within relative short depth-intervals. The HI differences may be as high as 140, or even more (well HGV-1). Figure 5 shows that the HI values in the South Limburg area are closely related to relative depth. The hydrogen index first shows a progressive increase up to an averaged maximum of approx. 210. These maximum values are measured on samples mainly derived from lower Westphalian C coals. The Westphalian A and B samples show a trend of decreasing values with depth. The HI values against relative depth of the three wells in the Achterhoek/Twentbe area form three different trends, not related to stratigraphy. For instance, a remarkable difference exists between the wells HGV-1 and JPE-1. These wells have a comparable stratigraphic range and present day depths, yet the HI trends against depth are almost opposite. This feature is obviously

due to the different maturity ranges of both wells.

Petrographic composition The average maceral group composition of 130 samples from both study areas is given in Table 4. Vitrinite is the most abundant maceral group. The averages for each well range from 67.7 to 82.9%. The average liptinite frequencies for each well range from 5.2 to 15.0%, and the average inertinite contents vary between 6.0 and 21.3%. The large fluctuations in maceral group distribution and the generally low sample density preclude the establishment of definite depth related trends in the maceral group composition within individual wells. Remarkably, there is a distinct difference between the inertinite content of the samples from the Achterhock/Twenthe wells and those from the South Limburg wells. The average inertinite contents amount to 19.3% in the Achterhoek/Twenthe area and 11.1% in the South Limburg area, respectively. This difference is considered highly significant. Consequently, the figures for the liptinitc and vitrinite percentages are reversed, due to the closed sum effect of the analyses.

668

HARRYVELDet al.

Tmax

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DISCUSSION Palaeogeography

The persistent higher variation within the vertical trends of vitrinite reflectance values and Tmax values encountered in the Achterhoek/Twenthe wells are difficult to explain with thermal modeling because this would imply large and long-lasting temperature differences at relatively short vertical distances (Fermont, 1988). Fermont (1988) suggested that the high variability of the Achterhoek/Twenthe maturity parm e t e r s is related to differentiation of the vitrinite precursors during the early diagenetic biodegradation processes induced by variations in the depositional circumstances. The higher variability of the vitrinite reflectance and Tmax values in the Achterhoek/ Twenthe area compared to the South Limburg area may be related to the relative position of these areas in the Variscan foredeep (Ziegler, 1982). Our data may support the hypothesis that towards the north there is a much wider tectonic instability during the Westphalian B/C. This is surprising because the front of tectonic activity and Variscan deformation is found in the present day Ardermes, south of South Limburg. A possible cause might be that the subsidence directly in front of the Variscan mobile belt was more continuous, or at least more regularly compensated by sedimentation. Thus reducing the actual

number of subenvironments. In contrast, in the Achterhoek/Twenthe area the sedimentation rate could be disturbed more frequently, for instance by small inverse basement movements or interruptions of sediment supply. This is supported by the occurrence of two recently discovered marine bands in the Westphalian B of the Achterhoek area (Van Amerom et al., 1986). Differences between the sedimentary regimes of the two areas are also supported by the differences in the percentages of inertinite which are significantly higher in the Achterhoek/Twenthe area. High abundances of the inertinite macerals may indicate more pronounced oxic conditions, mainly established as a consequence of a low or fluctuating groundwater level and more pronounced drainage patterns (Calder et al., 1991). A similar difference in the inertinite abundances was also noted by Strehlau (1990) in his comparison between the Ibbenbiiren region and the Ruhr region. The Ibbenb~ren region is situated north of the Ruhr region and east of the Achterhoek/Twenthe area. The higher inertinite contents of the Ibbenbiiren region were explained by Strehlau (1990) to be the result of a constantly lower groundwater level, due to a higher elevation of this area compared to the Ruhr region. The reduction of vitrinite reflectance values below the Aegir marine band in the South Limburg area has been observed many times. It has been related to the

Organic petrological characterization of Westphalian coals

669

shows that the increase of Tmax as a function of vitrinite reflectance is not linear or logarithmic. The averaged correlation curve derived from this data matrix is rather flexuous (Fig. 7). Up to a reflectance value of 0.85%Rm the correlation is more or less linear• The first flexure occurs at a reflectance value of approx. 0.85-0.90%Rm, and the second is observed

early diagenetic activity of sulphate-reducing bacteria (Veld and Fermont, 1990).

Correlation between Tmax, vitrinite reflectance and hydrogen index The correlation between Tmax and vitrinite reflectance for all samples is depicted in Fig. 6. This figure

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670

HMtR~FVELD et al.

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at a reflectance value o f 1.35%Rm. Within the reflectance range from 0.90 up to 1 . 3 5 % R m the correlation shows a curved path. Above a reflectance value of 1 . 3 5 % R m the correlation becomes linear again. F o r comparison, Fig. 7 also shows a part of the vitrinite reflectance-Tmax correlation curve of Teichmiiller and D u r a n d (1983). There are significant differences

between both correlation curves. U p to values of approx. 0 . 9 5 % R m our data shows lower T m a x values at comparable vitrinite reflectance values, although the general slope is similar. In the range between 0.95 and 1.35%Rm our data shows higher T m a x values. A b o v e 1.35%Rm no significant differences are observed. Although similar standard

671

Organic petrological characterization of Westphalian coals Rock-Eval analyses and vitrinite reflectance measurements were performed for both sample sets, the sample preparation and the calibration of the equipment may also partly contribute to the observed differences. It is also possible that the application of maturity data from one basin to another is of restricted value, because of differences in sedimentary regimes, and consequently in maceral composition or geochemical characteristics. No detailed maceral analyses are available for the samples of both correlation curves. The data set of the Teichm/iller and Durand (1983) correlation includes samples from a wide range of stratigraphically and geographically different coals, e.g. Miocene Mahakam coals, Jurassic North Sea coals, and Late Carboniferous coals from the Paris basin (Teichmfiller and Durand, 1983; Durand and Paratte, 1983; Espitalir, 1986). The material used for our correlation are samples exclusively from Westphalian coals. Therefore it is likely that some of the observed differences between both correlations are the consequence of different coal petrographic compositions. The cause of the flexuous pattern in the correlation between Tmax and vitrinite reflectance may be simply statistical. There is a relative overrepresentation of data points around a reflectance value of 0.90%Rm. However, it is already well recognized that variations in Tmax and vitrinite reflectance values may be related to the petrographic composition, notably the amount of liptinite. Elevated liptinite contents result in higher Tmax values. Up to

[--] D

reflectance values of approx. 0.95%Rm the liptiniterich kerogens have higher Tmax values compared to coal at similar reflectance values (Espitalir, 1986). The effect of elevated liptinite contents on vitrinite reflectance has also been reported many times. Coals with a high amount of liptinite display lower vitrinite reflectance values than coals with a low amount of this maceral group (e.g. Kalkreuth, 1982; Murchison et al., 1991). This implicates that intervals with a high liptinite content in a sequence with an otherwise moderate liptinite abundance will show suppressed vitrinite reflectance and increased Tmax values. Therefore variations in the petrographic composition (i.e. liptinite content) of the coals may provide a possible explanation for the flexuous pattern of our Tmax-vitrinite reflectance curve. Our data show that there is a clear trend of progressive decreasing liptinite percentages with increasing maturity (Fig. 8). Moreover, there appears to be a break in the liptinite abundance around vitrinite reflectance values of 0.95%Rm, approximately where a significant bend in the correlation curve occurs. It cannot be excluded, however, that the decrease of liptinite content with increasing maturity partly reflects the problems of identification of this maceral group at higher maturity levels. The ability of coals to generate liquid hydrocarbons has been acknowledged by many authors (Teichmiiller and Durand, 1983; Durand and Paratte, 1983; Tissot and Welte, 1984; Horsfield et al., 1988; Huc et al., 1986). It is of interest to note that a major

WESTPHALIAN

mE A

r3

0I

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Fault

Fig. 10. Geological outline of the Carboniferous abrasion surface of part of the South Limburg area, with the location of the seven investigated wells used in the burial history models.

672

HARRYVELD et al.

• //~//

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(b)

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Heafflow (mW/m2)

Fig. 11. Geohistory reconstruction of the South Limburg area. Situation 1: heatflow variations at constant overburden. (a) Based on our vitrinite reflectance data, and (b) based on our converted Tmax data. change in our correlation curve occurs between 0.85 and 0.95%Rm. This reflectance interval is generally considered to correspond with the zone of maximum oil generation in coals and with one of the classical coalification "jump" intervals (Teichmiiller, 1982). Explanations for the specific path of our correlation may thus be related to the formation of hydrocarbons in coals. The hydrogen index (HI), as derived from a Rock-Eval analysis, is a frequently used parameter for the assessment of the (residual) hydrocarbon potential of humic coals (Espitali6 et al., 1985, 1986). The evolution of the HI as a function of maturity (Tmax and vitrinite reflectance) for the present coals is illustrated in Figs 9(a) and (b). The HI-vitrinite reflectance diagram shows that the evolution of HI values with increasing maturity is complex. At low maturity (below 0.80%Rm) there is a broad scatter between 80 and 240 for the South Limburg samples. With increasing maturity this variation is reduced. At 1.05%Rm the HI values vary between 140 and 200. At higher rank there is an irregular decrease of HI with increasing rank. The pattern for the Achterhoek/Awenthe area is slightly different. At low maturity the HI values are markedly higher than those of the South Limburg area. Extrapolation of the HI values of South Limburg and the Achterhoek to immature intervals suggests an entirely different chemical composition of the samples from these areas. With increasing maturity the HI values of the samples of these areas converge to more identical values although the Achterhoek samples tend to remain slightly higher. A comparable pattern is found when the HI is plotted against the Tmax [Fig. 9(b)]. In this figure the differences between the samples of Achterhoek/Twenthe and South Limburg at lower maturities are also very pronounced. Furthermore there is a distinct maximum of HI values at Tmax of approx. 440°C for the South Limburg samples. It is suggested here that the large differences in HI values of different areas are related to the sedimentary environment. These differences gradu-

ally disappear with increasing maturity and reach a minimum at Tmax values of around 440°C. In Fig. 8(b) the HI values are plotted against the liptinite content of 130 samples from the Achterhoek/Twenthe and South Limburg areas. The HI shows a distinct increase with increasing liptinite content. The samples with the high liptinite percentages are predominantly the immature samples of the Achterhoek/Twenthe area which are plotted in Fig. 8(a). The immature Achterhoek/Twenthe samples with high HI values need some additional comments. These are the samples with significantly suppressed reflectance values on the general vertical reflectance trend of well HGV-1 (Fig. 3). Most of the coal seams from which these samples were taken have been designated by Fermont (1988) as so-called "erosive sequences". The high HI values, suppressed reflectance values and relatively high liptinite abundances of these coal samples are interrelated and possibly caused by differentiations in the depositional environment (Fermont, 1988). It is exactly this set of samples that is responsible for the large differences in the HI values at low rank in the Figs 9(a) and (b). Hence, if these samples are not considered, the diagrams show patterns more similar to previously published HI against maturity diagrams (e.g. Durand and Paratte, 1983; Espitali6, 1986; Bertrand, 1989). These diagrams show a maximum in the averaged HI curve at a vitrinite reflectance value of 0.70%Rm, or even lower. The maximum of the averaged HI values for the present coals is encountered at 0.90%Rm and at Tmax of approx. 440°C. These values are significantly higher than those of the published diagrams mentioned above, indicating that hydrocarbon release started at a much later stage of maturity for the present coals. The decrease in HI above a maturity corresponding to 0.90%Rm and 440°C correlates very well with the change in the pattern of the Tmax-vitrinite reflectance curve around 0.90%Rm.

673

Organic petrological characterization of Westphalian coals

Implications for geological reconstructions Many commercially available burial history programs translate maturity parameters into vitrinite reflectance equivalents. In a model study we imported our measured Tmax-vitrinite reflectance correlation curve and the Tmax-vitrinite reflectance correlation curve of Teichmiiller and Durand (1983). The consequences of the application of these different correlations between Tmax and vitrinite reflectance are illustrated in a burial history study of the South Limburg area. A summary of the geology of South Limburg and the position of seven wells in this area is given in Fig. 10. South Limburg is situated in the Variscan foredeep, approx. 20km north of the Variscan mobile belt. The Westphalian sediments have been buried deeply at the end of the Carboniferous. In this time the present-day maturity pattern has been established. After the Variscan orogeny intense erosion took place. The present-day Palaeozoic abrasion surface is overlain by a thin cover of Triassic-Tertiary sediments. The study area has been modified tectonically by Variscan up- and overthrusts and younger normal fault systems (Van Waterschoot-Van der Gracht, 1938; Kimpe et al., 1978). In this study a commercially available burial and thermal history modeling computer program (GAPS, Bury 5.4) was used. The algorithms used in the geohistory analysis are summarized in Tang Dazhen et al. (1990). The kinetic model describing the organic matter maturation used in this program is an adaption of that presented by Tissot and Espitali6 (1975) and Tissot and Welte (1984). This kinetic model uses a set of parallel first-order kinetic equations and the Arrhenius equation to calculate the time-temperature dependent transformation ratio of a specific type of kerogen. For the determination of vitrinite reflectance a default correlation between the transformation ratio of coal and vitrinite reflectance is used.

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,

(a): to~

The model serves well for the present purpose of comparing different maturity conversions. In the burial history study of the South Limburg area two variables are modeled: heatflow and overburden (maximum depth of burial). The combination of these two variables is considered of prime importance in thermal modeling. Other variables, like lithology, surface temperature, compaction, and conductivity are kept constant. The models were calibrated with measured vitrinite reflectance data. Optimization of the best combination of the two variables has been performed on the key-well Kemperkoul-I (KPK-1) because the youngest Westphalian C sediments are preserved here (Fig. 2). For our vitrinite reflectance data the best fit was found at a heatflow of 92 mW/m 2 and an overburden of 2100 m. A second optimization assumes that only a set of Tmax values is available. Calculated vitrinite reflectance equivalents are obtained by using the published Tmax-vitrinite reflectance conversion curve displayed in Fig. 7 (Teichmiiller and Durand, 1983). F o r our converted Tmax values a best fit was found at a heatflow of 130mW/m 2 and an overburden of 1000 m. The obtained heatflow and overburden values of KPK-1 have been taken as a reference for the whole area. Similar optimization of the other wells would likely result in somewhat different values for the heatflow and the overburden. Two situations are considered in which the effects on overburden and heatflow are calculated for the measured vitrinite reflectance values and the calculated vitrinite reflectance equivalents. In the first situation a heatflow map is constructed assuming a constant overburden for the area (2100 or 1000 m), with a correction for the additional amount of eroded sediments in the other wells as compared to KPK-1 (see Fig. 2). The two resulting heatflow maps in Figs 1 l(a) and (b) are based on our vitrinite reflectance data and the published Tmax-vitrinite reflectance conversion,

.._

SITTARD

(b)]

KPK1

• Well location 150 Relative overburden (meters)

~'100

Fig. 12. Geohistory reconstruction of the South Limburg area. Situation 2: relative overburden variations at constant heatflow. (a) Based on our vitrinite reflectance data, and (b) based on our converted Tmax data.

]

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HARRYVELD et al.

respectively. N o t only are the calculated Late Carboniferous heatflow values different, but the interpreted heatflow patterns of the area are significantly different also. In the second situation an overburden map is constructed assuming a regional constant heatflow (92 or 130 mW/m2). The two maps in Figs 12(a) and (b) display the calculated overburden relative to well KPK-1. Figure 12(a) displays the relative overburden map when our vitrinite reflectance values are applied in the model. Figure 12(b) displays the results using the measured T m a x values and the Teichm/iller and Durand (1983) correlation. The application of the two correlations between T m a x and vitrinite reflectance result in two significantly different maps. Despite the fact that both correlations between T m a x and vitrinite reflectance are established on similar types of organic material (coals), the interpretation results in two entirely different geological reconstructions of an area. It is concluded that the use of generalized calibration functions of maturity parameters for the purpose of geological modeling, which is c o m m o n practice in exploration geology, may lead to misinterpretations o f the geological history. The Tmax-vitrinite reflectance correlation curve which has been established for the Westphalian coals from The Netherlands may provide an alternative for burial history analyses as far as the local geology of the Carboniferous of the Variscan foredeep is considered. Acknowledgements--The drilling and coal petrological research was financed by the Dutch Ministry of Economic Affairs (Project Inventarisatie Nederlandse Kolenvoorkomens). The work was also supported by the Netherlands Foundation for Earth Science Research (AWON) with financial aid from the Netherlands Organization for Scientific Research (NWO). We gratefully acknowledge A. Burnham and H. B. Lo for critically reviewing the manuscript.

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