1994 Andresen Etal 1994

~ Pergamon 0146-6380(94)E0015-R Org. Geochem. Vol. 21, No. 12, pp. 1229-1242, 1994

Copyright 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0146-6380/94 $7.00 + 0.00

Thermal generation of carbon dioxide and organic acids from different source rocks

BJORG ANDRESEN,I* TORBJORN THRONDSEN,t TANJA BARTH 2 and JOHANNE BOLSTADI IInstitutt for Energiteknikk, Kjeller, Norway and 2Department of Chemistry,

University of Bergen, Norway

(Received 27 April 1993; returned for revision 27 September 1993; accepted in revised form 23 February 1994)

Abstract---Carbon dioxide and organic acids are claimed to play an important role in sandstone diagenesis. In the present work hydrous pyrolysis was used for a qualitative comparison of the relative potential of different source rocks to generate organic acids and carbon dioxide. In addition, the carbon isotopic composition of the generated carbon dioxide was determined. Experiments with three immature source rocks are described; a Miocene brown coal from Germany, a coal from the Middle Jurassic Ness Formation in the northern North Sea and a rich marine type II source from the Late Jurassic Kimmeridge Clay Formation onshore U.K. Both coal samples are typically humic. A stepwise heating program from 150 to 365C was used. Closed Pyrex tubes containing immature coal (%R m = 0.37) were heated in order to measure vitrinite reflectance evolution. The experimental results were related to vitrinite reflectance, as a maturity reference parameter, and to a kinetic model using kinetic parameters for carbon dioxide generation (Burnham A. K. and Sweeney J. J. (1989) Geochim. Cosmochim. Acta 53, 2649-2657). Differences in the yield and carbon isotopic composition were obtained between the marine type II source rock and the coals. Organic acid generation converted between 1 and 3% of the organic carbon. Carbon dioxide was the major oxygen containing component, with molar yields 5-10 times that of organic acids. The experimental yields of organic acids and carbon dioxide were compared to a first order kinetic model for carbon dioxide generation, and a reasonable correspondence in the transformation ratio curves was observed.

Key words--hydrous pyrolysis, organic acids, carbon dioxide, carbon isotopes, kinetic model

INTRODUCTION

The concentration of organic acid species in subsurface waters is highly variable, and maximum concentrations vary with temperature. Acetic acid (or more correctly acetate, depending on the formation water pH) is the most abundant species and the abundance of other organic acids decreases with increasing carbon number (Carothers and Kharaka, 1978; Fisher, 1987; Barth, 1991). Lower concentrations are found in low temperature waters. This may be a combined effect of bacterial consumption of organic acids, especially acetate, reduced generation of organic acids and dilution by mixing with meteoric water (Lundegard and Kharaka, 1990). Organic acid anions can dominate the total alkalinity of subsurface waters, and the anions of short chain aliphatic acids contribute the major portion. Organic acids can influence pH dependent reactions as pH buffering agents, and possibly also act as complexing agents for metals such as aluminium and iron (Lundegard and Kharaka, 1990). At higher temperatures (150-200C) the organic acids are thermally decarboxylated and may act as a source of natural gas. A good correlation

*To whom all correspondence should be addressed.

between the distribution of homologous aliphatic acid anions in formation waters, and the proposed decarboxylated gases in produced natural gas, is shown by Carothers and Kharaka (1978).

Carbon dioxide is an essential component in the biocarbonate system, and plays an important role in both early and late diagenesis. Carbon dioxide has several possible origins; thermal breakdown of kerogen, decarboxylation of organic acids either thermally or by bacteria (Kharaka et al., 1983, 1986), dissolution of carbonate minerals (Hutcheon et al., 1990), bacterial oxidation of hydrocarbons (Whiticar and Faber, 1986) and possibly deep crustal origins. Carbon dioxide and organic acids are both claimed to play an important role in sandstone diagenesis. However, relatively little is known about how, when, and to what extent these substances are formed. It is, though, generally accepted that they are formed in the thermal breakdown of kerogen.

Pyrolysis techniques are frequently used to study hydrocarbon generation under controlled laboratory conditions. Hydrous pyrolysis (Winters et al., 1983; Lewan, 1985) permits the study of water soluble compounds generated in the aqueous phase and has proved to be a useful method for investi- gating organic acid synthesis from organic matter

1229

1230 BJI~RG ANDRESEN et al.

Kawamura et al., 1986; Eglinton et al., 1987; Lunde- gard and Senftle, 1987; Barth et al., 1987). Pyrolysis experiments have also been used to study the generation of carbon dioxide (Harwood, 1977; Saxby et al., 1986; Cooles et al., 1987). The laboratory experiments provide convincing evidence for a thermal origin of the major dissolved organic acids, and carbon dioxide, and demonstrate that these low molecular weight compounds are the most important oxygen-containing products of the thermal degradation of kerogen.

In the present work, hydrous pyrolysis was used to study the generation of organic acids, and carbon dioxide, from different types of source rocks under controlled laboratory conditions, in order to provide information on patterns of generation and the quantities generated. In the pyrolysis procedure, progressive heating from 150 to 365C in steps of 25C per 24 h was used. A stepwise heating program, starting at a fairly low temperature (150C), was used because microscopic examination of vitrinite has shown that the petrographic structure, and appear- ance, of the vitrinite develop much more naturally than under isothermal conditions (Throndsen et al., 1993). Closed Pyrex tubes containing an immature, vitrinite-rich humic coal (% R m = 0.37) were heated in order to monitor the vitrinite reflectance evolution. The idea was to have a common and independent maturity scale for all three source rocks, and to avoid the difficulties of obtaining good quality vitrinite reflectance data from the residual sample material of the Kimmeridge Clay Formation source rock, The experimental results are discussed relative to vitrinite reflectance as a maturity reference parameter. In addition, the experimental data are related to a kinetic model for carbon dioxide generation (from Burnham and Sweeney, 1989), in order to investigate how well this relatively simple model corresponds to the data obtained. More rigorous thermodynamic modelling has not been attempted.

EXPERIMENTAL PROCEDURES

Three immature source rocks were used in the study; a Miocene brown coal from Germany, a coal from the Middle Jurassic Ness Formation in the northern North Sea and a rich marine type II source from the Late Jurassic Kimmeridge Clay Formation onshore U.K. Both coal samples are typically humic. Some initial organic geochemical data for the samples are given in Table 1. The immature coal sample, used to monitor the vitrinite reflectance evolution, is equivalent to the coal from the Ness Formation in Table !. All source rocks were homogenized and crushed ( < 1 mm) prior to the pyrolysis experiments. The marine type source (Kimmeridge Clay For- mation) was, in addition, treated with hydrochloric acid in order to remove the inorganic carbonate prior to pyrolysis.

The hydrous pyrolysis experiments were performed in 71 ml stainless steel autoclaves (Parr Instrument Company, IL, U.S.A.). In each experiment 3-5 g of the crushed sample, and sufficient distilled water to give 5 ml liquid water at maximum heating temperature, were used. In this way the pressure during heating equals the vapour pressure for water at each temperature level. Before closing, the autoclaves were purged with N 2 (g). After heating at 150C for 24 h, the samples were heated in steps of 25C (15C in the last step) up to the final temperature for each experiment (see Table 3). The samples were heated for 24 h at each step. Different lengths of time (25, 72 and 240 h) were used at the final temperature (365C). The heating and sampling program is illustrated in Fig. 1. The autoclaves were air cooled to ambient temperature overnight before sampling and analyses.

The amount of carbon dioxide and hydrocarbons generated was calculated from the measured over- pressure, determined with a pressure transducer and using the ideal gas law (pV = nRT), together with gas chromatographic analysis on a Porapaq QS stainless steel column connected to a hot wire detector

Table 1. Some initial organic geochemical data Miocene brown Kimmeridge Clay

coal, Ness Formation, Formation, Sample Germany North Sea U.K. TOC, wt% 54.4 61.2 51.3 Rock-Eval pyrolysis data

Si, mg/g rock 6.7 19.1 5.3 S 2, mg/g rock 64.6 102.9 333 $3, mg/g rock 17.0 9.8 12.5 Tmax, "C 411 425 416 Hydrogen Index, mg/g TOC 119 168 650 Oxygen Index, mg/g TOC 31 16 24

Vitrinite reflectance, %R m 0.25 0.37 0.29 ~3C kerogen, %~PDB - 26.0 - 26.4 - 20.8

Extractable bitumen, mg/g TOC 32 nd 55 6t3C bitumen, %oPDB - 27.6 nd - 23.5 Carbonate content, mg/g rock nd 0.1 180 nd--not determined.

Carbon dioxide and organic acid generation 1231

400

350.

o ~ 300-

250.

~, 200.

150

I00 0

I Heating program ~ 9

Samples / -

. . . . I0 . . . . I . . . . I . . . . I . . . . f . . . . I . . . . I . . . . I . . . . I . . . .

5 100 150 200 250 300 350 400 450 500 Duration, h

Fig. 1. Heating and sampling program for hydrous pyrolysis experiments.

(HWD). 10 ml of the overpressured gas phase was sampled with a syringe for isotopic determinations. Carbon isotopic determinations were performed on a Finnigan MAT 251 mass spectrometer. All isotopic values are reported as %oPDB. The uncertainty in the carbon isotopic value is estimated to +0.3%oPDB and includes the gas chromatographic separation together with the mass spectrometric determination. The present ~13C value on NBS 22 is -29.77 + 0.06%oPDB.

Most of the water in the autoclaves was removed using a Pasteur pipette. The solid residue, and the rest of the liquids were then carefully transferred to a filter funnel equipped with a glass fibre filter, using a spatula. Additional distilled water was used in the transfer. The aqueous phase was filtered off, and the residue washed with water to ensure complete re- moval of water soluble compounds. The collected water phases were mixed, and analysed by isotachophoresis for the determination of dissolved organic acids (Barth, 1987). The simpler carboxylic acids, from C2 (acetic/ethanoic acid) to Cs (octanoic acid), were determined. Aromatic and dicarboxylic acids can be determined by the use of isotachophoresis, however, these compounds were only observed in trace amounts (

1232 BJORG ANDRESEN et al.

Table 2. Kinetic parameters for release of carbon dioxide from vitrinite according to Burnham and Sweeney (1989)

Fraction of potential characterized by given

Activation energy (kJ/mol) activation energy (%)

175.5 5 184.1 15 192.5 25 200.8 25 209.2 15 217.6 10 225.9 5

Arrhenius factor: 1 x 10 13s-t.

where t is t ime, T is abso lute temperature , X~ is remain ing potent ia l for carbon dioxide assoc iated to react ion i, E~ is the assoc iated act ivat ion energy, R is the universa l gas constant and A is the Ar rhen ius factor. The amount o f carbon dioxide generated can thereby be expressed by:

Q=Z(X~0-X , ) fo r i= l . . .n (2)

where X~0 is the value o f X~ at t = 0. The kinetic parameters used by Burnham and

Sweeney (1989) are l isted in Tab le 2.

RESULTS AND DISCUSSION

Rock-Eva i data and TOC values f rom the residual sample mater ia l o f each exper iment are shown in

Table 3. Vitr inite ref lectance data, measured on the accompany ing coal sample, are also inc luded in the table together with calculated vitr inite ref lectance values us ing the method o f Burnham and Sweeney (1989).

As seen f rom Table 3, unreal ist ic h igh Tm~ x values were obta ined f rom the h ighest pyrolys is temperatures and longest heat ing t imes. Most p robab ly this

reflects the low S 2 values, and indicates that Tmax is not a wel l -suited matur i ty ind icator under the present condi t ions.

Increas ing TOC values were observed for both coal samples with increas ing matur i ty , and a product ion o f inert carbon or pyrob i tumen was indicated. A somewhat different p icture was found, for the K immer idge Clay source rock with a low TOC value, f rom the 325C exper iment . Th is may be due to uncer ta in TOC determinat ions in such a rich sample,

or it may be related to max imum generat ion o f b i tumen at the same temperature level (Andresen et aL, 1993).

Table 3. Rock-Eval data and TOC values measured on residual sample material from each experiment. Vitrinite reflectance values are measured on coal from accompanying Pyrex tubes, data in brackets are calculated vitrinite reflectance values after Burnham and Sweeney

(1989)

Pyrolysis temperature Tma x S l S2 S 3 HI Ol

(C) (C) (mg/g rock) (mg/g rock) (mg/g rock) (mg/g TOC) (mg/g TOC) TOC (%) %R m

Miocene brown coal, Germany Source rock 411 6.65 65 17.0 119 31 54.5 0.25

225 418 1.10 66 14.0 106 22 62.5 0.44 250 425 1.15 71 12.0 I11 19 63.8 0.56 275 430 1.03 60 7.1 92 11 65.7 0.49 300 439 0.95 86 5.9 123 8 69.8 nd 325 453 1.11 60 5.1 77 7 77.5 0.72 350 451 1.27 53 4.3 68 6 77.8 0.88 365,1" 463 0.59 35 3.1 42 4 81.2 1.06 365, 3 460 0.48 44 2.3 57 3 78.5 nd 365,10 543 0.20 16 2.3 20 3 79.7 1.20

NessFormafion coal, NoMh Sea Source rock 427 19.00 103 10.0 168 16 61.2 0.37

175 428 2.03 109 11.2 170 17 64.3 0.44 200 428 2.16 92 7.2 147 12 62.6 0.46 225 429 1.63 115 6.7 172 10 66.9 nd 250 432 2.04 112 9.1 161 13 69.5 0.56 275 428 1.00 101 143 70.5 0.53 300 448 0.23 62 82 75.3 0.61 325 449 1.22 77 3.1 105 4 73.1 0.70 350 466 0.58 39 48 81.6 0.95 365, 1 489 0.30 28 35 80.6 1.11 365, 3 531 13 16 81.2 1.53

KimmeddgeClayFormafion, U.K. Source rock 416 5.30 333 12.5 649 24 51.3 0.29

175 411 8.40 165 8.7 247 13 66.9 0.48 200 413 5.20 209 9.3 310 14 67.5 0.51 225 412 4.00 205 8.0 289 II 70.9 0.49 250 416 4.50 160 4.8 270 8 59.2 0.52 275 418 2.30 216 4.3 322 6 67.1 0.57 300 424 0.71 398 633 62.9 0.61 325 439 0.25 30 5.3 152 27 19.8 0.72 350 455 0.56 59 3.7 114 7 51.8 0.94 365,1 485 24 46 52.0 nd 365, 3 515 17 30 57.0 nd

(0.36) (0.44) (0.54) (0.65) (0.77) (0.96) (I.14) (I.29) (i.51)

(0.27) (0.30) (0.36) (0.44) (0.54) (0.65) (0.77) (0.96) (1.14) (1.29)

(0.27) (0.30) (0.36) (0.44) (0.54) (0.65) (0.77) (0.96) (I.14) (1.29)

nd--not determined. *365, 1; 365, 3; 365, IO---heated isothermally at 365C for 25 h; 3 days; I0 days.

2.0

175

Carbon dioxide and organic acid generation

200 225 250 275 300 325 350 365 Pyrolysis temperature, "C

1233

1.o-

> J O Calculated vitrinite reflectance, %R m Measured vitrinite reflectance, %R m 0.3 . . . . i . . . . I . . . . I . . . . I . . . . I ' ' ' I . . . . I . . . . I . . . . I . . . .

0 50 100 150 200 250 300 350 400 450 500

Duration, h

Fig. 2. Measured vitrinite reflectance evolution on a coal sample from accompanying Pyrex tubes and calculated vitrinite reflectance values after a modified version (Throndsen et al., 1993) of the model for

vitrinite reflectance evolution of Burnham and Sweeney (1989).

Vitrinite reflectance evolution

The measured and calculated vitrinite reflectance evolution pathways, for the accompanying coal sample, are shown in Fig. 2.

The measured vitrinite reflectance evolution shows a steady and continuous increase with increasing temperature, and duration, from about /oRm = 0.4 to more than %Rm= 1.5. Moreover, there is good agreement between measured and calculated values for maturities above approximately %Rm = 0.50. The lack of agreement for maturity levels below %Rm = 0.50 simply reflects the relatively high initial maturity (%Rm = 0.37) of the coal sample in the Pyrex tubes. These results indicate that the modified model of Burnham and Sweeney (1989) is valid for the time scale, and experimental conditions, used in the present study, and can be used to calculate vitrinite reflectance in experiments where measured values are not obtained. More details on the validity of the model of Burnham and Sweeney (1989) with respect to laboratory experiments is presented by Throndsen et al. (1993).

In the following all results are related to the calculated vitrinite reflectance values.

Carbon dioxide and organic acid generation

The yield of carbon dioxide and organic acids from the three different source rocks is shown in Table 4. Figures 3, 4 and 5 show the evolution of hydrocarbon gases (2~CI-C5), carbon dioxide, acetic acid and total organic acids (calculated as acetic acid equivalents) vs calculated vitrinite reflectance.

Carbon dioxide is the main gaseous product from the Miocene brown coal [Fig. 3(A)] and carbon dioxide comprises more than 60% of the total gas phase at all maturity levels. C2-C6 organic acids are found in the aqueous phase, and acetic acid (C2) is the main component comprising as much as 80% of the total organic acids at high maturity levels [Fig. 3(B)]. The Miocene brown coal shows a continuous, increasing generation of both carbon dioxide and total organic acids up to a vitrinite reflectance level of approximately %Rm = 1.0. For higher maturity levels there is a slight decrease in the yields. The carbon dioxide and the organic acid generation follows nearly identical trends. The maximum yields are high, reaching 6.3mmol carbon dioxide and 1.1 mmol total organic acids (acetic acid equivalents) per gram total organic carbon (TOC).

The Ness Formation coal (Fig. 4) shows trends similar to those of the Miocene brown coal except that the yields are lower, and the evolution is less pronounced, at low maturity levels. This is related to the fact that the Ness Formation coal has a higher initial maturity (%Rm=0.37) than the Miocene brown coal (%R m = 0.25) and has already exhausted part of its generation potential. However, for the maturity interval between %Rm =0.4 and 0.8-1.0 there is a continuous and significant generation of carbon dioxide and organic acids. As found for the Miocene brown coal, the carbon dioxide and the organic acids follow nearly identical trends. The maximum yields are 3.5 mmol carbon dioxide and 0.53 mmol organic acids per gram TOC. The slight decrease in the yield of carbon dioxide, observed from

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the two coal samples at maturity levels higher than 1.0 in vitrinite reflectance, may be due to generation of other undetected oxygen-containing components in this maturity range. However, experimental artefacts cannot be excluded.

The Kimmeridge Clay Formation (Fig. 5) is somewhat different. It shows a continuous and almost linearly increasing yield of carbon dioxide and total organic acids with increasing maturity, and no culmi- nation point is attained up to a maturity level of /0Rm= 1.3. Moreover, the yields are significantly lower than for the coal samples. The maximum yields from the experiments are 2.53 mmol carbon dioxide and 0.29mmol organic acids per gram TOC. A marine type source rock (kerogen type II) has a lower O/C ratio than coals (kerogen type III) (Tissot and Welte, 1984). In the present samples the Oxygen Index values (Table 3) are higher from the Miocene brown coal than from the Kimmeridge Clay source rock. This can explain the observed lower carbon

dioxide and acid yields from the Kimmeridge Clay source rock. Lower initial Oxygen Index values from the Ness Formation coal is related to the higher initial maturity of this source.

From the Kimmeridge Clay Formation, carbon dioxide is the main component in the gas phase at low maturity levels, but the molar amount of methane equals the carbon dioxide at higher maturity levels (Table 4). However, the carbon dioxide generated still comprises more than 30% of the gas phase at all levels. Organic acids up to C8 are detected in the aqueous phase at the higher maturity levels. Acetic acid is the main component at all levels, but it is noteworthy, and also in accordance with the kerogen type, that a smaller amount of the total organic acids is acetic acid (about 50%) than that for the corresponding coal samples [Fig. 5(B)].

In a thermal maturation process, the generation of organic acids is one of many more or less simultaneous reactions. Organic acids are generated

0.2

MIOCENE BROWN COAL (%Rm = 0.25) B

HC gases (C 1 - C5)

0.4-

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A

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Total acids

Y IELD, mmol /gTOC Y IELD, 10E-6 mol /gTOC

0.2

-0.4

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-0.8

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-1.4

1.6

E ,v

<

r,I

ta. r,I e~

Fig. 3. (A) Yield of hydrocarbon gases (S C I-C s) and carbon dioxide from Miocene brown coal, Germany. (B) Yield of acetic acid and total organic acids from Miocene brown coal, Germany.


NESS FORMATION COAL (%R m -- 0.37)

B

0.2-

0.4

0.6

Z O 0.8

L12

, . /

z

HC gases (C l - C 5)

CO 2

1.2

0 1 2 3 4 5 6 7 YIELD, mmol/gTOC

l .0.2

-.o-- Acetic acid

Total acids -0.4

-0.6

, .

-0.8 Z ~

.d k~

z

1.2

1.4

m

YIELD. 10E-6 mol/gTOC

1.6

Fig. 4. (A) Yield of hydrocarbon gases (Z'Cj-C5) and carbon dioxide from Ness Formation coal. (B) Yield of acetic acid and total organic acids from Ness Formation coal.

from kerogen at a temperature-dependent rate (3). At the same time the acids degrade into hydrocarbon gases and carbon dioxide with a slower but also temperature dependent rate (3). In a closed system such as hydrous pyrolysis, both processes will at any time determine the total amount of organic acids. Carbon dioxide and hydrocarbon gases will also be generated directly from the kerogen (4, 5).

Kerogen- -k t - - *RCOOH- -kz - - ,RH + CO2 (3)

Kerogen--k3--*CO2 (4)

Kerogen--k 4---, RH (5)

Organic acids may then be regarded as intermediate products in the generation of hydrocarbon gases and carbon doixide (Carothers and Kharaka, 1978). The short chain acids can also be minor products from the reaction of alkyl radicals with carbon dioxide rather than with a hydrogen donor (Barth et al., 1987). The decrease in organic acids, observed for the coal

samples for maturity levels higher than approximately %Rm = 1.0, most likely means that the thermal decarboxylation of the organic acids exceeds the formation rate (3). The decrease in organic acids is not reflected in a corresponding increase in carbon dioxide, since the thermal stability of acetate ensures that the contribution of carbon dioxide from cracking of acids is very small compared to the amount of carbon dioxide that is formed directly from the kerogen (4) (Palmer and Drummond, 1986).

Comparing the amount of carbon in the generated product with that in the initial kerogen, the generation of organic acids in the present experiments is a process converting between 0.7 and 2.7% of the organic carbon into organic acids (Fig. 6). Carbon dioxide is the principal oxygen containing component, with molar yields 5-10 times that of organic acids (Table 4), and 3.0-7.6% of the kerogen organic carbon is converted to carbon dioxide (Fig. 6). With the use of H2 ~sO, aqueous oxygen incorporation has


been demonstrated in pyrolysis products (Stalker et al., 1993), and hydrolysis of hydrocarbons in petroleum at the oil-water interface has been shown to be likely reactions, from thermodynamic con- siderations (Helgeson et al., 1993). Mass balance calculations by Lewan (1992) showed that under hydrous pyrolysis conditions the kerogen oxygen accounted for only 64% of the oxygen in the generated carbon dioxide, implicating water as the source of the excess oxygen. A complete oxygen mass balance is not available in the present work. However, if the amount of oxygen in the carbon dioxide and organic acids at maximum generation is compared to the initial Oxygen Index values (Table 3) an additional source of oxygen is indicated. Added water in the hydrous pyrolysis system is then a likely source of excess oxygen.

Normalized to the total gas generated in the artificial maturation experiments (sum of hydro-

carbon gases and carbon dioxide), carbon dioxide comprises more than 50% at all maturity levels. Non-hydrocarbon gases (CO2, N2, H2) are also found as dominant gaseous products from other pyrolysis experiments (Cooles et a/., 1987; Monthioux et al., 1985; Saxby et al., 1986). The amount of carbon dioxide generated in hydrous pyrolysis far exceeds the amount observed in most natural gases (Smith and Ehrenberg, 1989). In natural systems, however, carbon dioxide will not be conserved in the gas phase, but will dissolve in the formation waters and participate in solubility equilibria with carbonate mineral phases. These processes will be pressure dependendent, with a larger range of variations than used in hydrous pyrolysis.

The amount of carbon dioxide liberated from thermal breakdown of kerogen in the present experiments is indeed significant. This is exemplified in Fig. 7 for a simplified reservoir system. Consider i m 3

KIMMERIDGE CLAY FORMATION (%Rm = 0.29)

t~

~d

to

z

>

0.2

0.4

0.6

0.8

1.2

A B

HC gases (C l - C5)

-I I- CO2

1.4.

1.6 . . . . I . . . . I . . . . i . . . . / . . . . i . . . . i . . . .

0 1 2 3 4 5 6 YIELD, mmol/gTOC

--O- Acetic acid

Total acids

YIELD, 10E-6 mol/gTOC

0.2

-0.4

-0.6

~d -0 .8~

m m Lo

Z N

1.2

1.4

.I.6

Fig. 5. (A) Yield of hydrocarbon gases (-rcj-cs) and carbon dioxide from Kimmeridge Clay Formation. (B) Yield of acetic acid and total organic acids from Kimmeridge Clay Formation.

1238 BJORG ANDgESEN et al.

7, Z 0

~6 6 Z5

~4

~3

Z2 8

1

Carbon dioxide Total organic acids

[ ] Miocene brown coal

[ ] Ness Formation coal

[ ] Kimmeridge Clay Formation

Fig. 6. Comparison between the amount of carbon in carbon dioxide and total organic acids at maximum gener-

ation and initial amount of carbon in the kerogen.

of a sandstone with 25% intergranular porosity, and with 50% of this porosity cemented by calcite. This corresponds to about 3100 mol of carbon as carbonate. The generation potential of carbon dioxide from I m 3 of coal, typically containing 65% TOC, can be 6.3mmol/g TOC (corresponding to the Miocene brown coal in the present study). The yield of carbon dioxide from this type of coal is about 4100 mol/m 3 which is higher than the amount of calcite in the cemented sandstone. For comparison, 1 m 3 of a marine oil prone source rock with 10 wt% TOC and a generation potential for carbon dioxide of 2.5 mmol/g TOC (corresponding to the Kimmeridge Clay Formation in this study) will yield about 625 mol of carbon as carbon dioxide per m 3 of rock.

These numbers are significant, especially if one compares them with bicarbonate concentrations in pore waters which are typically in the range of 3-30 mol/m 3.

Comparison with a kinetic model

The results from the present pyrolysis experiments are compared with the calculated yields using the model and parameters of Burnham and Sweeney (1989) listed in Table 2. The same model and parameters are used for both carbon dioxide and organic acids. In order to compare the experimental data with the model, the yields are normalized to the maximum yield of carbon dioxide and organic acids respectively for each of the source rocks, and it is assumed that this corresponds to a transformation ratio of 1.0. The calculated and measured transformation ratios are shown vs the heating time in Figs 8, 9 and 10. Though the measured values are to some degree scattered, there is a reasonable correspondence between measured and calculated values for carbon dioxide and organic acid generation, with a few exceptions for the high maturity samples from the Miocene brown coal. For the marine type source rock the experimental yields are lower than the calculated yields in most of the experiments, but the measured generation follows the same pattern as the calculated generation. The results suggest that the kinetic model, and the parameters of Burnham and Sweeney (1989) for the generation of carbon dioxide, can be used as a first approximation to estimate the generation of carbon dioxide and organic acids for the heating rates employed in the present pyrolysis experiments, and can provide a basis for extrapolation to natural conditions.

Carbon isotopic composition of generated carbon dioxide

The carbon isotopic composition of the CO2 generated from the three source rocks is shown in Table 5. Figure 11 shows the evolution in carbon isotope composition, versus calculated vitrinite reflectance values, together with the initial kerogen isotopic composition (straight lines).

Table 5. Carbon isotopic composition of carbon dioxide from hydrous pyrolysis

Pyrolysis Miocene brown coal, Ness Formation, Kimmeridge Clay temperature Germany North Sea Formation, UK

(C) (313C%.PDB) (3 ~3C/~PDB) (~5 J3C~PDB)

Initial kerogen - 26.0 - 26.4 - 20.8

175 -27 .9 - 16.5 200 -26 .8 - 16.8 225 -25 .6 -25 .9 - 16.9 250 -25 .3 -25 .0 - 16.7 275 -25 .4 -25 .0 - 17.2 300 -25 .5 -23 .7 - 18.3 325 -25 .4 -23.3 - 17.6 350 -25 .3 -23.1 - 17.8 365, 1" -25 .2 -22 .9 - 18.1 365, 3 -24 .9 -22.8 - 18.6 365, 10 -25 .0

365, I - -heated isothermally at 365'C for 25 h; 365, 3--heated isothermally at 365C for 3 days; 365, 10--heated isothermally at 365C for 10 days.


The carbon dioxide released from the Miocene brown coal shows a slight increase in the heavy isotope from a 6'3C value of -25.69'00 at the lowest maturity level to -25.09'00 at the highest maturity level. The carbon dioxide is, in addition, slightly enriched in the heavy isotope compared to the associated kerogen (6 ~3C = -26.0%PDB).

The same trend in the isotopic composition of generated carbon dioxide is also found from the Ness Formation coal, but the trend is more pronounced. The carbon isotope values range between -27.0%o at the lowest maturity level to -22.89'oo at the highest maturity level. At the lowest maturity levels the generated carbon dioxide is also slightly depleted in the heavy isotope compared to the initial kerogen (6t3C =-26.4%oPDB), but at higher maturities all carbon dioxide is enriched in ~3C compared to the initial kerogen.

The carbon dioxide released from the Kimmeridge Clay Formation is significantly enriched in '3C

compared to the coal samples and is, in addition, increasingly depleted in the heavy isotope with increasing maturity. The 613C values range from -- 16.5 to -18.69'oo. These values are all somewhat heavier than the associated kerogen (~13C =-20.8%oPDB).

The largest variation in carbon isotopic composition of the generated carbon dioxide is found at maturities lower than %Rm = 0.6, when only a small amount of carbon dioxide is liberated. At higher maturities, with a higher proportion of carbon dioxide liberated, the isotopic composition is more constant. With the exception of a few low maturity samples from the Ness Formation coal, all carbon dioxide is enriched in 13C compared to the initial kerogen. Based on thermodynamic isotope factors, an enrichment of the heavy isotope in the liberated carbon dioxide is due to enrichment of '3C in O-containing groups compared to C-H bonds (Galimov, 1975). However, in addition to different isotopic trends with maturity between

1 m 3 SANDSTONE

25 % grain porosity

50 % calcite cementation of grain porosity

1 rn 3 COAL

65 wt % TOC

3125 mol calcite

Carbon dioxide- potential

6.3 mmol/gTOC

1 1 4095 mol

carbon dioxide

1 m 3 SOURCE ROCK

10 wt % TOC

Carbon dioxide- potential

2.5 mmol/gTOC

1 625 mol [

carbon dioxide

4095 mol C/m 3

3125 mol C/m 3

625 tool C/m 3

Typical water 3-30 tool C/m 3 [ I

pore

Fig. 7. Comparison between the amount of carbonate carbon in a calcite cemented sandstone and carbon dioxide liberated from coal and marine type source rocks.


MIOCENE BROWN COAL

1.0 O o

0.9 o ~ 0.8 .0

0.7 o 06 j . ~ 0.5

0.4

0.3

0.2 o

OA . .~

0 . . . .

0 50 100 150 200 250 300 350 400 450 500 Duration, h

- - Calculated transformation Measured transformation, carbon dioxide o Measured transformation, organic acids

Fig. 8. Comparison between measured and calculated generation of carbon dioxide and organic acids from Miocene

brown coal, Germany.

KIMMERIDGE CLAY FORMATION 1.0

0.9. o ~ 0.8.

~ 0.7.

g0.10"6. 0 ~ "

"~ 0.5-

~ o.4-

~0.3-

0.2-

0~. t . . . . , , .., .... ,. 0 . . . . . . , . i . . . . , . . . .

50 100 150 200 250 300 350 400 450 500 Duration, h


Fig. 10. Comparison between measured and calculated generation of carbon dioxide and organic acids from Kim-

meridge Clay Formation.

the marine source and the coals, a difference in isotopic fractionation between initial kerogen and liberated carbon dioxide is also observed. A rather small isotopic fractionation is found for the Miocene brown coal (0.4-1.0%o), a larger isotopic fractionation for the Ness Formation coal (-1.5-3.6%o) and an isotopic fractionation between 4.3-2.49/00 is found for the Kimmeridge Clay Formation.

Equilibrium conditions, relative to the hydrocarbon gases, are not indicated in the present experiments when the isotopic fractionation between

NESS FORMATION COAL 1.0 o

0.9 o 0.8

..q 0.7i

.~ 0.6

~ 0.5- ~ 0.4-

0.3- e /~/ [_, 0

0.2-

0.1 00 /# o

0 50 100 150 200 250 300 350 400 450 500 Duration, h


Fig. 9. Comparison between measured and calculated generation of carbon dioxide and organic acids from Ness

Formation coal.

methane and carbon dioxide is compared to theoretical values calculated after Bottinga (1969). Figure 12 shows the fractionation factor between methane and carbon dioxide as a function of calculated vitrinite reflectance values in the present experiments. A smaller isotopic fractionation is obtained in the pyrolysis experiments than indicated by the theoretical values. The largest variation is found at maturities lower than %Rm = 0.6. The slope of the fractionation

0.2 Initial kerogen: -20.8 -26.0-26.4

0.4

i

0.6 " I "~

0.8 I I - I I

1.0 I I

i~,. i * t ft. 1.2 I I

k 6

1.4 ~ Miocene brown coal I ] 'l - Ness Formation coal .... ,t.... Kimmeridge Clay Formation 1.6 . . . . . . . . . , . . . . . . . .

-10 -15 -20 -25 -30 Carbon isotopes, 813C %oPDB

Fig. I1. Carbon isotope composition of carbon dioxide released during hydrous pyrolysis experiments.


0.2

0 .4 ,,s

0.6

0.8

1.0

"r. 1.2

~ 1.4

1.6 . . . . i . . . . i . . . . , . . . . i . . . . i , , 1.005 1.010 1.015 1.020 1.025 1.030 1.035 1.040

Fract ionat ion fac tor , methane-carbon d iox ide

Theoret ica l value (Bot t inga, 1969)

--m- Miocene brown coal

Ness Format ion coal

-.i.- Kimmer idge C lay Formation

Fig. 12. Carbon isotopic fractionation between methane and carbon dioxide.

curves differs between the coal samples and the type II source rock. The fractionation between methane and carbon dioxide from the Kimmeridge Clay Formation has a slope somewhat similar to the theoretical values but with different numerical values. The fractionation from all three source rocks seems to approach the theoretical values at higher maturity levels. The lack of correspondence with isotopic equilibrium trends support the use of kinetic models for transformation and yield estimation, since a kineticaily controlled reaction system is indicated.

CONCLUSION

The results of the present study show that carbon dioxide, and organic acids, are products of the thermal breakdown of kerogen. The amounts are significant. Yields of up to 6.3 mmol carbon dioxide and 1.1 mmol organic acids per gram TOC have been registered. Both the amounts, and the temperature dependence, varies' with the initial source rock composition. Experimental and calculated results indicate that the release of components are at a maximum between a vitrinite reflectance of %R m =0.40 and 1.00, depending on source rock type. The generated carbon dioxide is isotopically slightly heavier (up to 4.3%o) than the parent kerogen. The liberation of carbon dioxide, and organic acids, from kerogen can be described with reasonable accuracy using a first order distributed type kinetic model.

Associate Editor- -M. RADKE

Acknowledgements--The present work is part of a larger diagenesis study financially supported by Elf Petroleum Norge AS, and Elf is thanked for permission to publish the data. Dag Karlsen, The University of Oslo, is thanked for his assistance with the Rock-Eval analysis and TOC determinations. The improvements provided by two anonymous reviewers are greatly appreciated.

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