geochemical characterization of sedimentary organic matter ... · advances in organic geochemistry...

Advances in Organic Geochemistry 1989Org. Geochem. Vol. 16, Nos 1-3, pp. 175-187, 1990Printed in Great Britain. All rights reserved

0146-6380/90 $3.00 + 0.00Copyright © 1990 Pergamon Press pic

Geochemical characterization of sedimentary organic matter bymeans of pyrolysis kinetic parameters

D. DELVAUX I,2 * , H. MARTINI, P. LEPLAT2 and J. PAULET2

'Geology Laboratories, University of Louvain-la-Neuve, PI. L. Pasteur 3, 1348 Louvain-la-Neuve,Belgium

2Labofina, Research Center of Petrofina, Chee, de Vilvorde 98, 1120 Brussels, Belgium

(Received 19 September 1989; accepted 22 February 1990)

Abstract-A new means to classify organic matter in rock samples using kinetic parameters is described.Kinetic parameters are computed for Rock-Eval comparative pyrolysis curves, using a single heating rateexperiment. The kinetic parameters are computed by an improved Freeman-Carroll method, assumingan overall nth-order reaction. Kinetic parameters are used for characterizing the type and maturity ofsedimentary organic matter.

For kerogens, the kinetic parameters are obtained on extracted-rock samples. They provide acomplementary determination of organic matter type, by using the S2 peak shape which is not describedby the classical Rock-Eval parameters.

For resins + asphaltenes, the kinetic parameters are computed on the S2' pyrolysis curve obtained bycomparative pyrolysis. They allow the determination of the type of kerogen from which they were derivedand also their maturity level.

The kinetic characterization can be applied directly to current Rock-Eval analysis and used in routineanalysis, without changing the standard procedures but care should be taken for bitumen rich samples.

Key words-oil shales, pyrolysis, nth-order kinetics, kerogen and bitumen characterization, activationenergy

INTRODUCTION

Kinetic parameters are seldom used for geochemicalcharacterization of sedimentary organic matter. Theymay prove very useful however, as it is well-knownthat the shape of pyrolysis curves is dependent on thetype and maturity of the kerogen (Espitalie et al.,1985/86). In their Rock-Eval method, Espitalie et al.(1977) paid no attention to this aspect of pyrolysis,although the results are convenient for kinetic analysis. More recently, Espitalie et al. (1985/86) describedthe use of the S2 peak shape for a better organicmatter determination and for kinetic computations.

Kinetic models were mainly developed for coalsand kerogen, to describe the mechanism of pyrolysis.At present, kinetic models for kerogen degradationare frequently used in the mathematical modelingof hydrocarbon generation in sedimentary basins(Ungerer et al., 1986; Burnham and Braun, 1985;Sweeney et al., 1987, Ungerer and Pelet, 1987).

This paper describes a new means to classifyorganic matter using kinetic parameters. It providesa complementary determination of organic mattertype from the data directly available in currentRock-Eval analysis. It uses the S2 peak shape, whichis not described by the classical Rock-Eval parameters HI (hydrogen index), 01 (oxygen index) and

"Present address: Department of Geology/Mineralogy,Royal Museum for Central Africa, 1980 Tervuren,Belgium.

Tmax • In order to avoid any change in the Rock-Evalanalytical procedure, the kinetic parameters are to becomputed on a single constant heating rate experiment. The nth-order Freeman and Carroll (1958)method is used, with some improvements, for thedetermination of kinetic parameters from pyrolysiscurves. In a recent work, Delvaux (1988) havedemonstrated that the activation energies and reaction orders for the pyrolysis of kerogen andresins + asphaltenes (R + A) in rock samples are,within certain limits, representative of their originand maturation rank. The activation energies andreaction orders are then plotted in reference diagramsin order to classify the organic matter type.

In order to increase the possibilities of this kineticcharacterization technique, it is desirable to computekinetic parameters on pyrolysis curves obtained bythe new comparative pyrolysis method. (Delvaux etal., 1990), which is a recent development of theRock-Eval method of Espitalie et al. (1985/86). Forkerogens, kinetic parameters are computed on extracted rock samples to avoid contamination byheavy products of the bitumen fraction. For R + A,the kinetic parameters are computed on the S2' peakobtained by the comparison of the whole-rock pyrolysis curve and the extracted-rock pyrolysis curve(Fig. 1). With the combination of comparative pyrolysis and kinetic analysis, it becomes possible toclassify the R + A and to assign them to the kerogentype from which they are derived. This is of great

175

176 D. DELVAUX et al.

COMPARATIVE PYROLYSIS ~IKINETIC ANALYSIS I

2.0 %HC/oC

550450

KEROGEN

RESINS + ASPHALTENES

350

.5

1.5

.5

1.0

1.5

1.0

2.0 %HCj"C

r T m a x

r T ma x '

AI I I ~roc

82Kerogen

82'Res.+ Asph.

4-

2-

o- "';-I---:--'-~_-:-------=::'~_----l,+255

I

81Light

HCC, - 25

%HC/5·C

350 450 550

Fig. 1. Kinetic analysis is carried out on S2 curves for kerogen and S2' curves for R + A. These areobtained by comparative pyrolysis (see Delvaux et al., 1990).•••• , Experimental curve (one dot forevery 5°C). --, Theoretical curve, computed from kinetic parameters obtained by the Freeman and

Carroll (1958) method.

interest because these products cannot be characterized by the standard Rock-Eval method.

The kinetic characterization can also be applied towhole-rock samples, but it needs to be used withcaution on rock samples with a high bitumen contentand the advantages of the R + A determination arelost.

We assume here that the overall pyrolysis reactioncan be represented by a single nth-order kineticreaction. The reaction parameters should not betaken literally. Instead, they are apparent reactionprofile parameters that reflect differences in kerogenstructure and reactivity. Our kinetic parameters arederived from a single heating rate experiment, so itshould be emphasized that the kinetic parametersobtained here are not meant to reproduce pyrolysisat other heating rates, and certainly not to reproduce hydrocarbon generation at geological heatingrates.

DETERMINATION OF KINETIC PARAMETERS

The pyrolysis curves were first obtained by RockEval pyrolysis analysis, using the method describedby Espitalie et al. (1985/86). But, as shown elsewhere(Delvaux et al., 1990), this method presents someinadequacies for the study' of kerogen for rocksamples with high bitumen content, and it is not at

all suitable for the study of R + A. ComparativeRock-Eval pyrolysis, which has been developed recently (Devaux et al., 1990), was used to obtain twowell-defined reaction curves: S2 for the kerogen(without contamination by products of heavy bitumen); S2' for R + A (Fig. 1).

The pyrolysis reaction can be represented by twodifferent kinetic models:

-a statistical distribution of the activation energyof first-order reactions (Anthony and Howard,1976). This method is widely used to providekinetic data for the modeling of hydrocarbongeneration at geological heating rates(Burnham and Braun, 1985; Ungerer et al.,1986).

-a single nth-order overall reaction (Freemanand Carroll, 1958; Coats and Redfern, 1964;Friedman, 1965). The latter was used for computation of kinetic parameters of kerogenbearing rocks and coals by Leplat et al. (1983),Wen and Kobylinsky (1983) and Yang andSohn (1984).

Recently, Jiintgen (1984) and Braun and Burnham(1986) have demonstrated that, in some circumstances, a single nth-order reaction model canprovide a comparable fit to experimental data as amultiple first-order reaction model.

Classification of organic matter in rock by kinetic parameters 177

The Freeman and Carroll (1958) method waschosen for a number of reasons. Its main advantageis that it gives simultaneously the activation energy Eand reaction order n, without preliminary assumptionof the reaction order. Sharp and Wentworth (1969)have, however, reported a lack of precision in theresults given by this method. In fact, the Freeman andCarroll method produces several possible results andthe choice between them is rather subjective. Toovercome this cause of imprecision, a computermethod has been developed for selecting the valuesof activation energy E and reaction order n, whichbest represent the experimental reaction as a whole(Delvaux, 1988).

In a few words, for each set of E and n values givenby the Freeman and Carroll equation, the relatedfrequency factor A is computed. A theoretical pyrolysis curve is then obtained by a numerical integrationmethod, including E, n and A kinetic parameters, aswell as the true experimental temperature gradient.This theoretical curve is quantitatively comparedwith the experimental one. The best-fitting curve isconsidered and its corresponding E and n values areretained. These results are then used for the complementary determination of organic matter type, atthe experimental heating rate of 25°C/min. In practice, not every sample will give representative results.By experience, the minimum yield of S2 and S2'products required for the kinetic calculation is between 0.5 and 1 kg HC/tonof rock.

The mathematical processing, which also comprises comparative pyrolysis calculations, is carriedout on a personal computer. The reproducibility ofthe selected results is good enough for further use ingeochemical characterization. Standard deviationsfor E range between 0.9-2.8% for kerogen and up to5.3% for R + A. For n, standard deviations rangebetween 1.4-4.5% for kerogen and up to 11.2% forR+A.

KINETIC CHARACTERIZATION OF KEROGEN INEXTRACTED SAMPLES

Rock samples of various origin and maturationrank were selected as reference samples (Table 1). Thepyrolysis and kinetic parameters were obtained onextracted rock samples, except for HI, which isobtained separately on decarbonated samples. Theirkerogen type is characterized by the classical HI- Tmax

and HI-OI diagram (Fig. 2). The samples aregrouped together, according to their origin and organic matter type:

-Type Ia: lacustrine organic matter (Green Riverformation, U.S.A.).

-Type Ib: lacustrine algal organic matter (Cretaceous, Lower Zaire-Angola).

-Type IIa: marine planctonic organic matter(Cretaceous-Tertiary, Lower Zaire andKimmeridgien-Toarcien).

-Type IIb: mixed marine-terrestrial organicmatter (Miocene, Angola).

-Type III: terrestrial deltaic organic matter.-Type IV: terrestrial origin, high rank.-Lingnites and coals of terrestrial origin.

(a) Typical kinetic results for different kerogen types

A first examination of the kinetic results for onesample of each kerogen type and of nearly similarmaturation rank shows the existence of a qualitativerelationship between the S2 peak shape and thekinetic parameters. From Fig. 3, it can be seen thathigh orders are linked with tailing peaks (e.g. type IIIor IV) and high energies are associated with narrowpeaks (e.g. type Ib and IIa). The shape of the peaksis also very well represented by the ratio Eln whichdecreases regularly from type I to type IV kerogen.This ratio has no physical meaning, but it will be usedas a distinctive parameter in the determination oforganic matter type.

Except for the type Ia sample (Green River), theenergies obtained by the nth-order Freeman andCarrol method are in the range of 40-65 kcal/rnol.This range is consistent with the results provided byseveral laboratories using the multiple heating rateexperiments and statistical distribution of first-orderactivation energies model (Campbell et al., 1980;Burnham and Braun, 1985; Ungerer et al., 1986;Braun and Burnham, 1986). In the case of the typeIa sample, the activation energy is significantly lowerthan the 50-55 kcal obtained for this material byCampbell et al. (1980) and Ungerer et al. (1986). Thereaction order less than the unity should indicate aperturbation in the rate of volatile release at thebeginning of the experiment. Consequently, this result seems unrealistic and this kind of sample needsto be studied by another, more appropriate method.

(b) Kinetic results for various reference samples

From the complete results of Table 1, it can be seenthat the activation energies and reaction orders aremarkedly different from one kerogen type to another.However, they show significant evolution with thematurity level (given by Tmax and Ro ) , for type IIa, IIIand IV kerogen, as well as for lignites and coals. Thisphenomenon will be discussed later.

For a better examination of the results, the meanvalues and standard deviations of E, nand E [n aregiven for each kerogen type in Table 2. In order toavoid the influence of immature kerogen, only thesamples with Tmax values 430°C (or 425° for type III)are retained. As in Fig. 3, the mean activation energyis decreasing from 56 kcal (type Ib) to 42 kcal (typeIII) and the reaction order is increasing from 0.72(type Ia) to 3.11 (type IV). In the meantime, the ratioEln is regularly decreasing from 56.6 (type Ia) to 16.0(type IV). Type Ia samples have the lowest (and mostunusual) mean activation energy and reaction order.Coal samples show the highest mean activation energy but also a high reaction order.


(c) Relation between kinetic and pyrolysis parameters

The kinetic parameters E and n for kerogen decomposition are related to the pyrolysis parametersHI (Espitalie et al., 1977), 01 (Espitalie et al., 1977)and PI (paraffin index; Larter and Senftle, 1985).Three main conclusions are readily drawn from thediagrams in Fig. 4:

-the apparent activation energy depends on thetype of organic matter (diag. HI-E), and in-

creases with decreasing oxygen content (diag.E-oI);

-the apparent reaction order increases with decreasing hydrogen content (diag. HI-n), as wellas with decreasing paraffin content, and thuswith increasing aromatic content (diag. PI-n);

-with increasing maturation rank, the activationenergy rises to a maximum level but the reaction order still increases to values as high as 4(diag. HI-E and HI-n).

Table l. Rock-Eval and kinetic pyrolysis data from selected samples of different origin, type and maturation rank

ROCK-EVAL and KINETIC DATA for ROCK SAMPLES (Part I)

E Ii

53.13

77.66

61. 79

55.37

62.7·+

n Eln

n

1.03 43,00

0.93 .'13.711.1038./+50.94 44.841 • ] 'j 4(; . ~:;

1.08 37.7(")

0.94

0.75

E

44.29

40.6542.30':'2.1547.9040. 7~

54.71 0.54 101.353.55 0.60 89.2550.26 0.56 89.7550.79 0.69 73.60

58.25

51.29 0.83

55.84 0.89

42.64 0.77

436

441421426431

410

IIII

I 438

II 426

!1

IiIIii"

:!

II I:

!: RESINS'" ASPHALTE~ES IS2';::'L:::::. ._.._---l!,r-----r---.--------I'11 T~:ax: EnE;n :i

II:: 425 ! 50.84 0 .84 60.52ilII":;!!

:i

:!ii !dO ,

ILlil IIi TmaxE/n

E/n

48.7342.8338.8045.5448.0249.8750.3751. 3251.4244.1844.9050.10'+46.9454.6857.48'+5.40 I'

45.6::Ji

64.8164.8151. 5443. 7562.7341.3365.2368.8461.1541.48

n Evn '! Trnax E___,.._. ._--lL.~ ~ . .._ ------..---.....--.

I ~ I

n

n

1. 141.19

1. 360.961. 211. 341.031. 120.811 .311. 431. 371. 581.461.341. 651. 551. 501.371.09

1. 161. 231. 220.971.040.981.000.991.131. 351. 161. 201. 381. 201.031. 301. 29

0.730.600.680.680.630.B70.630.580.900.91

E

E

E

50.5945.01

22.5220.3324.5824.3924.9023.2618.4541.5546.7540.2365.8161. 5355.2565.0858.2054.1452.6743. 23

56.5352.6747.3444.2049.9448.8750.3750.8158.1159.6452.0855.8564.7.';65.6259.2059.0258.87

PI

PI

PI

11.30

11. 02

16.04

14.54

19.6522.75U).97L:l.70

16.5421.1820.3118.2024.1720.7720.0131. 7132.6929.5941.6542.1441. 2339.44 43537.5536.0938.45 44639.66 421

441~ '+ • .3.s '! 44137.82 '~:16

====='='======== .JL===f==== ======--447

4083934014074114154154')')

420426432433440441443444445443445

438437435441439442441443439441445445443:+ 51450450452

Tmax

Tmax

or

54401919311771

85935331622

1415141214

916IS211313511437

)3

332733413557

01

2113

79434643354141i+O

HI

HI

HI 01 Tmax

771

941861764720797800775733691720782715767709620496446

792966892378908900883

803820

CHARACTERIZATION

4.479.95

13.6416.7225.1913.5521.115.98

16.0016.80

OF KBROGEN (S2 peak)-~=======~-=========;==

3.16 5436.62 5788.15 5864.58 6239.21 6.315.96 634

12.45 66411.266419.29 556

13.73 6192.88 4791. 36 5283. 35 4h61. 79 4351.17 4282.25 3374.36 3951. 91 4616.12 5314. f,S 3,+~,

3.94 387=== ji__. ..=_=_==

--~~~68-;;-' iiAAP 0786 IIAAP 0672 IiAAE 0840 iiAAP 1046 ij

AAE 0760 i,!.11KKF 1070KKF 2050FEe 4892FEC 7505AAF 3134i\..:\F 3126MO 3537AM 2870AM 2005AM 2865MO 3558MG 2246AAO 35')0AAD 3564AAO 31)15

'I SA."1PLE".;==:.::-.':::"~': •.~-

:; Type I a Rc (-.~ , TOe::-..__ ._._.._

A.!\D 0100 "AAD 0200A.'\D 0300AAD 0400AAD 0500

I' AAD 0600AAD 0700A.!\D 0800AAD 0920

!~AAD 0~1!1-·--------:ll===================f==========U====!=========~

" '!YI!§.-l!!. !! -i. o ( %) TOC

Ii AM 2023 11--- 11.07AM 2019 II 6 • ] 5AAG 2329 !! 6 •27AAE 2736 II 21. 03AAE 2762 I! 7.02ME 2766 !I 7•66ME 2774 !I 5 . 54ME 2770 II 6.80AAE 2789 !i 1. 75A..<\G 2270 ii 4.73AAG 2272 ~ 9.45

j!AAF 3128 10.85MF 3132 3.67AAO 3463 5.29AAD 3482 3.31AAO 3561 4.95A.<\O 3485 1.83

i' ..JL_. _;;---~!.._--

.. Type IIa . RoO:" TOC

Continued


Table I-Continued

ROCK-EVAL and KINETIC DATA for ROCK SAMPLES (Part ID

CHARACTERI ZATIONSA.."'1PLE

Type rIb Ro(%) TOC HI 01

OF

PI E n E/n

E J1

Efn

E n

n

n

n

E

E

E

4 1 • 40 ~ . !+ 1'j • ) 5 ;,

+=

435 38.29 1. 41 27.1')

435 40.49 1. 14 35.52~~o 34.85 1.04 -j 5.51

420 47.51 1. 51 31 .4(:>

420 39.98 1. 39 28.7fJ420 36 .84 1. 39 36.50425 44.25 1 .52 2'3.11

420 49.52 1.45 34.15

:; T:'~ax :; Jl---_._----_._--,._-

21.79

11--·--

1') .41 i:

18.2/:1 l!20.57 ::

Ein

18.6919.6720.4720.2122.4321. 44 "

20.2118.4321. 3928.6830.0726.0125.822'j.3423.43 "3: '1. .,

J._O

33.2129.7434.15

nEPI

32.9136.5534.6537.8531 .::;'j ;,

33.95=================---,~~~ ~~=--=======.~:

01 ImaxHI

2.29 407 35 413 22.43 1. 110.82 44'3 53 419 22.12 1. 203.05 361 96 425 22.25 1.042.82 319 67 423 27.25 0.'35:2 • Cj!+ 360 84 425 5.37 34.23 1. 143.55 2% 63 425 31.48 1. 213. 1'3 261 59 425 3.90 37.45 1.453. 33 309 5) 425 32.57 1.11

30.49 351 14 426 4.5,8i

43.35 I.S51. 62 354 17 42'3 40.91 1 • 1tl3. 25 354 429 6.50 43.18 1.300.26 350 29 433 )7.56 1. 292.S) '3..,.) 39 ,~ 32 6.31 49.52 1. 45'-

2.22 328 43 434 46.08 1. 401.86 2':'9 38 436 3,,% 53. 73 1. 47.2.08 265 34 438 49.21 1. 422.29 19<:.1 35 439 2.74 51.48 1. 362.53 210 34 442 50.07 1. 570.58 333 28 441 4.89 51. 27 1. 51

0.37

0.420.38

0.32

A..;\B I,S15 0.39MB 2143A..o\B 2143AAB 2487 0.51AAB 2483 0.60AA..t". 2015

AAP 0850 3. 31 377 41 3i39 27.59 1.79EZH 1875 1. 36 38 74 414 26. ,49 1. 45EZH 1380 0.% 44 88 416 2<3.39 1.38EZH 2390 9.97 113 23 426 1. 93 41. S6 1 • '37EZH 2310 11.73 li4 42 428 1. 54 47.85 2.56ElH 2290 S. IS 81 37 i.2? 1.00 44.07 2.24EZH 2420 8.38 Sl 32 428 1. 18 45.46 2.22EZH 2330 5. ':;4 78 36 430 J..06 45.27 2.24EZH 2470 3.64 5'3 !d 430 0.86 47. 11 2.10EZH .'2360 4.6.3 ") .35 431 1.00 1+6 • .3 :2 2.16I .J

EZH 2510 2.77 77 33 ld2 1.00 42.41 1. 99EZH 2589 3.85 83 45 432 1. 26 4.3.13 2.00EZH 2450 4.02 62 39 4.32 0.·47 41.80 1. 'j 1EZH 2541 :! 3.7·S 57 66 432 0.68 44.5(, 2.30i!

fL.........-

"

21 .3121. 5621.88

19.37 :'~:

:: Type IV!! RoC'() TOC HI 01 Tmax PI E n £in !! 'h:a.x:~.._._-----,~---- ---+----.--------.--.---.--.--------t-----.--,----------~~---.- .-'-..-;----.----.-------.----- .. ,~~,; AAT 0290 .: 0.83 0.'37 'j5 60:+51 65.17 3.17 20.55:!

MT 0570 "1.11 1.10 44 44 460 , 49.74 3.86 17 . .39!:AAT 0 786 1. 2 '3 1. 2 ') 33 33 4 73 5 2 • 6 1 3. 40 15 • 4 7AAT 0960 1.37 1.10 40 40 477 ~1.08 2.87 15.00".:\AT 1190 'r 1.73 0.'32 24 499 i 36.82 2.26 16.2'3

::::::.-::.=-.-=-~_':--;-===--~P_-==-===----===-.==-~-=--:':':.:=-:===--===::=..--=---=-==f=.-=-.=.-==---=:=-:ri: Lignites ;; Roi%l TOC HI 01 Tmax PI' E n £in ;i Tmax;: ,'< Coals " Ii::-----------+.-- --- -----;---_._--_._----------;-;---:-------_._----- ----:,, AAS 5100 iI 0 • .31 54.68 130 74 395 ! 18.22 1.2') 14.57 il 45.50 2.49 18.:n"

AAS 5200 55.80 124 393 20.27 1.31 15.47 I 3'3.67 2.23 17.7'1.MS 3'307 0.75 51.5,4 255 434 5'3.91 2.25 26.62AAS 4 370 0 • 77 44 1 I 4'). 1 I 1 • 5 7 28 • 7.3AAS 3507 0.92 79.14 165 433 64.12 2.87 22.34AAS 3':' 7 i) 0 . 'j 2 7 9 • 14 2 13 4 3'3 6 3 . 10 2 • 6 7 2 3 • 63A..~S 10 17 " I. 1 5 76 • 57 1oj4 454 50 . 2 I 2 .0'3 2·~ .02AAS 252,S " :.38 .38.40 153 461 45.56 2.12 2[,4'3A..~S 2031 .: 1.70 '38.37 [')'-' i~i'2 50.59 2.11 23.97 iiA..L\S 142) S9.12 68 4 (j:j 65.S0 2.41 27.30 'I

AAS IS0b 2.00 ,sc).12 65 ~,08 69.35 3.41 20.33!i~ ...::.:.::'=.j~_==_-..=:-:'===_~_==:_.=_=:==:::::=__=_~.:.==---~--==-- Il-

;; Type I I I :; Ro I:::! TOe

The kerogen pyrolysis curve is obtained from extracted rock (S2 peak) and R + A pyrolysis curve (S2' peak) is obtained by the new methodofcomparative pyrolysis described by Delvaux et al. (1990). Kinetic parameters are computed by the modified nth-order Freemanand Carroll (1958) method. R; = vitrinite reflectance; TOC = total organic carbon (wt%); HI = hydrogen index (mg HC/g org.C);01 = oxygen index from carbonate-free samples (mg C0 2 jg org.C); Tmax= temperature of maximum hydrocarbon release duringpyrolysis of kerogen (on the S2 peak); T~,ax = temperature of maximum hydrocarbon release during pyrolysis of R + A (on the S2'peak); E = activation energy (kcal/rnol); II = reaction order; Ejn = kinetic ratio.


1'000

Cl •(;;l) 800 •Cl<,

o 0J:ClE

I600

xUJCl

~

ZUJo~ 400o>-J:

480..400 420 440 460-------TEMPERATURE Tmax ('C) --.-

* lib Mixed

• 1\1 Terrestrial

, A Lignites & Coals

:- ;/ .s.:.~10 7~ ~bIVV~'* '* ~ * *° * * **

** *

Cl800

zUJeoa:c>-J: 200

110 0 0

xUJoz 400

O-+-..::::ilI::::r---,..-----r----,..__--,-__,......__.,-I

ooCl<,

oJ:

ClE 600

I

o 50 100 150

OXYGEN INDEX 01 (mg CO2

/ 9 Corg.)-.-

Fig. 2. Classification of the reference samples in classical HI-Tmall and HI-DI diagrams (data in TableI, R; = vitrinite reflectance).


• 2.0 %/"C

I0

"'U 1.5 n=O.58 n=1.20"""'I

0Q.

E=39.9c E =65.6o 1.0

0 E/n=68.8 E/n=54.7::J

JJ .5Ol.....CD

I 0 +---+...........-fC--+--+---+..l-..,;-4'--!--250 350 450 550 Tc--- Tem peratu re ~

n=1.57

E=50.1

E/n=31.9

n=2.56 n =3.40

E=52.6

Fig. 3. 82 peak shapes, kinetic parameters obtained by the nth-order Freeman and Carroll (1958) methodand theoretical curves obtained .by numerical integration for a selected sample of each kerogen type. Datafrom Table 1: type Ia = sample AAD 0800; Ib = AAO 3463; IIa = AAO 3537; lIb = AAB 2487; III = EZH2310; IV = AAT 0786. Kinetic parameters; n (reaction order); E (activation energy, kcal/mol); EIn (kinetic

ratio) .•••• , Experimental curve (one dot for every 5°C). --, Theoretical curve.

With regard with these data, it is suggested that theglobal pyrolysis mechanism should be influenced bythree main partial reactions. Each of them seems tocorrespond to one end-member in kerogen composition:

-for aliphatic hydrocarbon-rich kerogen (type Iand mature type lIa), the global reactionmechanism is dominated by carbon-carbonbreaking in the aliphatic chain (first-order.reaction with high activation energy dependingon the length of the chain: 57-70 kcal/mol).

-for aromatic hydrocarbon-rich kerogen (maturetype III and type IV), the global reaction isrelated to the pyrolysis mechanism of coalspresented by Jiintgen (1984): reaction orders

between 1.3 and 2.0 (and possibly higher), withactivation energy depending on the degree ofsaturation).

-for oxygen-rich kerogen (immature type II andIII), the global reaction is dominated by thebreaking of low-energy oxygen-bearing bonds.

(d) Kinetic characterization of kerogen in extractedrock samples

The kinetic data of Table 1 are used for theconstitution of reference diagrams for the determination of kerogen type. When using E and n values,it appear that a good way of discrimination betweenthe various types of kerogen is to represent them inan n-E diagram. The reference samples are plotted in


Table 2. Mean values and standard deviations for the kinetic parameters of Table I

MEAN VALUES AND STANDARD DEVIATION OF KINETIC DATA

1.39 ±O.19

1.15 ±O.13

0.72 ±O.1.3:: I a :! 38. 0 ±5 . 0:~-4---- 1b :i 5/ - 6 0ii i! '4. ':J ± •;;_.._---+- ----

I I a -, 55 • I ±7 • 7._---_.

lIb id.2 ±4.9 1. 44 ±O.OSl~·--

III 42.6 ±2.1 2.15 ±O.19

IV 43.5 ±iO.7 3. 11 ±O.GO

Coals 57.1 ±9.3 2.39 :to.53

56.6 ±IO.u

51.8 ±4.248.3 ±5. I

)9.S ±2.5 43.0 ±2.7

3!+.8 ±2.5 41.5 ±5.2

:20.8 ±1.2 'J

16.9 ±2.2 J'/ ') ±3.0'+""".L.

2ifo 3 ±2.8

Only data for samples with Tmax > 430°C (425°C for type III) are retained, to avoid the influence of immaturekerogen. E = activation energy (kcaljmol); n = reaction order; Ejn = kinetic ratio.

such a diagram [Fig. 5(a)] and the boundariesbetween different types of kerogen are traced. Linesof equal maturity rank are drawn with referenceto their known position in the HI-Tmax diagram ofFig. 2. This way, the boundaries delineate threemain evolution paths along which E and n valuesprogressively increase with increasing maturity rank,depending on the type of kerogen. This evolutioncould be explained by the progressive loss of oxygenbearing groups and the relative increase in aromaticity with thermal maturation.

For high maturation rank, at the end of the oilwindow (1% vitrinite reflectance), type I and IIkerogen are almost entirely degraded and the fewresidual organic matter remnants should exhibitkinetic parameters similar to type III and IV kerogen.

This n-E diagram is not very precise for thedetermination of organic matter type in the case ofimmature samples. The HI-E In diagram seems therefore complementary to the first one" including theclassical HI which is more discriminatory for immature samples [Fig. 5(b)].

Despite the unusual low E and n values for type laGreen River kerogen, the use of these data stillprovide a complementary way for the determinationof their organic matter type.

KINETIC CHARACTERIZATIONOF RESINS +ASPHALTENES (R + A)

Resins + asphaltenes (R + A) in bitumen andreservoir oil are usually considered to have beenproduced by early kerogen decomposition, throughthe breaking of low-energy oxygen-bearing bonds.They have a chemical structure and compositionsimilar to their related kerogen (Behar and Pelet,1985). They differ from it, mainly through theirsolubility in polar organic solvents and by theirchemical composition, poorer in oxygen and richer inhydrogen.

For R + A characterization, the kinetic determination by the modified Freeman and Carroll (1958)method is carried out on the bitumen pyrolysis curveobtained by comparative pyrolysis (Delvaux et al.,1990). In the 28 samples selected the R + A arethought to have been produced directly from thekerogen contained in the host sample. Results arepresented in Table 1, and mean values with standarddeviations in Table 2. Similar ranges of values arefound as for mature kerogen, but there are noenergies < 36 kcal. The activation energies for R + Aare in the same range of 41-43 kcal for all the samplesof types IIa, lIb and III, but the reaction order isincreasing from type I (0.74) to type III (2.29). Thereare also great differences from one type to another inthe E In ratio.

The plot of n vs E gives an n-E diagram similar tothe one for kerogen, though much simplified [Fig.6(a)]. There also appears to be good differentiationbetween R + A of different origins. Due to the similarity of composition and structure between R + Aand kerogen, this diagram indicates the type of kerogen to which the R + A are related. The kineticmechanism may be somewhat comparable to the onefor kerogen, the main difference being the lack of lowE values, which is due to the depletion of low-energyoxygen-bearing bonds. Unfortunately, this characteristic disables a systematic evolution of kinetic parameters with maturation rank for R + A. Thus, datafor R + A define no kinetic path but kinetic zones.

For a complete characterization of R + A, oneshould use the T~ax in combination with kineticvalues in the form of a (E In)-T~ax diagram[Fig. 6(b)]. This kind of diagram provides useful informations about the type, origin and rank of R + A.

CONCLUSIONS

Kinetic analysis is applied in a new way, for therapid and complementary characterization of sedi-


N

ozm

r

o

R o =1.0 "lo

o/. 0o.• 0

0'0\

I

oI

112

•la•-..

lIaOl

E 600

UI

~ 800Ol

oUOl<,

Ixw0

~400

zw<..?0a:0>-I

200

o 20 40 60 80-- ACTIVATION EN ERG Y E ( k cal / mole) ---.-

120100

*

Anomalous?

80604020

\

100 \

• 0 "".. 0o • 0 '-\~. i" 0-. ..:;.~~~~ """ * * * * .. "* ..

o ~~O D.

---------

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rou

-'"

W60

>-oa:UJzw 40

Z

2I--c>~ 20et:

I 00

------ OXYGEN INDEX 01 (mg CO 2 / 9 Corg.) ...

Fig. 4(a) Caption overleaf

mentary organic matter. Kinetic parameters are computed on comparative pyrolysis curves for kerogen(S2) and R + A (S2'), using an nth-order modifiedFreeman and Carroll (1958) method and a singleheating rate experiment.

Activation energy alone is insufficient for theclassification of organic matter. The characteristicranges of energies for each organic matter type aretoo poorly defined (Table 2) and the standard devi-

ations are too high. This necessitates the use of asecond parameter. The reaction order shows betterdefined ranges for each organic matter type, but it isstill insufficient when used alone, because of someoverlapping between types Ib, lIa and lIb. For a gooddiscrimination between organic matter types, thecombination of the activation energies and reactionorders in a single n-E diagram appears to be appropriate for mature kerogens and also for R + A. The


1000

800

OJ

ooOJ.....oIOJ 600E

Ixw~ 400

zwC}

oa:o>I200

\ialI·...e.

1.0 2.0

------ REACTION ORDER n3.0 4.0..

\

l::;. Lignites & Coals

e la Lacustrine

• Ib Lacustrine

o lIa Marine

* lib Mixed

.... 11\ Terrestrial

'* ..-4---------------.:>_ ~ -.t.~-,_'<_-------_1

.t. .t.

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o 10-+---.::>.....-----=---=.~_.:__---------------_1IoC')

I

0> 5oc;/<

oo~

1

10 0

50

.5 1.0 , 1.5 2.0------ REACTION ORDER n

Fig. 4(b)

2.5 3.0

Fig. 4. Relation between kinetic parameters and pyrolysis parameters for kerogen analysis in extractedrock samples (data in Table 1). Activation energy (E) is related to the hydrogen (HI) and oxygen content(01). Reaction order (n) is related to the hydrogen (HI) and paraffin-aromatic (PI) content. HI and 01

are defined by Espitalie et al. (1977), and PI by Larter and Senftle (1985). R; = vitrinite reflectance.


(a)

3.0

Ro=1.0%_2: t ..

2.5

I 2.0

ca:wCla:oz 1.5

oI-o~

wa:

1.0

0.5

0+---.---,---,---,---,--...,--....,....-....,....---'o 20 40 60 80

-- ACTIVATION ENERGY E(kcal/mole)~

RO = 0.5'"

(b)fOOO

:, 800

0oCl

......UJ:

Cl

.5 600

I

xwCl

=400zwo0a:c>-J:

200

,A \"... - Ro=0.3 .~~\ ./.f' / \

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I • i..• I

I

• la Lacustrine

• Ib Lacustrine

o lIa Marine

* lib Mixed

.. III Terrestrial

l::. Lignites & Coals

20 40 60

--------- E (kcal) / n80

Fig. 5. Complementary determination of kerogen type from kinetic parameters obtained by the modifiednth-order Freeman and Carroll (1958) method. Symbols are chosen according to the type of kerogendefined in the HI-Trnax diagram of Fig. 2 (data from Table 1, R; =vitrinite reflectance). (a) n-E diagramon which E and n values show a typical evolution path with thermal maturation, for type I, II and IIIkerogen. (b) HI--(E In) diagram, for a better distinction between organic matter type for immature kerogen

(vitrinite reflectance < 0.5%).

OG 16jl-3-D


(a)

2.5

2.0

C 1.5

a: /w0a:0

z0;:: 1.0o~

wa: I

0.5 /

o 20 40 60 80- ACTIVATION ENERGY E (k c a l zmote)~

fJ. Lignites & Coals

• la Lacustrine

• Ib Lacustrine

o lIa Marine

* lib Mixed

A III Terrestrial

460380

IMMATUREZONE 7~ OIL ZONE--1

- ?J!. ~ - 0l!-e? It) 3c:i c:iII II /I

. 0 0 0

a: - a: a:

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- II I

TYPE I • ,-- • II • I- I

I- I-47.5

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0_1 _ - 3 7. 5

* *I * Ilib I I I I- ~ * 25

- ... ~ I I I

III I I I. - I I I

I I I

I I I I I I I 1o360

80

20

100

wI 40

C 60<,

(b)

400 420 440

--------- TEMPERATURE Tmax' (OCI-----4I..-

Fig. 6. Complementary determination of kerogen type from kinetic parameters obtained by the modifiednth-order Freeman and Carroll (1958) method. Symbols are chosen according to the type of kerogen ofthe host sample, defined in the HI-Tmax diagram of Fig. 2 (data from Table 1, R; = vitrinite reflectance).(a) n-E diagram on which E and n values show typical zones for types I, IIa, lIb and III, but no significantevolution with maturity level. (b) E/n-T~ax diagram, which allows an estimation of R + A type and

maturity. T~ax is the temperature of maximum hydrocarbon release by R + A during pyrolysis.


E In ratio has well-defined ranges and can be useddirectly for the determination of R + A type. The EInratio is also useful for the classification of immaturekerogen, in combination with the HI.

Maturation rank is given approximately by then-E diagram for kerogen and by the temperaturer:Uax for R + A.

The combination of comparative pyrolysis andkinetic analysis appears to be a new and complementary method for the study of sedimentary organicmatter:

-for kerogens, it provides an alternative andcomplementary determination of organic mattertype, which is particularly useful for mixtures oforganic matter of various origin;

-for R + A, it offers a practical mean for the rapidstudy of these pyrolysis products which is unlikethe classical pyrolysis method of Espitalie et af.(1977).

We suggest that this method should be applied foreach routine Rock-Eval analysis, using an appropriate solftware, to provide complementary results. Thebest results are obtained when the comparative pyrolysis method is used first. For whole-rock samples,kinetic results should be regarded with care.

Acknowledgements-This work issues from a Ph.D. thesispresented by D. Delvaux and sponsored by the LabofinaCompany (Pertrofina's Research Group). We thank thePetrofina and Labofina Companies for permission to publish this work. We also thank J. Espitalie, P. Ungerer, K.Peters and A. Burnham for helpful comments.

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