The kinetics of sterane biological marker release and degradation processes during the hydrous pyrolysis of vitrinite kerogen

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Gewhimica er Cosmochimica Aaa Vol. 54, pp. 2451-2461 Copyright 0 1990 inU.S.A.

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Pergamon Pressplc.Printed

The kinetics of sterane biologi~1 marker release and d~dation during the hydrous pyrolysis of vitrinite kerogen

processes

G. D. ABBOTT,* G. Y. WANG, T. I. EGLINTON,~A. K. HOME, and G. S. PETCH Newcastle Research Group (NRC) in Fossil Fuels and Environmental Geochemistry, Drummond Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK (Received November 28, 1989; accepted in r~ised~~

June 20, 1990)

Abstract--The hydrous pyrolysis of a mineral-free vitrinite kero8en (Dinantian coal, Lower Carbonifemus, North East England) has been carried out at four temperatures (270,300,330, and 3SOOC) for heating times ranging from 2 to 648 h. No significant differences in the epimer-based maturation parameters 2OS/(2OS + 20R)-5~(H),14a(H),17a(H)C29non-rearrangedsteranesand22S/(22S + 22R)-17a(H), 2 1/3(H) homohopanes were found for a comparison between “expelled oil” and “bitumen” fractions in the resulting pyrolysates. A deuterated model compound (( 20R)-5ar(H),14cx( H),17a( H)-[2,2,4,4-Q] cholestane) was added to a number of pre-extracted kerogens (vitrinite, Kimmeridge, Messel, and Monterey), and the mixtures were heated under typical hydrous pyrolysis conditions. These experiments showed that direct chiral i~rne~~tion at C-20 in the non-rearranged steranes appears to be relatively unimportant during hydrous pyrolysis which has also been suggested by other recent studies on geological samples. A kinetic model comprising consecutive release and degradation processes was derived to measure first-order rate coefficients from the bi-exponential concentration-time functions of both the (20R)- and (2OS)-5cw(H), 14~( H), 17cr(H) Cz9 “‘free” steranes in the vitrinite kerogen pyrolysates. This data was then used to calculate preliminary Arrhenius parameters for release (( 20s): AEO = 125 t 30 kJ mol-’ , A a 4.7 X 10’s_‘; (20R): AE, = 151 t 39 kJ mol-‘, A = 2.7 X lo9 s-‘) and degradation ((20s): A& =104 f 22 kJ mol-‘,A TZ?5.8 X lo3 s-‘; (20R): A& = 87 z!z6 k.i mol-‘, A = 2.2 X lo2 s-‘) of the above individual isomers and the values were found to be consistent with a free-radical chain m~ha~sm. This work helps in the greater undemanding of the important biomarker reactions that prevail in hydrous pyrolysis experiments. INTRODUCTION CHANGESIN BIOLOGICAL MARKERdistributions, such as those

of steranes and hopanes, during the hydrous pyrolysis of kerogens and whole rock samples are now well documented ( LEWAN et al., 1986; COMET et al., 1986; EGLINTON and DOUGLAS, 1988; RULLK~TTER and MARZI, 1988). Despite the fact that hydrous pyrolysis is one of the most widely used methods in artificial maturation studies (e.g., LEWAN, 1985; PETERSet al., 1989), the fundamental physical chemistry of the processes controlling these changes is not very well understood. MACKENZIEet al. ( 1982) have suggested that monitoring the changes in the relative abundance of certain biological marker isomers (biomarker maturity ratios) down the sedimentary column can assist in providing information about the thermal history of sediments. This application has been based on the kinetic analysis of assumed, single-step reactantproduct (i.e., A --, B) biomarker reactions, e.g., isomerisation at C-20 in the non-rearranged steranes (MACKENZIE and MCKENZIE, 1983). A similar approach has been adopted in the hydrous pyrolysis studies of RULLK~TTER and MARZI ( 1988, 1989). There are, however, potential compli~ting factors which need to be borne in mind when attempting to derive kinetic models of this type. Firstly, a frequent observation of kerogen pyrolysates is that their biomarker distri-

* To whom correspondence should be addressed. t Present address: Fye Laboratory, Department of Chemistry, Woods Hole OceanographicInstitute, Woods Hole, MA 02548, USA.

butions are commonly dominated by relatively “immature” isomers ( SEIFERT, 1978). The reason for this is not clear but it may be a result of restricted sensitivity to thermal alteration for compounds bound into the macromolecular network ( MICHAELISet al., 1990). If the same behaviour is followed in sediments then the generation of these “immature” biomarkers from the kerogen could influence the apparent maturity assessment of a source rock/oil (as determined by its biomarker content). This is particularly the case for those samples in which the keropn represents a major potential source of these compounds ( EGLINTONand DOUGLAS,1988). Biomarker degradation reactions may also be important in controlling changes in biomarker maturity ratios, if the mechanisms of sedimentary reactions are the same as those observed in the model compound heating expe~ments that have been described by ABBOTTet al. ( t 985a,b) and BEACH et al. ( 1989). In this respect, dramatic decreases in absolute biomarker concentrations with increasing maturity have been observed in sediments (e.g., RULLK~TTER et al., 1984). Regarding the apparent isomerisation of the steranes, PEAKMAN and MAXWELL( 1988) and PEAKMANet al. ( 1989) have cited the possibility of alternative mechanisms, occurring at the sterene stage, as an explanation for the gradual relative increase of other configurations relative to the precursor “biological” configuration with increasing thermal stress. The major objectives of this study are ( 1) to assess the importance of the presumed chiral isomerisation at C-20 in the non-rearranged steranes for a range of kerogens during hydrous pyrolysis, and (2) to derive a kinetic model which is able to rational& the observed changes in absolute “free” 2451

2452

G. D. Abbott et al.

sterane concentrations with time during the isothermal hydrous pyrolysis of a vitrinite kerogen. In this work “free” hydrocarbons are discrete molecular species which are amenable to GC analysis. The development of this kinetic model has involved the determination of preliminary Arrhenius parameters for the release and degradation processes under lab-

Nz was repeated (3X) to ensure that molecular O2 had been removed.

oratory conditions. These are especially pertinent given that RULLK~TTER and MARZI ( 1988) have derived kinetic con-

Pyrolysate

stants based on the evolution of the 2OS/( 20s + 20R) sterane isomerisation ratio during hydrous pyrolysis and used these to reconstruct the geothermal history of the Michigan Basin ( RULLKOTTERand MARZI, 1989). Vitrinite kerogen (Type III kerogen) was chosen specifically because it could be prepared in a mineral-free state without any acid treatment. This, therefore, allowed us to focus on the molecular properties of the released pyrolysate in isolation from the potential complicating effects of some of the variables which are encountered in whole rock experiments. Such effects, which arise primarily from the influence of the mineral matrix, have been studied in detail by other workers (e.g., HUIZINGAet al., 1987; LU et al., 1989). This work helps to fulfill the need, noted by other workers (SMITH et al., 1989), for a fundamental understanding ofthe prevailing chemical reactions and physical processes occurring during hydrous pyrolysis. EXPERIMENTAL Sample Preparation The following kerogens were used in this work: (i) vitrain, Dinantian coal (Lower Carboniferous) which was collected from an exposure in NE England; (ii) Kimmeridge kerogen, Kimmeridge shale (Blackstone Band, Upper Jurassic) which was collected from an outcrop at Clavell’s Hard, Dorset, England; (iii) Messel kerogen, Messel shale (Eocene) from a quarry near Darmstadt, West Germany; and (iv) Monterey kerogen, Monterey shale (Miocene) from an outcrop near Vandenburg Air Force Base, Ventura County, California, USA. The vitrinite reflectances of the samples are: Vitrain (0.55%), Kimmeridge (0.35%), Messel (0.27%), and Monterey (0.33%). The isolation of the organic material began with the removal of the outer weathered surfaces with a hard wire brush, and then the samples were washed with successive aliquots of methanol and dichloromethane in an ultrasonic bath for periods of I5 min each. At this stage the vitrain bands of the Dinantian coal were visually separated from the other macerals and checked for purity both by use of reflected light microscopy and elemental analysis. Essentially, this resulted in mineral-free organic matter. The dried vitrain was ground to a powder with a particle size of between 100 and 200 mesh. The shales were also powdered (in a disc mill, Tema) and all samples were Soxhlet extracted, for 72 h, using a solvent mixture of dichloromethane/methanol (93:7 v/v) yielding a bitumen I fraction. The kerogens (ii), (iii), and (iv) above were isolated from their respective shales by acid washing in the manner described by FOWLER (1984). The resulting organic-rich material was then visually inspected by reflected light microscopy for any obvious mineral contamination. Before use in the pyrolysis experiment, a final extraction of the residual bitumen (bitumen II) was carried out using the Soxhlet method described above.

After the final purge with N,, the pressure was reduced to ambient and heating commenced. The temperature of the reaction vessel was controlled to +2.5”C. The second method for hydrous pyrolysis involves the use of four purpose-built “bomblets” and has been described previously by EGLINTONet al. ( I988 ) Analysis

After rapid cooling the organic material was removed from the reactor. The gases were collected in a purpose-built gas cylinder. The “expelled oil” occurs in the reactor, as a floating liquid pyrolysate layer on the water surface and as a weakly bound liquid pyrolysate film on the solid particles, whereas the “bitumen” is more strongly bound to the residue ( LEWAN et al., 1986). Since powdered mineralfree samples were used in the present hydrous pyrolysis experiments, the terms “expelled oil” and “bitumen” do not strictly adhere to the definitions of LEWAN et al. ( 1986). One of the major objectives of this work, however, was to determine whether any differences could be observed between these two phases in our experiments which consequently might influence our kinetic calculations. The “expelled oil” was obtained by washing with a solvent mixture of light petroleum ether/dichloromethane ( I:4 v/v; 4 X 200 cm3). The washed kerogen was then allowed to dry before the bitumen was extracted using the Soxhlet extraction method described above. The extracted kerogen was dried and retained for optical and elemental analysis. The concentrated extracts were fractionated by silica gel thin layer chromatography giving aliphatic, aromatic, and polar fractions. An appropriate quantity of a suitable internal standard [( 20R)5a( H),14a( H), I~Lu(H)-[2,2,4,4-d4] cholestane (Fig. 1 )] was added to quantify the non-rearmnged steranes. These extracts were analysed on a Carlo Erba Mega Series 5160 fitted with either a DB-5 or an OV- 1 coated, 30 m fused silica capillary column (0.25 pm film thickness and 0.32 mm i.d.). The column oven programme for the analysis was as follows: 50°C for 2 min, 50-300°C at 4”C/min, and 300°C for 20 min. Cold on-column injection of the sample was employed and a detector temperature of 3 10°C was maintained. Gas chromatography-mass spectrometry (GC-MS) analyses were carried out on a Hewlett Packard 5970 mass selective detector (electron energy, 70 eV; filament current, 220 PA; source temperature, 220°C) interfaced to a Hewlett Packard 5890 GC fitted with a HP1 coated fused silica capillary column (25 m X 0.2 mm i.d. X 0.11 pm film thickness). Some further GC-MS analyses were carried out on a VG TS250 mass spectrometer (electron energy, 35 eV; filament current, 4200 mA; acceleration voltage, 4 kV; source temperature, 220°C) interfaced with a Hewlett Packard 5890 GC fitted with a HP-5 coated fused silica capillary column (25 m X 0.2 mm i.d. X 0.11 Frn film thickness). Data Analysis

A modified Gauss-Newton algorithm (E04FDF from the Numerical Algorithms Group (NAG) Library) was used to find the unconstrained minimum of a sum of squares of m non-linear functions in n variables (Eqn. 1). The program was run under double precision on an Amdhal mainframe computer with IBM 370 architecture (under the MTS system). Briefly, the program takes a set of concentra-

Hydrous Pyrolysis

Hydrous pyrolysis was carried out by one of two methods. The majority of the work used a purpose-built stainless steel open vessel which was placed in the full chamber of a 1 1general purpose stainless steel reactor (Parr Instrument Co.). This open vessel was filled with vitrain (25 g) and distilled water (300 cm 3). The bomb was sealed, evacuated, and purged with Nz. The evacuation and purging with

FIG. I. Structure of (20R)-5a(H),14a(H),17a(H)-[2,2,4,4d,]cholestane ( dq standard).

2453

Kinetics of vitrinite pyrolysis tion-time data, and then initial estimates of values for k,, kz, and [Xl, were incremented and decremented iteratively in Eqn. (4) (see below: Results and Discussion) until values were yielded that minimized the sum of the squares of the residuals between the calculated and the experimentally observed data points. When k, , k2, and [Xl, had been determined, the value, termed by the authors Mean Percentage Error (MPE, Eqn. 2), was computed and compared with the calculated systematic experimental error for the whole heating and work-up procedure. If the residuals between the computed curve and the data points were significantly greater than the systematic experimental error then the computed values for k,, k2, and [Xl0 were rejected:

270% . “expelled oif +

“bitumen”

300°C l

*

I

“expelled oil” “bitumen”

I

200

400

600

Time (h)

sum of squares = 2 [Y(t,) - Y&l* r-l of residuals MPE = z

I,

330%

,t ][Y(t,) - Yti]l/Y(tJ

8

“expelled oil”

+

“bitumen”

350°C

where m is the number of experimental data, Y( t,) is the calculated biomarker concentration at time ti, and Yt, is the experimental biomarker concentration at time tt.

l

“expelled oil”

RESULTS AND DISCUSSION

Biomarker Maturity Ratios

0.25

There have been many studies using hydrous pyrolysis to determine biological marker maturity parameters in the “expelled oil” fraction (e.g., COMET et al., 1986). In this work, the changes in the biological marker maturity ratios have been compared over a broad range of temperatures (270, 300,330, and 35O’C) and heating times for both the “expelled oil” and “bitumen” fractions. The vitrinite kerogen results

for the epimer-based molecular maturation parameters 2OS/ (20s + ~OR)-~~(H),~~~(H),I~CI(H)C~~ steranes (2OS/(2OS -t 20R)) are presented in Fig. 2, and a similar comparison 270°C

:,

0.35

-

t

*

“expelled oil”

+

“bitumen”

300% +

0

400

200

l

“expelled oil”

0

“bitumen”

0

50

was made for the 22S/(22S + 22R)-l7~(H),2i~(H) homohopanes (22Sf( 22s + 22R )) (Fig. 3). Although there are sometimes small differences between the “expelled oil” and “bitumen” fractions, there are no significant trends. This, therefore, suggests that if resistance to mass transfer is important in the hydrous pyrolysis experiment ( BURNHAMet al., 1988) then it does not appear to have had an effect on the 2OS/( 205 + 20R) and 22S/( 225 + 22R) epimer-based molecular maturation parameters in these particular experiments. LEWAN et al. (1986) have noted that there may be differences in some of the sterane maturity parameters between these two fractions whilst others ( RULLK~TTER and MARZI, 1988) have suggested that these are not significant. Both of these studies, however, used unextracted rock chips in their experiment as opposed to the powdered pre-extracted vitrain employed in this work.

600 0.4

0.35

+

(f

t

n

“expelled oil”

+

“biiumeii’

350°C * “expelled

*

0

8 ‘= v)

.

A 270°C A

330%

0.25

200

FIG. 3. Comparison of the 22S/(22S + 22R) ratio for 1701(H),2 Ip( H) homohopanes in “expelled oil” and “bitumen.”

Time(h)

B a

150

100

Time (h)

oil”

“bitumen”

300%

. 330°C Q 350°C ,.-.“A _.--

ii? 8 (t, z =: cn z?

A ,_/-.-.***.f

0.15

z?J

0.05’ 0

.

’ 50

.

*

’ 100

I

’ 150

.

.

’

200

0

200

400

Time

600

800

(h)

Time (h)

2. Comparison of the 2OS/( 205 + 20R) ratio for 5ry(H), 14ru(H),17n(H) Cz9steranes in “expelled oil” and “bitumen.” FIG,

FIG. 4. Variation in the 2OS/(2OS + 20R) ratio for 5a( H ), 14a( H ) ,17cu(H) Czs steranes (“expelled oil” and “bitumen” combined) with heating time at various temperatures.

G. D.

2454

Abbott et al.

The 2OS/(2OS + 20R) and 22S/(22S + 22R) maturity ratios for the combined “bitumen” plus “expelled oil” fractions are plotted in Figs. 4 and 5, respectively, as a function of increasing heating time at 270,300, 330, and 350°C. There is an increase in both ratios with time at all temperatures, apart from the values at 120 and 144 h for the 350°C experiment. The rate of change of these ratios are both enhanced significantly as the temperature is raised from 270 to 300°C. The values of both ratios appear to be more dependent on time than temperature, for temperatures greater than 300°C. It is noticeable that for the 2OS/( 20s + 20R) ratio the end point (ca. 0.55, MACKENZIE, 1984) is never reached under the conditions employed in this study. In contrast the 22S/ (22s + 22R) ratio approaches the end point (ca. 0.6, MACKENZIE, 1984) at 300, 330, and 350°C. It is interesting to note that the 2OS/(2OS + 20R) ratio is less than the 22S/ (22s + 22R) ratio in the unheated extract (0.17 and 0.60, respectively) and continues to lag behind in the total pyrolysis extract during heating. A similar type of behaviour in the relative progress of these two ratios has been observed by MACKENZIEet al. (1980) in the sedimentary column. Steranes released by hydrous pyrolysis show lower values of the 2OS/( 20s + 20R) ratio than in the unheated bitumen for most of the samples heated at 270°C and for the early time points at the higher temperatures. The 22S/( 22s + 22R) ratio is consistently lower in the pyrolysates as compared to its value in the unheated extract. This effect has been observed previously in both pyrolysis and chemical degradation studies on coals, kerogens, and asphaltenes ( SEIFERT and MOLDOWAN, 1980; JONES et al., 1988; MICHAELISet al., 1990). There is a reversal in both the 2OS/( 20s + 20R) and the 22S/( 22s + 22R) ratios at 350°C for heating times greater than 72 h. A similar change in the 2OS/( 20s + 20R) ratio has been reported by LEWANet al. (1986) as well as RULLKOTTER and MARZI ( 1989). Lu et al. ( 1989) observed reversals in the 2OS/(2OS -t 20R) ratio as well as the 22S/( 22s + 22R) ratio for the Cjl and C3z 17~u(H),2la(H) hopanes during the anhydrous pyrolysis of both a Rocky Mountain coal and an immature Cretaceous black shale. A reversal of the 2OS/( 20s + 20R) ratio may be one explanation for the observations made by STRACHANet al. ( 1989) on a sample suite from the Carnarvon Basin, Australia.

0.6 A A . 0

270% 300°C 330°C 350%

_.-._.*.-*

I 0

200

400

600

.

_

600

Time (h)

F’rc. 5. Variation in the 22S/(22S + 22R) ratio for 17a(H),21@(H) homohopanes (“expelled oil” and “bitumen” combined) with heating time at various temperatures.

Mass Chromatograms, m/z 221 (20R)- [2,2,4,4-d4) - Cholestane (before heating)

Kimmeridge + (20R) - dq - Cholestane

E

(300%/63h) 8 E

i

______Monterey + (20R) - d4 - Cholestane (300W63h)

Time

-

FIG. 6. Partial mass chromatograms (m/z = 22 1) for the Q standard (unheated) and “expelled oils” after heating various kerogens at 300°C for 63 h with added 4 standard.

Derivation of Kinetic Scheme

A deuterated model compound was added to a number of pre-extracted kerogens. This mixture was then used in experiments, the objective of which was to derive a kinetic model that could be used to describe the concentration-time behaviour of individual sterane isomers during hydrous pyrolysis. Of particular concern was the possibility of isomeri&ion at any chiral centre within the “free” non-rearranged steranes in the saturated hydrocarbon fraction, The sterane used in this work was the d4 standard (Fig. 1) which was shown to be analytically pure (Fig. 6). A deuterated standard was used since the deuteration allowed any isomers which were formed from it to be resolved by GC-MS from the nondeuterated steranes released by the kerogen. The vitrinite kerogen/deuterated sterane mixture (25 g kerogen with 1.1 mg d4 standard) was heated at 300°C for 80 h under hydrous pyrolysis conditions. Further experiments using a lower quantity of standard added to the vitrinite and other kerogens (for amounts see Table 1) were also carried out. The levels of d4 standard used in these subsequent experiments were such that, following pyrolysis, its absolute concentration was of the same order as the absolute concen-

Kinetics of vitrinite pyrolysis Table 1.

2455

Amounts of kerogen (g) and d4 standard (pg) used for hydrous pyrolysis experiments at 300°C for 63 hours

Kerogen

wt. of kemgen

(i) (ii) (iii) (iv)

5.0 0.5 0.5 0.5

Vitrinite Messel Monterey Kimmeridge

Wt of added std. 0.2000 4.6283 4.6283 4.6283 Mass/Charge

trations

of the non-deuterated

steranes released by the ker-

ogen. Initial analyses, using the m/z = 221 partial mass chromatogram of the pyrolysates, showed no significant isomerisation (Fig. 6). The small peaks visible on the Monterey kerogen partial mass chromatogram were not sterane isomers. Further investigation, however, showed other peaks could be resolved from the baseline noise of the m/z = 22 1 mass chromatogram for the “bitumen” and “expelled oil” fractions of the vitrinite experiment with the larger quantity of d, standard added. This was achieved by increasing the amount of the aliphatic fraction from the pyrolysate being analysed to such an extent that the dq standard began to overload the GC column capacity. The partial mass chromatogram for the “bitumen” (Fig. 7) showed that of the peaks that could be resolved from the baseline noise there were two (A and B) whose mass spectra (Fig. 8) were consistent with assignments to isomers of the (20R)-S~u(H),l4u$H),l7a(H)-[2,2,4,4d4] cholestane standard. Definitive stereochemical assignments of A and B were not possible, however, due to the lack of suitable synthetic standards. Since the peaks A and B were only detectable when the GC column capacity for the dq standard was overloaded a second check on the purity of the standard was carried out using a similar overload of column capacity for the pure compound. This showed no evidence of any other steranes being present. Therefore, the most likely explanation is that isomerisation has occurred. The extent of this isomerisation and its importance to the kinetic scheme was then assessed. The vitrinite “bitumen” and “expelled oil” ( 1.1 mg dq

B

M+-15 361

I

Mass/Charge RG.

8. Mass spectra for the two unknown compounds A and B.

standard added) each required two separate analyses to quantify both A and B formed and the & standard remaining: (i) for compounds A and B the GC column capacity relative to the d4 standard was overloaded in order to quantify the very small peaks, and (ii) for the & standard a normal loading was used since overloading would have resulted in an underestimate (Table 2). In both analyses the internal standard used for the quantification was 9-n-dodecylperhydroanthracene (appropriate response factor being used). The “expelled oil” from the other experiments collated in Table 1 were also analysed to determine the amount of 4 standard remaining as well as testing for the presence of A and B. The absolute quantities of the non-deuterated (20R)-5a( H), 14a( H), 17a(H) CZ9 sterane were also determined using the same internal standard to allow a comparison with the quantity of deuterated sterane present (Table 3 ) . The 20s /( 20s + 20R) for the 5a(H),l4a(H),l7cu(H) CZ9 non-deuterated sterane and the (BLY(Y+ a/3/3)/( (YOI(Y + polar + a/3@) for the

ciaa 20R Table 2.

Amounts of isomers formed by heating Ihe standard (1.1 mg) with vitrain (25.0 g)

&

A

Peak identity

Fraction

Amount

(pg/25 g vitrain) 6)

A

iij B 0) (i) TimeFIG. 7. Partial mass chromatogram (m/z = 22 1) for the “bitumen” of a mixture of vitrinite kerogen (25 g) and d., standard ( 1.1 mg) after heating at 300°C for 80 h (with the region containing minor peaks expanded in inset).

A B A B (ii) d4 standard (ii) d4 standard a4 standard

Bitumen Bitumen Expelled oil Expelled oil Total Total Bitumen Expelled oil Total

1.295 4.093

0.261 0.505 1.556 4.678 393.484 452.960 846.444

d4 standard = (20R)-5a(H).14a(H).17a(H)-[2,2,4.4-d~] cholestane

G. D. Abbott et al.

2456 Table 3. Comparison

of quantitative

Kerogen 20s (20S+20R) W5aC29)

(a) Vitrain+ (b) Vitrain+ Monterev Kimmerihge Messel

11 9 4 I 20s NOT

data for non-deuterated

Non-deuterated steranes (!3aa+a!3!3) @aa+c@3+aaa) Amo”“1 Of (20R)aaa-C2g SC29 released (!Jg) 30 31 21 24 34

0.25 0.16 3.00 0.13 0.73

and deuterated

A (A+(20R)-d4 Std) %Q?.l

steranes

in “expelled

oil”

Deuterated steranes B (B+(20R)-d4 Std) %c27

0.06

Amount of d4 standard remaining @g)

0.11

452.96 0.06 2.4X 2.88 3.19

ND ND 0.65’

0.63* ND

+ (a) 25 g of vitrinite kerogen and 1.1 mg of d4 standard. + (b) 5 g of. vitrinite kerogen and 0.2 pg of d4 standard. * Tentative assignment based on relative retention times due to very weak mass spectra ND = A and B were not detected. dq standard = (20R)-5a(H).14a(H).l7a(H)-[2,2,4,4-d~]choleslane.

5p( H ) ,14a( H ) ,1701( H ) ; 54 H ) ,14p( H ) ,17P( H ) and 5c~(H ) ,14a( H ) ,17a( H ) CZ9 non-deuterated steranes together with A/(A + (20R)-d4 Std) and B/(B + (20R)-d4 Std), as appropriate, for the (20R)-5a( H),14a( H), 17a( H), as well as A and B CZ7 deuterated steranes were calculated from the above experiments (Table 3). A comparison of the data in Table 3 shows that although in certain cases some isomerisation of the d4 standard has occurred, as evidenced by the values of A/(A + (20R)-d., Std) and B/(B + (20R)-d, Std), a similar process cannot account for the maturity ratios of the non-deuterated steranes released in the same pyrolysates; e.g., for the vitrain experiment (a) the above ratios for the deuterated components were of at least two orders of magnitude less than for the ratios of the non-deuterated components. When the amount of d4 standard added to the kerogen was of a similar magnitude to the amounts of non-deuterated steranes, then little, if any, isomerisation of the deuterated sterane was detectable, e.g., for the pyrolysis of vitrain (b), Monterey, Messel, and Kimmeridge kerogens. In these same kerogen pyrolysates, however, there were significant quantities of (2OS)5a(H),14a(H),17a(H); 5/3(H),14a(H),17ujH) and 5a(H), 14p(H), 17p( H) Cg9 non-deuterated steranes. It is possible that the d4 standard may have isomerised to a greater extent than is apparent from the A/(A + (20R)d4 Std) and B/(B + (20R)-d4 Std) ratios in the m/z = 221 mass chromatograms. Instead, simple homogeneous deuterium-hydrogen exchange with the water may have taken place, and, in fact, A and B are a fraction of the total amount of isomer formed. By studying the appropriate mass chromatograms corresponding to the loss of 1 or 2 deuterium atoms, the ratios A/(A + ( 20R)-d4 Std) and B/( B + (20R)d4 Std) were measured, and it was found that there were no significant differences between their values and the values of the same ratios observed in the m/z = 22 1 mass chromatogram. This indicates that any deuterium-hydrogen exchange which may have occurred has been to the same extent in each of the A and B peaks as well as in the d, standard. The mass chromatograms corresponding to the loss of 1 or 2 deuterium atoms were also extremely weak showing that simple, homogeneous deuterium-hydrogen exchange was relatively unimportant. This has also been noted by HOERING ( 1984)

who conducted model compound heating experiments on kerogen in the presence of heavy water. The conclusion which can be drawn from the pyrolysis experiments with added deuterated standard is that chiral isomerisation of the (20R)-5a( H), 14a( H), 17a( H) saturated non-rearranged sterane in the “free” state is relatively unimportant. In other words, the (20R)and (2OS)Sc~(H),14@(H),17@H) and (20S)-5a(H),l4cu(H),17a(H) non-deuterated steranes mustftherefore, arise largely from a source other than the “free” (20R)-5a( H), 14cu(H), 1701(H) isomer. It is interesting to note that recent absolute quantitative studies in sedimentary basins have suggested that little evidence exists for epimerisation of saturated steranes at C20 (NOBLE et al., 1989; REQLJUO, 1989). PEAKMAN and MAXWELL ( 1988) and PEAKMAN et al. (1989) have also suggested a mechanism for the formation of the (20R)and (~OS)-~L~(H),~~P(H),~~P(H) and (20S)-5n(H), 14a( H ), 17c~(H) epimers which does not have to involve direct chiral isomerisation at C- 14, C- 17, and C-20 in the (20R)5a(H),14a(H),17a(H) steranes. On the other hand, anhydrous laboratory experiments have shown that direct chiral isomerisation can be brought about in “free” steranes when elemental sulphur is present (ABBOTT et al., 1985b). The kerogens used in these experiments were pre-extracted. therefore the presence of “free” biomarkers in the total extract following pyrolysis was due solely to release from the “bound” state as a result of breaking chemical bonds and/or physical interactions. The term “bound” state in this work could apply to one or more of the asphaltene, resin (which are all generated during hydrous pyrolysis), and kerogen fractions. These various fractions of sedimentary organic matter may be regarded, to some extent, as part of the same continuum (S. R. LARTER, pers. comm.). Since the results for the vitrain experiment (a) (see Tables 2 and 3) also reveal that ca. 20% (w/w) of the & standard was degraded, then the kinetic scheme (Eqn. 3) can, therefore, be postulated: X Y k, “FREE” “BOUND” BIOMARKER BIOMARKER DEGRADATION

k2

PRODUCT

(S)

(3)

Kinetics of vitrinite pyrolysis where k, is the rate constant for the release process whereas k2 is the rate coefficient for the degradation process. Equation 3 may not be a “true” chemical mechanism, i.e., all the individual elementary processes involving molecules that take place simultaneously or consecutively in producing the observed overall reaction (FROST and PEARSON, 196 1), but needs to be regarded as an empirical reaction model. If it proves to be a useful mode1 (see below), then further work, using precise experimental data, could be used to improve Eqn. (3) step by step ( ISBARNet al., 1981).

2457

Vitrain

(“expelled oil”+“bitumen”) T=300”C

I

Kinetic Analysis of Sterane Data from the Hydrous Pyrolysis of Vitrinite Kerogen

100

kt k*

_

k,

(exp(-kd)

- exp(-&))

3

Time (h)

The above kinetic scheme, Eqn. (3), is a classic example of a series of consecutive first-order reactions (LAIDLER, 1987) . If k, and k2 are assumed to be first-order rate constants, analysis of such a scheme predicts that the concentrationtime function for the absolute concentration of a “free” biomarker [Y] behaves as follows:

[Ylr = [Xl0

200

Vitrain

(“expelled oil”+“bitumen”) T=300”C

(4)

where [Xl, is the initial concentration of the biomarker, Y, in its “bound” form. The total pyrolysate extract, rather than a specific fraction, has been used to derive the kinetic information since the

I

I

100

200

Time (h) Table

4.

Heating conditions “C/hours

Quantitative data for sterane isomers released from hydrous pyrolysis of vitrinite kerogen (27O”C,3OO”C, 33O’C and 35O’C) aaa-C29 (20s) (wglg

aaa-C29 (2OR) kerogen)

aaa-C29 (20S+20R)

270172 270/168 2701216 2701264 27013 12 270/360 2701648

0.076 0.152 0.193 0.144 0.332 0.224 0.219

0.460 1.586 1.537 1.600 1.801 1.250 0.806

0.536 1.738 1.730 1.745 2.133 1.473 1.025

300/l 1 300/24 300/48 300172 300/120 300/192 300/264

0.043 0.082 0.157 0.220 0.332 0.331 0.311

0.456 0.552 0.879 1.283 1.087 0.916 0.529

0.499 0.634 1.036 1.502 1.419 1.247 0.840

33015 330112 330124 330/48 330172 330172” 330/72* 330196 330/120

0.168 0.290 0.409 0.421 0.246 0.276 0.273 0.134 0.061

1.345 1.903 1.937 1.519 0.882 0.834 0.833 0.407 0.177

1.513 2.193 2.346 1.940 1.128 1.110 1.106 0.541 0.238

35015 350/12 350/18 350124 350172

0.261 0.299 0.228 0.215 0.044

2.090 1.465 0.90 1 0.821 0.081

2.351 1.764 1.129 1.036 0.125

* These experiments were carried out utilising the ‘bomblets’ rather than the whole bomb in order to assess the reproducibility of the work-up procedure. These particular data points were not used in the minimization routine.

FIG.9.Variation of the absolute concentrations (0) of (2OS)- and (20R)-5cr(H),l4a( H),17a( H)(& steranes (“expelled oil” and “bitumen” combined) with time at 300°C. The curve corresponds to the computer calculated concentration-time function based on Eqn. (4). biomarker distributions were so similar in the two fractions. If it is assumed that the kinetic scheme in Eqn. (3) is obeyed by the (20R)- and (20S)-5cu(H),l4a(H),17a(H) C29 steranes, then Eqn. (4) predicts that the concentration-time functions, for each of these isomers, will be a composite of an exponential growth followed by an exponential decay. When the experimental data (presented in Table 4) were used in the minimization procedure described in Data Analysis (Experimental) above, then good correlations with this type of bi-exponential function were obtained at 300 and 330°C as shown in Figs. 9 and 10, as well as at 270°C. The values of the rate constants and [Xl, values derived in the above manner at 270, 300, and 330°C are presented in Table 5. If the [Xl, values are examined in detail then for the (20R) isomer these are the same at 300°C as at 330°C. At 270°C [Xl, is 1 pg/g kerogen greater than its value at the two higher temperatures. The [Xl, values for the (20s) isomer are 0.9 pg/g kerogen with an associated variation of 50.3 pg/g kerogen. Since [Xl, is an initial concentration, the source of these differences is unclear. There are, however, two possible explanations: (a) It may be that the differences are the result of systematic experimental error combined with statistical variation arising from the minimization procedure. The systematic experimental error for the absolute quantification of the steranes was estimated to be about f 11%-this includes

G. D. Abbott et al.

2458

Vitrain (“expelled oil”+“bitumen”) T=330”C

(b) The proposed model, whilst achieving the above-stated aim of rationalising the absolute concentration-time behaviour and related changes in maturity parameters for the “free” steranes, does result in real differences when [Xl0 is calculated at different temperatures. The source of such differences is beyond the scope of this study but the model has stimulated further questions. Such questions may be answered by carrying out further experimental work, which may result in a step-by-step improvement of the model as stated above. The absolute concentrations of the (20R) isomer and its (20s) counterpart at 350°C were also measured. There is a problem at this temperature in that the rise time for both concentration-time functions falis in the same regime as the heat-up and cool-down times ofthe Parr bomb. It is, therefore, difficult or even impossible to define, with a reasonable degree of precision, the temperature at which the release process occurs. At reaction times in excess of the initial rise time, however, the degradation of the “free” biomarker (Y) will dominate the total concentration-time function. In such a time regime, therefore, if the degradation is first-order in sterane con~entmtion then the rate of loss of Y will follow:

Time (h) Vitrain ~exp~il~~~i~biturnen”) 0 =

-4Yl -

dt

Separating variables and integrating gives

Time (h) RG . 10. Variation of the absolute concentrations (0) of (2OS)and (20R)-5cr( H), 14=( H), 1fcvfH)Cm steranes (“expelled oil” and “bitumen” combined) with time at 330°C. The curve corresponds

to the computer calculated ~ncen~tion-time Eqn. (4).

function based on

potential variation from weighings, transfer losses during work-up procedure, and integration of peak areas. Although we have taken a great deal of care to determine these biomarkers quantitatively, there is an unavoidable error associated with the data. Table 5.

Kinetic parameters from minimization non-rearranged steranes

Temperature

Isomer

W)

f20Rf-aaa-C29

procedure

kt (IO-7 s-1)

for (20R)-

and (2OS)-5a(H).14a(H).l7a(H)-C29

kz (IO-7 s-1)

Irg/g

[Xl0 kerogen

Sum of of

Squares

residuals

300

29.08 (29.0717-29.0816)

I,

330 350

60.64 (60.6411-60.6412) 133.00

3.2

II

297.30 (297.2988-291.2995) * 5.83 (5.8255-5.8334) 13.97 (13.9619-i3.9668) 92.63 (92.6408-92.6.510) *

5.83 (5.8306-5.8368) 13.97 (13.9653-13.9721) 92.65 (92.6412-92.6543) 80.39

0.7

1.63596x10-*

0.9

1.68480x10-3

1.2

3.57770x10-3

300 0

11

330 350

(6)

Equation (6) predicts that when the natural logarithm of the concentrations of either (20R)- or (20S)-5a(H), 14a( H ), I7a( H) Cz9 steranes is plotted versus time then a linear function will result. The experimental data showed good linear correlation (Fig. 11) where the gradients of the two plots, obtained by linear regression analysis, yielded the degradation rate constant, k2, for the (20R) ( 132.57 X lo-’ s-‘) and the (20s) (SO.39 X lo-’ s-i ) epimers, respectively. The activation energies (A&,) and pre-ex~nential A-fac-

*t

270

10.43 (~0.4183-10.4312)+

ln[Y], = ln[Ylo - kzt.

10.44 (10.4309-10.4432) 29.08 (29.0746-29.0791)

(ZOS)-aaa-C*9

270

= kz[YJ.

4.2

3.32307x10-’

3.2

5.91485x10-~ 7.22076~10.~

*

-

* Data is not available ‘Values of kt and k2 in brackets represent the range obtained for the minimized

*

*

sum of squares of

c

residuals given.

Kinetics of vitrinite pyrolysis

2459

-11

-3 -4 0

-14 I

I

I

20

40

60

*

I 80

.60

Time (h)

I

1.70

I

.\

1.80

(1/T)x103 (K -1)

FIG. 1I. Plot of the natural logarithm of the sterane concentration against time for the (2OS)- and (20R)-5cz(H),l4a(H),l7a(H) C19 steranes at 350°C to determine the degradation rate constants.

FIG. 13.Arrhenius plots for the generation of the (2OS)-and (20R)~~I(H),~~cY(H),~~u(H)& steranes.

tots for the processes identified in Eqn. (3) were determined by application of the Arrhenius expression:

13. The slopes yielded activation energies of 15 1 f 39 and

k = A exp( - AE,/RT)

(7)

where R is the gas constant and T is the absolute temperature. AE,, and A-factors for the degradation of both the (20R)and (20S)-isomers were determined from Arrhenius plots presented in Fig. 12. The slopes gave activation energies equal to 87 + 6 and 104 + 22 kJ mol-’ for the (20R)- and (2OS)isomers, respectively. These two are similar when the error (+ 1 standard deviation) is taken into consideration. The associated A-factors are ca. 220 s-’ (20R) and ca. 5800 s-’ (20s); however, if all the errors implicit in the measurement of A-factors (BENSON, 1976) are taken into account then it is difficult to ascertain their “true” relative magnitudes. The values for the above Arrhenius parameters are consistent with a complex chain mechanism. HOERING ( 1984) has cited the possible importance of free radical chain mechanisms in the hydrous pyrolysis of kerogens. The degradation of the biomarkers under investigation in this work is, thus, more than likely a system in which two or more elementary reaction steps are coexisting. Despite the complexity of chain mechanisms it is still possible for them to follow simple first-order kinetics ( FROST and PEARSON, 196 1) . Forty-two concentration-time data points were measured to generate values of kl at 270,300, and 330°C. The Arrhenius plots for k, , using data from Table 5, are presented in Fig. -11

. 0

20R 20s

-12 '; 22 N Y S

-13

-14 -15i 1

I

1.68 (l/l)x

1.78

1.06

103(K-1)

FIG. 12. Arrhenius plots for the degradation of the (2OS)- and (~OR)-~~(H),I~~Y(H),I~~(H) Cz9steranes.

125 t 30 kJ mol-’ for the (20R)- and (2OS)- isomers, respectively. The associated A-factors are ca. 2.7 X 10gs-i and ca. 4.7 X 10’ s-‘. If Fig. 13 is examined in detail, however, the shape of the “true” plots for both isomers may be concave upward ( BUNNETT, 1986 ). This may be the result of a mechanism whereby there are two or more parallel reactions occurring in the generation process, e.g., if there is more than one type of chemical bond being cleaved in the kerogen resulting in the release of either biomarker species. According to the Arrhenius equation the increase in the rate constant with increasing temperature is greater when the activation energy is greater. Therefore, if two or more reaction pathways are in competition, then the pathway with the higher activation energy carries an even greater share of the reaction as the temperature increases-the Arrhenius plot is, hence, steeper at higher temperatures. On the other hand, because both Arrhenius plots for release of the (20R)- and (2OS)epimers are composed of only three temperature points, we can only speculate about their possible non-linearity. There is, of course, an alternative approach which would use a distribution of activation energies (cf. BURNHAM,1989) to analyse these kinetic data. The degradation of a single molecular type (either (20R)- or (20S)-_5cx(H),l4a(H), 17c1(H) Cz9 non-rearranged steranes), however, is a relatively simple chemical process and, in all likelihood, will involve only a few reactions with very specific activation energies. It is, therefore, doubtful as to whether the application of an activation energy distribution to this problem will reveal any new information beyond that discussed above. The release process generating non-rearranged steranes from the “bound” fraction could be a result of several different chemical bond types being cleaved. In this case an activation energy distribution could be useful. There is, however, the problem that the hydrous pyrolysis experiments employed here were isothermal and, hence, cannot adequately calibrate an activation energy distribution. The activation energies for the release process correspond to the cleavage of chemical bonds, rather than those which might be expected for the release of physically trapped steranes. This is confirmed by carrying out quantitative analyses of the possible trapped sterane content of the pre-extracted vitrinite kerogen and comparing this with the maximum

G. D. Abbott et al.

2460

yields obtained during hydrous pyrolysis. MONTHIOUX and LANDAIS( 1987) have indicated that preliminary results obtained at the lnstitut Francais du Petrole (IFP) show that Soxhlet extraction of Mahakam coals can give extraction rates 3-4 times higher than those obtained using the standard chloroform extraction procedure employed at IFP. They also showed that a Soxhlet extraction of a Mahakam coal yielded an extract which was comparable in quantity with a chloroform extract of an artificially matured coal of the same rank. They believe that the artificial maturation releases all of the physically trapped hydrocarbons due to the opening of porosity in the type Ill coal. This infers that the use of Soxhlet extraction removes all, or most, of the physically trapped hydrocarbons. The amount of “free but trapped” hydrocarbons is, thus, dependent upon the extraction procedure used. BEHAR and VANDENBROUCKE(1988) have suggested that the bitumen II fraction, using the standard IFP one-hour chloroform extraction method on an acid (HF/ HCl) treated pre-extracted coal, is composed of all the physically trapped components. Whilst the vitrinite kerogen used in this work was Soxhlet extracted with dichloromethane / methanol, which should remove all the physically trapped steranes according to MONTHIOUX and LANDAIS( 1987), a further assessment of the possible contribution to the pyrolysate from trapped steranes was made by quantitative analysis of the sterane content in the bitumen Il. This was obtained following HF/HCl acid treatment of the unheated Soxhlet extracted vitrinite. If these steranes were physically trapped then their contribution to the pyrolysate can be gauged by comparing their absolute concentrations to those of the total “expelled oil” and “bitumen” fractions of the hydrous pyrolysate. The summed (20R)- and (20S)-5a( H), 14a(H),17a( H), Cz9 sterane concentration in the bitumen II (0.010 pg/g kerogen) was at least 2 orders of magnitude less than the maximum 5a(H), 14~u(H), 17~u(H) Cz9 sterane yields in the hydrous pyrolysates (2.133 pg/g kerogen (270°C); 1.502 pg/g kerogen (300°C); 2.346 pg/g kerogen (330°C); and 2.35 1 Fg/g kerogen (350°C)). Further evidence for the release of chemically bonded steranes from kerogen during hydrous pyrolysis can be found from the work of HOERING( 1984) who used heavy water with extracted Messel kerogen and found deuterium incorporation into either the A or B rings of the sterane residues. MICHAELISet al. ( 1990) has shown that chemical degradation of an extensively extracted coal yields steranes which were chemically bonded to the organic matrix. CONCLUSIONS The hydrous pyrolysis of a vitrinite kerogen reveals the following:

1) Differences in the epimer-based

molecular maturation parameters, 2OS/(2OS + 20R)-Cz9,5~(H),14a( H), 1701(H) non-rearranged steranes, and 22S/( 22s + 22R)C3,, 17a(H),2 I@(H) hopanes appear insignificant between the “expelled oil” and “bitumen” fractions. If resistance to mass transfer is important (BURNHAM et al., 1988) then it, thus; has little or no effect on these ratios for these particular experiments. 2) Pyrolyses with an added deuterated standard indicate

that direct chiral isomerisation of the (20R)Sa(H), 14a( H), 17a( H) non-rearranged sterane in the “free” state is relatively unimportant. Similar experiments with certain Type II kerogens gave the same result. This apparent insignificance of the “isomerisation reaction” needs to be more fully explored in the sedimentary column (cf. PEAKMANet al., 1989) to confirm the initial field observations of NOBLEet al. ( 1989) and REQUEJO( 1989). 3) A kinetic model comprising consecutive release and degradation processes is able to explain quantitatively the concentration-time behaviour of the “free” (20R)5a(H),l4a(H),l7al(H) C29 sterane and its (20s) counterpart. 4) Preliminary Arrhenius parameters for the release (( 20s): AE, = 125 + 30 kJ mol-‘, A = 4.7 X lo5 s-l; (20R): AE, = 151 ? 39 k.I mol-‘, A = 2.7 X lo9 s-l); and degradation ((20s): AE, = 104 + 22 kJ mol-‘, A = 5.8 X103s-‘;(20R):AE,=87+6kJmol-‘,A=2.2X lo* s-‘) processes probably represent composite values which are consistent with a free radical chain mechanism (HOERING, 1984). 5) Kinetic evidence, as well as comparisons of absolute quantitative data between the bitumen-11 fraction of the unheated kerogen and the pyrolysate extracts, suggests that sterane release is primarily a result of chemical bond cleavage from the “bound” state. Acknowledgments-We are grateful to Mr. P. Donohoe for technical assistance with the GC-MS analyses; and Dr. D. M. Davies (Newcastle upon Tyne Polytechnic) and Dr. R. Archer (British Petroleum p.1.c.) for valuable discussions. We are also grateful to the Natural Environment Research Council (NERC) for studentships (to TIE and AKH), the Research Committee of the University of Newcastle upon Tyne and B&oil p.1.c. for a University Fellowship (to GSP). We are indebted to Ms. C. Jeans for preparation of the figures and Mrs. Y. Hall for typing the manuscript. Reviews by Lloyd R. Snowdon, Torren M. Peakman, and an unknown reviewer were helpful and appreciated. Editorial handling: J. RullkBtter REFERENCES

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