Heating rate dependency of petroleum-forming reactions:
implications for compositional kinetic predictions
V. Dieckmann
a,b,*, B. Hors®eld
a, H.J. Schenk
a aInstitut fuÈr ErdoÈl and Organische Geochemie (ICG-4), Forschungszentrum Juelich GmbH,D-52425, JuÈlich, Germany
bENI div. AGIP Spa, 20097 San Donato Milanese, Milan, Italy
Abstract
The generation of bulk petroleum, liquid and gaseous hydrocarbons from the Duvernay Formation was simulated by heating immature kerogens in a closed system (MSSV pyrolysis) at four dierent heating rates (0.013, 0.1, 0.7 and 5.0 K/min). Using the established parallel reaction kinetic model, temperature and compositional predictions were tested to be suitable for geological conditions by comparing the laboratory results with natural changes in source bitumens and reservoir oil maturity sequences from the Duvernay Formation. In the case of bulk liquid and gaseous hydrocarbons, the above kinetic calculations can be considered valid because their maximum yields are independent of laboratory heating rates. In contrast, the contents of parans, aromatics and sulfur compounds show a pronounced heating rate dependence. Extrapolated to geological heating rates, the compositional predictions are consistent with the bulk composition of natural products in the Duvernay-petroleum system showing an increase of paranicity and hydrogen content. In contrast to that, the ``hump'' decreases with decreasing heating rate, a trend which is con®rmed by the low amounts of unresolved compounds in natural high maturity products. Because of these heating-rate dependent compositional changes, geological predictions of natural molecular composition by the commonly used kinetic models are not suitable.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Duvernay Formation; Bulk kinetics; Compositional kinetics; Aromaticity; Heating-rate dependency
1. Introduction
Diagenesis, catagenesis and metagenesis are the three consecutive alteration stages of the carbon cycle; irre-versibly changing the composition of sedimentary organic matter. Petroleum formation takes place mainly in the second and third stages, leading to the formation of oil/gas and gas, respectively (for review, see Hors®eld and RullkoÈtter, 1994). While petroleum-forming reac-tions are exceedingly complex and unknown in detail, it is clear that cracking, aromatisation and condensation are all involved in maturation processes leading to the formation of an organic hydrogen-rich volatile fraction which may then migrate from the source rock and the organic hydrogen-poor solid residues which remain behind. Pressure exerts a strong in¯uence on the prop-erties of the solid residues, as exempli®ed by the
bire-¯ectance of vitrinite (Carr and Williamson, 1990), but as far as the actual generation of petroleum from kerogen is concerned, temperature is generally considered to be the main driving force (Philippi, 1965; Louis and Tissot, 1967; Burnham and Singleton, 1983; BeÂhar and Van-denbroucke, 1996; Michels et al., 1995; cf. Price and Wenger, 1992).
Because temperature is the main driving force of pet-roleum generation, laboratory pyrolysis is used routi-nely to simulate the process. For instance, Rock-Eval S2 and Hydrogen Index values predict petroleum yields as a function of kerogen type and maturity (Larter, 1984; Pelet, 1985; Cooles et al., 1986; Espitalie et al., 1988) and pyrolysis-gas chromatography can provide infor-mation on petroleum type (for review, see Hors®eld, 1997). Similarly, and especially pertinent to the present article, the kinetics of product generation calculated for kerogen pyrolysis are used to predict the rate and timing of petroleum generation in sedimentary basins (Tissot, 1969; Tissot et al., 1971; Burnham et al., 1987; Ungerer
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* Corresponding author.
and Pelet, 1987; Espitalie et al., 1988; Braun and Burn-ham, 1990; Schaefer et al., 1990; Ungerer, 1990; BeÂhar et al., 1992; Schenk and Hors®eld, 1993; Dieckmann et al., 1998). The underlying assumption in all of these methods is that the reactions occurring in nature are closely similar to those brought about by pyrolysis, despite at least 10 orders of magnitude dierence in heating rate. Criticisms of this assumption have been voiced for speci®c cases by Lewan et al. (1995), Muscio and Hors®eld (1996) and Schenk and Hors®eld (1998), because the role of cracking can be overemphasised relative to aromatisation and condensation in labora-tory (anhydrous) pyrolysis. Nevertheless, it remains a fact that kinetic modelling is a routine part of petroleum exploration and that laboratory predictions are good, given the uncertainties in kerogen type distribution and temperature histories in natural settings (Quigley et al. 1987; Ungerer and Pelet, 1987; Braun and Burnham, 1990; Burnham et al., 1995; Pepper and Corvi, 1995; Schenk and Hors®eld, 1998).
The current contribution is concerned with the reac-tion kinetics of petroleum formareac-tion at the molecular level (compositional kinetics). Most kinetic models assume a ®xed number of parallel reactions (or Gaus-sian distribution), each of which is assumed to be of ®rst-order (e.g. Tissot, 1969). Predicting petroleum compositions, rather than bulk yields, is important because the initial gas±oil ratio of expelled petroleum strongly in¯uences the phase state (unsaturated oil, gas-saturated oil with gas cap, gas with dissolved con-densate) and hence the volume of that petroleum as it migrates through carrier systems as pressure and tem-perature gradually fall. Espitalie et al. (1988) used an open pyrolysis system with selective trapping to deter-mine the kinetic parameters of primary (direct from kerogen) C2ÿ5, C6ÿ14 and C15+ products from type II
and type III kerogen from the Viking Graben. Closed system experiments have since been employed to simu-late both primary and secondary (e.g. oil -> gas) cracking reactions (Braun and Burnham, 1990; Ungerer 1990; BeÂhar et al., 1992, 1995; Hors®eld et al., 1992; Schenk and Hors®eld, 1993; Schenk et al., 1997; Dieck-mann et al., 1998). Using open and closed system pyr-olysis con®gurations, BeÂhar et al. (1997) and BeÂhar and Vandenbroucke (1996) have applied the kinetic concept of the parallel reaction model to dierent hydrocarbon fractions. Finally, Tang and Stauer (1995) and Tang and BeÂhar (1995) used exclusively single compounds for their kinetic evaluations; ®nding out that these kinetics and resulting temperature predictions are closely similar to those from bulk kinetics. Compositional kinetic pre-dictions are dicult to validate because the success of the prediction can only be assessed by studying product distributions, unlike bulk kinetic predictions which can be tested using residues. Their value therefore remains uncertain.
The aim of this paper is to consider predictions of pet-roleum composition with reference to reaction mechan-isms in nature versus the laboratory, and the utility of simple parallel reaction models. The eect of heating rate is a notable issue because, to be valid, reaction models should not be rate-dependent, and yet earlier works from Van Heek (1982) on coals, Burnham and Happe (1984) and Burnham and Singleton (1983) on oil shales and Hors®eld (1997) on petroleum source rocks, have demonstrated that the observed yields of alkanes, aro-matics and NSO compounds are strongly heating-rate dependent. However, total yields, at least in the case of marine source rocks and oils, are not heating-rate dependent (Schenk and Hors®eld, 1993; Dieckmann et al., 1998). Here, we report the results of simulation experiments under open and closed system pyrolysis conditions on samples from the Duvernay Formation, Western Canada Basin. A natural sample set from the Duvernay Formation (reservoired oils and source rock samples) was used to calibrate the compositional evolu-tion of arti®cially generated products in order to show if respective compositional kinetic evaluation is realistic.
2. Materials and methods
2.1. Sample origin
The upper Devonian Duvernay Formation is one of the most proli®c marine source rocks in the Western Canada Sedimentary Basin (WCSB). It contains pri-marily type II kerogen and is thought to be the source of oil and gas reservoirs in the Leduc and Nisku reservoirs along the Rimbey-Meadowbrook reef trend in central Alberta (Creaney and Allan, 1990) (Fig. 1). The main hydrocarbon formation phase from the Duvernay For-mation is thought to be linked to a subsidence phase associated to the Laramide orogeny. According to Stoaks and Creaney (1984, 1985) and a more recent study of Chow et al. (1995) two basic lithofacies units can be recognised within in the Duvernay Formation; Litho-facies 1, dominated by nodular to nodular-banded lime mudstones, exhibits varying degrees of bioturbation and was deposited under relatively oxygenated (dysoxic) conditions.Lithofacies 2is consisted of laminated lime mudstones (up to 14% TOC) and was deposited in deep water oxygen-starved euxinic conditions. Li et al. (1998) have illustrated that these lithofacies are manifested in composition of the Leduc oils.
high maturity gas/condensates (Fig. 2). The source rock sample set selected for this study covers the complete maturity pro®le (Table 1), as does the suite of reser-voired oils with API gravities in the range 36 to 52
(Table 2). From a molecular point of view, this broad maturity range is also re¯ected by the MPT±2-ratio (Radke et al., 1980) which shows a clear increase up to an API-gravity of 46 and then decreases again in the higher maturity oils (Dieckmann, 1998). The crude oils were reported not to be aected by migration phenom-ena (Li, unpublished).
3. Experimental approach
After solvent extraction using ¯ow-blending (dichlor-omethane, 15 min; Radke et al., 1978), each source rock sample from the maturity sequence was pyrolysed under open-system conditions and the kinetic parameters of bulk petroleum formation were calculated from the rate curves (Schaefer et al., 1990). After normalisation to initial carbon (immature stage) (Pelet, 1985; Schenk and Hors®eld, 1998), the rate curves were overlain in order to ascertain whether facies variations were present and to verify that the main reaction pathway was that of cracking rather than aromatisation/condensation (Schenk
and Hors®eld, 1998). This also allowed the most suitable immature sample to be selected for closed system pyr-olysis experiments. The kinetic parameters of primary and secondary gas generation were calculated from the closed system pyrolysis data. Additionally, detailed compositional data from the experiments were com-pared with thermovaporisation data from the natural source rock maturity sequence and with whole oil gas chromatograms. Experimental details are given below.
3.1. Open system bulk ¯ow pyrolysis±FID
100 mg of each ®nely ground sample were pyrolysed at heating rates of 0.1, 0.7 and 5.0 K/min using a fur-nace set up described by Schaefer et al. (1990). Temperatures were measured by a thermocouple located immediately above the sample. A constant ¯ow of argon (45 ml/min) was maintained in order to transport all pyrolysis products to the ¯ame ionisation detector for the continuous registration of bulk formation rates.
3.2. Closed system (MSSV) pyrolysis±gas chromatography
The principles of microscale sealed vessel (MSSV) pyrolysis have been described by Hors®eld et al. (1989).
For the current study, 90 aliquots of immature kerogen concentrate (isolated according to the method of Dur-and, 1980) were sealed in glass capillary tubes and pyr-olysed in batches to selected temperatures at three heating rates (0.1, 0.7 and 5.0 K/min) within a massive cylindrical brass block which served as sample holder. For a selected number of analyses, lower heating rate (0.013 K/min) was also used. Temperatures were mea-sured with a thermocouple located in the brass block where the temperature errors during the heating pro-grams were <0.1C. To obtain an accurate database
involving enough samples for the primary formation as well as for the secondary cracking temperatures, the ®nal temperatures in this study were in the range of 540±610C. The composition of oil and gas formed in
each tube was determined by a single-step on-line gas-chromatographic analysis. An HP-1 column (1.65 mm
®lm thickness, 25 m0.31 mm i.d.) connected to a ¯ame ionisation detector was used, employing helium as car-rier gas. The GC oven was programmed fromÿ10C (2
min isothermal) to 320C at 8C/min. The major
indi-vidual components were quanti®ed (n-alkanes, aromatic hydrocarbons and thiophenes), as were boiling ranges
(C1ÿ5, C6ÿ14, C15+). The latter included both resolved and
unresolved components in the respective ranges. Aromati-city was calculated as the ratio of the sum of benzene, toluene, xylenes, ethylbenzene, naphthalene, methyl-naphthalenes and dimethylmethyl-naphthalenes to the sum of the C6+n-alkanes. The quanti®cation of the products was performed usingn-butane as external standard.
3.3. Kinetic model
The mathematical model has been described by Schaefer et al. (1990). It is based on the kinetic analysis of formation rate dM=dT vs. temperature (T) curves assuming twenty ®ve ®rst order parallel reactions with activation energies Ei regularly spaced between 46
(192.7) and 70 (293.3) kcal/mol (kJ/mol) and a single pre-exponential factor A. A total number of 26 para-meters, namely 25 potentials (partial yields) associated with 25 activation energies and the pre-exponential fac-tor A were optimised by a least squares iteration method that compares measured and calculated forma-tion rates at 600 temperatures (200 per heating rate) until the corresponding error function (sum of squared dierences) presented a well-de®ned absolute minimum. In the case of open system pyrolysis, the curves were measured directly. As described by Hors®eld et al. (1992), rate curves for MSSV data were generated by placing a spline through measured cumulative data points and then dierentiating the curve.
3.4. Thermovaporisation±gas chromatography
The gas chromatographic analysis of freely occurring volatile organic matter in the source rocks and crude
Table 1
List of Duvernay rock samples and related rock TOC and Rock-Eval data used in this study in order to establish a nat-ural source rock maturity sequencea
Well Sample Tmax HI TOC MPI 2 GOR
RED-2 42714 414 543 6.24 0.23 0.05 TOM 42721 415 520 6.09 0.11 0.06 42725 428 599 7.56 0.92 0.1
LE 42728 433 578 7.91 0.67 0.03
42731 436 575 4.47 0.8 0.05 42732 436 580 5.14 0.82 0.07
NOR 42735 464 37 3.56 1.88 0.02
42736 465 40 4.36 1.82 0.01 FOB 42738 442 385 9.71 0.71 0.02 42739 443 419 10.3 0.72 0.04 42740 442 364 10.7 0.77 0.01 42742 436 292 1.65 0.76 0.02 IMP3 42748 427 547 5.05 0.83 0.08 IMK 42765 423 527 5.01 0.49 0.04 FEB 42768 436 271 4.31 0.86 0.01 42769 437 273 4.02 7.2 0.01 42771 437 304 4.88 0.78 0.01
IMC 42777 771 303 4.19 1.5 0.05
42779 450 100 3.44 1.4 0.01
SAP 42786 418 551 6.9 0.7 0.03
42787 419 517 7.33 0.91 0.04 42790 420 604 8.58 0.39 0.03 BARR 42797 548 2.46 5.63 0.22 0.02
BI 42802 425 609 10.9 0.39 0.11
CAM 42807 430 450 2.25 0.62 0.02 42810 424 645 10.7 0.72 0.13 42811 425 611 4.84 1.09 0.06
a The locations of related wells are given in Fig. 1.
Table 2
List of reservoired oil samples used in this study in order to establish a natural maturity sequence of reservoired oils gener-ated from the Duvernay Formationa
Sample Well API MPI 2
East-shale basin
oils was performed without prior liquid chromato-graphic separation by means of the MSSV pyrolysis instrument described above. Rocks were only coarsely crushed to minimise evaporative loss prior to sealing in the capillary tubes. Crude oils were injected onto pre-cleaned quartz wool within the tubes and the tubes then sealed (Schenk et al., 1997). Volatile materials were released from the tubes by cracking them open at 300C,
and analysed under the same conditions as described above for MSSV pyrolysis.
4. Results and discussion
4.1. The natural maturity sequence
In an earlier study (Schenk and Hors®eld, 1998), we showed that petroleum generation from the Posidonia Shale both in nature and during simulated maturation resulted mainly from cracking reactions, and therefore that petroleum generation over geological time could be reliably extrapolated from pyrolysis of the appropriate immature sample. By contrast, signi®cant deviations were observed between the natural and arti®cial coali®cation series of vitrains, inferring that petroleum generation could not be reliably extrapolated from open-system pyrolysis of the appropriate immature sample. The main criterion for distinguishing these dierent behaviours was the temperature envelope of the matured samples relative to that of the least mature
sample, cracking being characterised by curves within the original envelope and aromatisation being char-acterised by curves extending beyond it. It was with this in mind that the natural Duvernay series was evaluated in order to establish whether laboratory pyrolysis was indeed the best way of determining kinetic parameters for petroleum generation.
As described above, three heating rates were employed for bulk kinetic analysis of the Duvernay maturity sequence. Fig. 3 shows the bulk hydrocarbon formation-rate curves (mg/g*K) for 0.1 K/min experi-ment after normalisation to initial carbon (Schenk and Hors®eld, 1998). The well abbreviations and Rock-Eval
Tmaxvalues are from Table 1. It is clearly seen that the
curve maxima are shifted to higher temperatures with increasing maturity. This is thought to be a result of the ongoing elimination of the labile parts of the kerogen structure. The end-temperature of generation is about the same for all samples and there is no major shift of hydrocarbon formation curves to temperatures exceed-ing the envelope de®ned by the least mature sample.
We conclude from the foregoing that the main pro-cesses leading to the formation of hydrocarbons from the Duvernay Formation in nature are characterized by thermal cracking of chemical bonds in the kerogen structure, rather than aromatisation and condensation. This means that geochemical data for this sample series can be directly compared with the results of simulated maturation, a process considered to proceed via mainly cracking reactions.
Sample LED is slightly anomalous in that its curve extends beyond the others, and is possibly attributable to a more stable kerogen type. However, this is rela-tively minor, especially when compared with the big shifts displayed by Carboniferous coal with increasing rank (Schenk and Hors®eld, 1998).
Fig. 4 illustrates the activation energy distribution and respective frequency factors calculated by the kinetic model. According to this ®gure, both frequency factors and dominant activation energies increase with increasing maturity of the samples. In each case the single frequency factor Aof the kinetic model will be some kind of weighted average of the high and low fre-quency factors associated with high and low activation energies (Burnham et al., 1995). This model frequency factor must increase along a maturation sequence because the samples become more and more depleted in pyrolytic low-energy potentials as a consequence of natural generation. In going from the least mature (SAP) to the most mature Duvernay sample (IMC b), the value ofAincreases by about 60 times which is more pronounced than the increase found for Toarcian Shales in the maturity range of 0.48 to 1.44% Rr (Schenk and Hors®eld, 1998). The increase of the average frequency factor implies that low-energy potentials do not only decrease in the course of natural maturation (Tissot et al., 1987), but that they are shifted to higher activation energies, because the relationship between A and the activation energies andTmaxtemperatures of individual
reactions must be maintained,
E individ:; mature=lnA mature ÿlnr
E individ:; immature=lnA immature ÿlnr
with ln r0 for laboratory heating ratesr. This eect generally enhances the maturity induced shift of
poten-tial versus activation energy distributions (Schenk and Hors®eld, 1998).
4.2. The simulated maturity sequence Ð bulk compositional predictions
Fig. 5a illustrates the cumulative evolution (mg/g TOC) of C1ÿ5, C6ÿ14 and C15+ boiling ranges during
programmed-temperature closed-system (MSSV) pyr-olysis of the immature Duvernay Formation sample at heating rates of 0.1, 0.7 and 5.0 K/min. In accordance with the theory of non-isothermal kinetics, all evolution pro®les are shifted towards higher temperatures with increasing rate of heating. The apices of the cumulative C6ÿ14- and C15+-evolution curves mark the
tempera-tures where degradation processes exceed primary for-mation processes.
Fig. 5b shows the cumulative formation of C6+and
C1ÿ5fraction at three laboratory heating rates (0.1, 0.7
and 5.0 K/min). The secondary formation of gas from oil to gas cracking reactions in the Duvernay Formation previously was shown to start when the cumulative for-mation of C6+ compounds comes to an end (Dieck-mann et al., 2000).
The amount of secondary gas can therefore be calcu-lated from the fall in yield below the maximum observed value multiplied by a stoichiometric constant (Dieck-mann et al., 1998; also see Ungerer, 1990; Pepper and Dodd, 1995). The result is shown in Fig. 6. Also shown is the yield of primary gas, generated directly from the kerogen structure, calculated by subtracting the yield of secondary gas from total gas.
Typically for marine source rocks, the pyrolysates were dominated by the C6+ fraction, which make up to 160 mg/g TOC. With 120 mg/g TOC, the primary gas formed directly from the kerogen dominates the total gas composition at higher maturities, while 90 mg/g TOC of the generated gas can be related to the cracking of previously generated oil. Importantly, all boiling range yields are the same, irrespective of heating rate, a prerequisite for kinetic modelling.
Two issues arise from the foregoing results, namely the yields from closed system pyrolysis, and distin-guishing primary from secondary reaction products for kinetic modelling.
Firstly regarding yields, it is noteworthy that the cumulative sum of all boiling ranges (C1ÿ5+C6ÿ14
+C15+=C1+) amounts to about 200 mg/g TOC at all
heating rates for this sample (SAP). This total closed-system yield is considerably lower than the correspond-ing open-system Hydrogen Index of that sample (ca. 600 mg/g TOC; Table 1). Lower yields from the MSSV con®guration are not caused simply by dierences in mechanisms of open- versus closed-system pyrolysis. Indeed, bulk yields from both the open- and closed-system pyrolysis of Posidonia Shale and Duvernay
Formation kerogens under the same analytical con®g-uration with GC-interface are identical (Dieckmann et al., 1998, 2000). Rather, the dierences in yield appear due to very heavy pyrolysis products condensing in the GC-interface (irrespective of whether the experiment was performed under open- or closed-system condi-tions) whereas they simply pass through (either intact or are cracked to smaller molecules) the high temperature (550C) split of the Rock-Eval and are detected by FID.
It is obviously important to know what eect con-densation has on kinetic parameters. For instance, pro-duct rate curves for pyrolysis systems employing a GC-interface (in this case, MSSV pyrolysates) should be skewed to higher mean temperatures if early-formed products were particularly enriched in materials of high
molecular weight that readily condense in the GC-interface. While this phenomenon cannot be ruled out for all samples, it does not apply to the Posidonia Shale. For this source rock, we have already shown that the kinetic parameters of C1+ product generation (MSSV
pyrolysis) and total pyrolysate generation (bulk-¯ow open-system pyrolysis) are identical (Schenk and Hors-®eld, 1993). It does not seem to apply to the Duvernay Formation either. While the kinetic parameters for C1+
generation (MSSV) have not been calculated for the analysed Duvernay Formation sample, we note that predictions of C6+product generation from the kinetic
model are closely similar to those for bulk-¯ow pyr-olysis (see below). These cases strongly argue that the proportion of heavy condensable pyrolysate is constant
for each of the pseudo-reactions of kerogen pyrolysis, and that kinetic predictions from the MSSV method are valid. Secondly, the simplistic dierentiation of primary and secondary reaction products needs justifying. In Fig. 5a, total gas yields converge to a plateau at higher tem-peratures and remain there because any cracking of wet gas components is compensated by the enhanced gen-eration of methane. By contrast, the concentrations of all liquid compounds ®rstly increase and then decrease as a consequence of secondary cracking which over-compensates primary generation at higher temperatures. From the evolution pro®les of total C6+ compounds
(=C6-14+C15+; Fig. 5b) the predominance of
second-ary oil cracking over primsecond-ary oil generating reactions becomes obvious at temperatures exceeding 405C (0.1
K/min), 430C (0.7 K/min) and 460C (5.0 K/min). It is
quite impossible to deconvolute a measured generation curve into primary and secondary curves in a reliable manner. For the purpose of kinetic modelling we have therefore assumed that the overlap between primary and secondary reactions can be neglected, the latter operat-ing only when the former have come to an end. The validity of this fundamental assumption has already been veri®ed for the Duvernay Formation by comparing C6+evolution under closed (MSSV) and open (Py-GC)
conditions (Dieckmann et al., 2000), using the same
analytical con®guration and employing a GC-interface. Consequently, yields of secondary gas could be calcu-lated from the decrease of C6+concentrations
accord-ing to the procedure outlined by Dieckmann et al., (1998). Furthermore, because open- and closed-system C1+yields from these experiments are essentially
iden-tical throughout the entire range of pyrolysis tempera-tures (350±540C), all primary and secondary sources of
measured gas yields have been taken into account. The resulting activation energy distributions and the individual frequency factors are shown in Fig. 7. Here it can be seen that there is a systematic shift in activation energy distribution when going from the parameters
evaluated for the oil (C6+) (Emain=55 kcal/mol) to the
primary gas (C1ÿ5prim.)(Emain=57 kcal/mol) and
sec-ondary gas (C1ÿ5 sec.) (Emain=60 kcal/mol), while the
frequency factor did not change signi®cantly (9.2E+15
ÿ6.2E+16 1/min).
By using these kinetic parameters, the temperature of C6+, C1ÿ5 prim. and C1ÿ5 sec. were calculated for a
geological heating rates of 1 K/my. As shown in Fig. 8, the oil generation begins at 80C (T
max 140C). The
onset of gas generation was predicted to take place at 100C (T
max165C) and is clearly related to the primary
formation of gas from the source rock kerogen. These predictions are in accordance with the kinetic studies of Braun et al. (1991) and Espitalie et al. (1988) on marine clastic type II source rocks, both coming out with a temperature range for primary gas generation in a geological interval between 110 and 200C. In
con-trast, Pepper and Corvi (1995) predicted the main for-mation of primary gas for much higher temperatures in the range of 140±210C. In their study primary gas
for-mation took place parallel to the forfor-mation of second-ary gas. However the author pointed out that their gas generation kinetics are based on a poor raw data-set and that they have especially a low con®dence in their primary gas generation kinetics and resulting predictions.
The signi®cant generation of secondary gas from the cracking of unexpelled oil in the Duvernay Formation was predicted to set in at around 150C (T
max=180C),
while the end of secondary gas formation is predicted at 250C. These results con®rm the previous predictions of
Dieckmann et al. (1998) and Pepper and Dodd (1995) for Toarcian Shales (type II kerogens), that the oil to gas cracking processes start earlier in source rocks than in reservoirs.
4.3. The simulated maturity sequence Ð compositional predictions at the molecular level
The MSSV pyrolysis technique allows the generation of individual compounds in the C1±C30 range to be
measured. In principle, any or all of these data may be modelled to predict bulk petroleum compositions in nature. This section considers the n-alkanes and aromatic hydrocarbons because they make up a high percentage of natural crude oils in general, and the unresolved complex mixture (UCM) because it makes up a consistently high proportion of pyrolysis products (Hors®eld, 1997).
The summed yields ofn-alkanes in the range C6ÿ14are
shown as a function of pyrolysis temperature in Fig. 9. The displacement of curves to higher temperatures occurs with increasing heating rate, as expected, but, importantly, the maximum yield is heating rate depen-dent, with highest yield occurring for the slowest heating rate. Yields of compounds contained in the unresolved
complex mixture are Ð to a lesser extent Ð also heating rate dependent, but in this case there is a decrease in the maximum yield as heating rate is lowered.
Extrapolated to even slower heating rates, beyond the practicalities of laboratory measurement into rates that are typical of sedimentary basins (e.g. 5.3 K/Ma, which is equivalent to about 110ÿ11K/min), it can be
antici-pated that maximumn-alkane yields would be even higher and UCM yields even lower than suggested by direct laboratory measurements. This phenomenon has also been reported earlier forn-alkanes and alkylthiophenes in pyr-olysates of the Posidonia Shale (Hors®eld, 1997).
``Aromaticity'' is considered in Fig. 10. Shown on the right-hand side is aromaticity as a function of tempera-ture for dierent heating rates. In addition to the usual three heating rates (0.1, 0.7 and 5.0 K/min), note that a very slow experiment was also performed to extend the database to the limits of what is practicable in labora-tory (0.013 K/min). For any given heating rate, aroma-ticity starts o high at low temperatures, decreases during intense n-alkane generation and then increases again as n-alkanes are cracked to gas at higher tem-peratures. Absolute aromaticity at the in¯ection point decreases as heating rate falls. The left-hand side of the ®gure presents aromaticity data for naturally occurring volatile compounds in the Duvernay Formation source rocks as a function ofTmax. It can be seen that the
nat-ural compositional changes with maturity seem to be comparable to the changes in the pyrolysates in that aromaticity ®rst decreases then increases. The minimum values of natural aromaticity evolution are lower than corresponding values measured in the MSSV-products. While expulsion eects may also have in¯uenced the natural system (Sandvik et al., 1992) it is considered beyond coincidence that the minimum value ®ts with the aforementioned trends extrapolated from the heating experiments.
Minimum values (in¯ection points) of the aromati-city-trends from the heating experiments and the natural source rock bitumen (Fig. 10) have been highlighted and plotted separately on a semi-logarithmic plot in Fig. 11. The logarithmic scale extends from laboratory heating rates on the right hand side to geological heating rates on the left hand side. Furthermore, the numerical value 110ÿ11 K/min for the geological heating rate is
gen-eralised for presentation purposes. The minimum aromaticities of the natural bitumens, which correspond to the infection area of the natural sample set are shown as a range rather than a single value in Fig. 11 because of the data scatter at this maturity level, as shown in Fig. 10. A logarithmic regression was applied on the labora-tory in¯ection points from aromaticity trends shown in Fig. 10, in order to extrapolate the heating rate depen-dent compositional changes in the in¯ection-points to natural heating conditions of 110ÿ11 K/min. The
results of this prediction purpose are shown in Fig. 11,
where the aromaticity values at the in¯ection point of the natural trend (from Fig. 10) coincides with the pre-dictions by extrapolated laboratory trend of in¯ection points (Fig. 11).
Heating-rate dependent changes in the aromaticity of arti®cially generated products have been reported earlier by Stout et al. (1976); Maciel et al. (1979); Fausett and Mikinis (1981), Campell et al. (1978), Rubel et al. (1983) and Burnham and Singleton (1983). However, it has to be pointed that high amount of sample material as well
as the generally open system used in these studies did not allow a clear separation between generation and expulsion processes. Due to the experimental con®gura-tion of MSSV-pyrolysis and the extremely small amounts of sample material envolved during the heating and GC-online detection of generated products, expul-sion eects can be completely excluded here (Hors®eld et al., 1989). Only the transformation of kerogen to hydrocarbons can be seen to be responsible for the observation made in Figs. 10 and 11.
Fig. 8. Formation rate (mg per g of kerogen concentrate and per degree) vs. temperature (C) of liquid hydrocarbons (C6+), total gas
(C1ÿ5tot.), primary gas (C1ÿ5prim.) and secondary gas (C1ÿ5sec.) for the slowest laboratory heating rate of 0.1 K/min and an average geological heating rate of 1 K/ma.
Fig. 9. Cumulative formation (mg/g TOC) vs. temperature (C) of then-C6
Keeping in mind that the H/C ratio of aromatic structures are lower than the respective ratios in alipha-tic hydrocarbons, the ®ndings from Fig. 11 indicate a hydrogen enrichment of the generated hydrocarbons with decreasing heating rate. This is in agreement with the general higher H/C ratio observed in reservoired oils from the Duvernay formation of corresponding matur-ity (Dieckmann, 1999).
A signi®cant heating-rate dependency of maximum yield can also be seen for the unresolved compound mixture of arti®cial products. In Fig. 12, this heating rate dependent trend of the maximum yields of the hump is shown relative to the corresponding changes of maximum yields ofn-alkanes in the C6+ fraction. Hereby, it can be seen that the relative amount of the unresolved part in MSSV pyrolysis gas chromatograms systematically decrease with decreasing heating rate while the max-imum amounts ofn-alkanes increase with lower heating rates, which leads to a crossover when extrapolating these compositional changes to geological heating con-ditions. Natural crude oils from the Leduc reservoirs provide a means for con®rming these extrapolations. The oils exhibit a broad span of totaln-alkane and total unresolved components in accordance with a wide maturity range, the contribution of n-alkanes being highest for the most mature crude oils. It is the highest mature oils that are directly comparable with the
experimentally determined n-alkane maximum because the latter feature is equivalent to a vitrinite re¯ectance of 1.2%Roaccording to EasyRopredictions (Sweeney
and Burnham, 1990). Interestingly this crossover is con®rmed.
The hydrogen-poor character of arti®cially generated products with respect to reservoired oils is a well known phenomenon and was discussed earlier by Hors®eld (1997) by comparing the composition of pyrolysates and natural oils. Hereby, the arti®cially generated products are consistently higher in polar and aromatic com-pounds, while reservoired oils are dominated by hydro-gen-rich compounds (saturates). The systematic increase of hydrogen enriched compounds with lower heating rates can be interpreted as an eect of the higher avail-ability of free hydrogen during the transformation of kerogen to hydrocarbons at lower heating rates. Both the natural and arti®cial formation of hydrocarbons are mainly controlled by free radical formation processes as a result of the thermal cracking of respective precursor molecules in the source rock kerogen. In nature, where free hydrogen is available in large amounts, most of the C bonds cracked during hydrocarbon generation are ``healed'' by recombination with hydrogen. This feature was illustrated by Patience et al. (1992), who balanced quantitatively the number of free radicals and free hydrogen generated under natural conditions. Schenk et
al. (1989) illustrated for laboratory heating rates (0.1 and 2.0 K/min) that the polycondensation processes obviously are set in after the main phase of hydrocarbon formation. Because aromatisation and polycondensa-tion processes are thought to be the main source for hydrogen, these late polycondensation processes under laboratory conditions might be responsible for the hydrogen de®ciency during the main phase of hydro-carbon formation. The need of free radicals to recom-bine with each other, if not enough hydrogen is available leads to the formation of branched, more complex molecules (Lewan, 1994). Probably, these types of molecules are not resolvable in the gas chromato-graph and are detected as part of the unresovable hump. Van Heek (1982) has reported that the quantity of free hydrogen increases signi®cantly during laboratory pyrolysis experiments with lower heating rates. Espe-cially he noticed that the amount of hydrogen increases by 80% when decreasing the heating rate from 35 to 3 K/min.
Although these experiments were performed on coal samples, these observation together with the changes of H/C ratios in the Duvernay pyrolysates underline the importance of hydrogen availability during closed sys-tem pyrolysis.
Based on the results of the present study, it is obvious that future kinetic studies dealing with the prediction of natural products must consider the heating-rate dependent changes of hydrocarbon forma-tion.
One possibility could be to develop more complex kinetic models, which consider the heating rate depen-dent compositional dierences between fast and high temperature and slow and low temperature heating conditions. Of course this can be seen as a big challenge for further studies dealing with heating-rate dependent compositional changes of hydrocarbons generated from dierent type of source rocks.
An alternative approach was shown in the recent study. Hereby, the timing predictions were performed by bulk gas and oil kinetic evaluation, while more detailed compositional predictions can be performed by the application of a logarithmic regression as a simpli-®ed mathematic link between laboratory and natural compositional changes. Although this approach was illustrated only for a restricted maturity window in the present work, the novel approach of extrapolating the laboratory compositional changes with the real natural compositional ®ndings clearly illustrate, that the con-cept is valid.
5. Conclusions
. For the Duvernay Formation the parallel reaction
kinetic model can be applied to bulk hydrocarbon formation and the formation of boiling ranges for GOR prediction because the total quantities are independent of the heating rate.
. For a geological heating rate of 1 K/ma the
for-mation of oil was predicted to take place between 80 and 160C (T
max140). Primary gas formation
directly from the Duvernay kerogen was predicted in a temperature interval between 100 and 230C
(Tmax160C).
. The cracking of oil to secondary gas in the
Duvernay source rock starts at 150C and is
completed at 250C (T
max190).
. On a molecular level, a clear heating-rate
depen-dency can be observed leading to the enrichment ofn-alkanes with decreasing heating rate. Because of this heating-rate dependency the commonly used ®rst order parallel reaction model is not sui-table, because single compounds show heating rate dependency.
. This heating-rate dependent compositional
chan-ges of laboratory products could be con®rmed by
natural occurring source rock bitumens and crude oils.
. A pragmatic way of overcoming this heating-rate
dependency is to make timing predictions using bulk kinetics, and then infer compositions by semi-logarithmic extrapolations.
Acknowledgements
This work was carried out as part of V.D.'s PhD at the Forschungszentrum Juelich (Germany). We wish to thank Anne Richter and Franz Leistner for their technical assistance in the pyrolysis experiments. This work also has signi®cantly bene®ted from the improve-ments and suggestions by Dr. Chris Clayton and an anonymous reviewer.
References
BeÂhar, F., Kressmann, J.L., Rudkiewicz, J.L., Vandenbroucke, M., 1992. Experimental simulation in a con®ned system and kinetic modelling of kerogen and oil cracking. Organic Geo-chemistry 19, 173±189.
BeÂhar, F., Vandenbroucke, M., Teermann, S.C., Hatcher, P.G., Leblond, C., Lerat, O., 1995. Experimental simulation of gas generation from coals and marine kerogen. Chemical Geol-ogy 126, 247±260.
BeÂhar, F., Vandenbroucke, M., Tang, Y., Marquis, F., EspitalieÂ, J., 1997. Thermal cracking of kerogen in open and closed systems: determination of kinetic parameters and stoichio-metric coecients for oil and gas generation. Organic Geo-chemistry 26, 321±339.
Braun, R.L., Burnham, A.K., 1990. Mathematic model of oil generation, degradation and expulsion. Energy and Fuels 1, 153±161.
Braun, R.L., Burnham, A.K., Reynholds, I.G., Clarkson, J., 1991. Pyrolysis kinetics for lacustrine and marine sources by programmed micropyrolysis. Energy and Fuels 5, 192±204. Burnham, A.K., Happe, J.A., 1984. On the mechanics of
kero-gen pyrolysis. Fuel 63, 1353±1356.
Burnham, A.K., Singleton, M.F., 1983. High pressure pyrolysis of Green River oil shale. In: Miknis, F.P., McKay, J.F. (Eds.), Geochemistry and Chemistry of Oil Shales. Am. Chem. Soc. Symposium Series 2, pp. 335±351.
Burnham, A.K., Braun, R.L., Gregg, H.R., 1987. Comparison of methods for measuring kerogen pyrolysis rates and ®tting kinetic parameters. Energy and Fuels 1, 452±458.
Burnham, A.K., Schmidt, B.J., Braun, R.L., 1995. A test of the parallel reaction model using kinetic measurements on hydrous pyrolysis residue. Organic Geochemistry 23, 931±939. Campbell, J.H., Koskinas, G.H., Stout, N.D., Coburn, T.T., 1978. Oil shale retorting: eects of particle size and heating rate on oil evolution and intraparticle oil degradation. In Situ 2, 1±47.
Carr, A.D., Williamson, J.E., 1990. The relationship between aromaticity, vitrinite re¯ectance and maceral composition in coals: Implications for the use of vitrinite re¯ectance as a maturation parameter. In: Durand, B., BeÂhar, F. Advances in Organic Geochemistry, Pergamon Oxford, pp. 313±323.
Chow, N., Wendte, J., Stasiuk, L.D., 1995. Productivity versus preservation controls on two organic-rich carbonate facies in the Devonian of Alberta: sedimentological and organic pet-rological evidence. Bull. Ca. Petr. Geol. 43, 433±460. Cooles, G.P., Mackenzie, A.S., Quigley, T.M., 1986.
Calcula-tion of petroleum masses generated and expelled from source rocks. In: Leythaeuser, D., RullkoÈtter, J. (Eds.), Advances in Organic Geochemistry. Pergamon, Oxford, pp. 235±245. Creaney, S., Allan, J., 1990. Hydrocarbon generation and
migration in the Western Canada sedimentary Basin. In Brooks, J. (Ed.), Classical Petroleum Provinces. Geological Society Special Publication 50, pp. 189±202
Dieckmann, V., 1998. The prediction of oil and gas composi-tion by the integracomposi-tion of laboratory experiments and case studies. PhD-RWTH±Aachen. Berichte des For-schungszentrum Juelich 3648.
Dieckmann, V., Schenk, H.J., Hors®eld, B., Welte, D.H., 1998. Kinetics of petroleum generation and cracking by pro-grammed-temperature closed-system pyrolysis of Toarcian shales. Fuel 77, 23±31.
Dieckmann, V., Schenk, H.J., Hors®eld, B., in press. Asssessing the overlap of primary and secondary reactions by closed-versus open-system pyrolysis. Journal of Analytical and Applied Pyrolysis.
Durand, B., 1980. Kerogen-Insoluble Organic Matter from Sedimentary Rocks. Editions Technip, Paris.
EspitalieÂ, J., Ungerer, P., Irwin, H., Marquis, F., 1988. Primary cracking of kerogen. Experimenting and modeling C1, C2ÿ5, C6ÿ15and C15+classes of hydrocarbons formed. In: Matta-velli, L., NoMatta-velli, L. (Eds.), Advances in Organic Geochem-istry. Pergamon, Oxford, pp. 893±899.
Fausett, D.W., Mikinis, F.P., 1981. Simpli®ed kinetics of oil shale pyrolysis. Colo. School of Mines Press, Golden, CO. Hors®eld, B., 1997. The bulk composition of ®rst-formed
pet-roleum in source rocks. In: Welte, D.H., Hors®eld, B., Baker, D.R. (Eds.), Petroleum and Basin Evolution, Insights from Petroleum Geochemistry, Geology and Basin Model-ing. Springer Verlag, Berlin/Heidelberg, pp. 337±402. Hors®eld, B., RullkoÈtter, J., 1994. Diagenesis, catagenesis and
metagenesis of organic matter. In: Magoon, L.B., Dow, G. (Eds.), The Petroleum System from Source to Trap. American Association of Petroleum Geologists, Tulsa, pp. 189±199. Hors®eld, B., Disko, U., Leistner, F., 1989. The microscale
simulation of maturation: outline of a new technique and its potential application. Geol. Rund 78, 361±374.
Hors®eld, B., Schenk, H.J., Mills, N., Welte, D.H., 1992. An investigation of the in-reservoir conversion of oil to gas: com-positional and kinetic ®ndings from closed-system programmed temperatur pyrolysis. Organic Geochemistry 19, 191±204. Larter, S.R., 1984. Application of Analytical Pyrolysis
Techni-ques to Kerogen Characterisation and Fossil Fuel Exploration/ Exploitation. Analytical Pyrolysis, Methods and Application. Butterworth, London 212-275.
Lewan, M.D., 1994. Assessing natural oil expulsion from source rocks by laboratory pyrolysis. In: Magoon, L.B., Dow, W.G. (Eds), The Petroleum System Ð from Source to Trap. AAPG Memoir 60, pp. 201-210
Lewan M.D., Comer, J.B., Hamilton-Smith, T., Hasenmueller, N.R., Guthrie, J.M., Hatch, J.R. et al., 1995. Feasibility study of material-balance assessment of petroleum from the New Albany Shale in the Illinois Basin. US Geol. Surv. Bull. 2137.
Li, M., Yao, H., Fowler, M.G., 1998. Geochemical constraints on petroleum secondary migration models: revisit the Rim-bey-Meadowbrook Reef Trend in WCSB. In: Hors®eld et al. (Eds.), Advances in Organic Geochemistry. Pergamon, Oxford, pp. 163±182.
Louis, M.C., Tissot, B., 1967. In¯uence de la temperature et de la pression sur la formation des hydrocarbures dans les argiles a kerogene. In: Proc. 7th World Petrol. Cong., Vol. 2, Mexico City, pp. 47±60.
Maciel, G.E., Bartuska, V.J., Mikinis, F.P., 1979. Correlation between oil yields of oil shales and 13C nuclear magnetic resonance. Fuel 57, 505±506.
Michels, R., Landais, P., Torkelson, B.E., Philp, R.P., 1995. Eects of euents and water pressure on oil generation dur-ing con®ned pyrolysis and high-pressure hydrous pyrolysis. Geochimica et Cosmochimica Acta 59, 1589±1604.
Pelet, R., 1985. Evaluation quantitative des produits formes lors de l'eÂvolution geÂochemique de la matieÁre organique. Rev. Inst. Fr. PeÂt. 40, 551±562.
Pepper, A.S., Corvi, P.J., 1995. Simple kinetic models of pet-roleum formation. Part 1: oil and gas generation from kero-gen. Marine and Petroleum Geology 12, 291±319.
Pepper, A.S., Dodd, T.A., 1995. Simple kinetic models of pet-roleum formation. Part II: oil±gas cracking. Marine and Petroleum Geology 12, 321±340.
Price, L.C., Wenger, L.M., 1992. The control of pressure on petroleum generation, maturation and thermal destruction as delineated by hydrous pyrolysis. In: Eckardt, C.D., Maxwell, J.R., Larter, S.R., Manning, D.A.C. (Eds.), Advances in Organic Geochemistry. Pergamon, Oxford, pp. 141±160. Philippi, G.T., 1965. On the depth, time and mechanism of
petroleum generation. Geochimica et Cosmochimica Acta 29, 1021±1049.
Radke, M., Sittard, H.G., Welte, D.H., 1978. Removal of soluble organic matter from rock samples with a ¯ow-through extraction cell. Anal. Chem. 50, 663±665.
Radke, M., Willsch, H., Welte, D.H., 1980. Preparative hydrocarbon group type determination by automated med-ium pressure liquid chromatography. Anal. Chem. 52, 406± 424.
Rubel, A.M., Margolis, M.J., Haley, J.K., Davis, B.H., 1983. Bench scale ¯uid retorting of Kentucky oil shale. In: Eastern Oil Shale Symposium, 13±16 November, pp. 399±405. Sandvik, E.I., Young, W.A., Curry, D.J., 1992. Expulsion from
hydrocarbon sources: the role of organic absorbtion. Organic Geochemistry 19 (1±3), 77±87.
Schaefer, R.G., Schenk, H.J., Hardelauf, H., Harms, R., 1990. Determination of gross kinetic parameters for petroleum formation from Jurassic source rocks of dierent maturity levels by means of laboratory experiments. In: Durand, B., BeÂhar, F. (Eds.), Advances in Organic Geochemistry. Perga-mon, Oxford, pp. 115±120.
Schenk, H.J., Hors®eld, B., 1993. Kinetics of petroleum gen-eration by programmed-temperature closed- versus open-system pyrolysis. Geochimica Cosmochimica Acta 57, 623± 630.
Schenk, H.J., Hors®eld, B., 1998. Assessing the reliability of kinetic timing predictions. In: Hors®eld et al. (Eds.), Advances in Organic Geochemistry. Pergamon, Oxford, pp. 137±154. Schenk, H.J., Witte, E.G., Littke, R., Schwochau, K., 1989.
Structural modi®cations of vitrinite and alginite concentrates during pyrolytic maturation at dierent heating rates. A combined infrared,13C NMR and microscopical study. In:
BeÂhar, F., Durand, B. (Eds.), Advances in Organic Geo-chemistry. Pergamon, Oxford, pp. 943±950.
Schenk, H.J., Di Primio, R., Hors®eld, B., 1997. The conver-sion of oil into gas in petroleum reservoirs. Part 1: com-parative kinetic investigation of gas generation from crude oils of lacustrine, marine and ¯uviodeltaic origin by pro-grammed-temperature closed-system pyrolysis. Organic Geochemistry 26, 467±481.
Stoaks, F.A., Creaney, S., 1984. Sedimentology of a carbonate source rock: the Duvernay Formation of Alberta, Canada. In: Longman, M.W., Shanley, K.W. (Eds), Rocky Mountain Carbonate Reservoirs Ð A Core Workshop. Society of Economy Paleontologists and Mineralogists, Core Work-shop No. 7, Golden, CO, pp. 343±375.
Stoaks, F.A., Creaney, S., 1985. Controls on the accumulation and subsequent maturation and migration-history of a Car-bonate source rock. In: SEPM Core Workshop Proceedings, Golden, CO, p. 56.
Stout, N.D., Koskinas, G.J., Raley, J.H., Santor, S.D., Opila, R.L., Rothman, A.J., 1976. Pyrolysis of oil shale: eects of thermal history on oil yield. Colo. Scho. Mines. Quart. 71, 153. Sweeney, J.J., Burnham, A.K., 1990. Evaluation of a simple model of vitrinite re¯ectance based on chemical kinetics. Am. Ass. Pet. Geol. Bull. 74 (10), 1559±1570.
Tang, Y., BeÂhar, F., 1995. Rate constants ofn-alkane genera-tion from type II-kerogen in open and closed pyrolysis sys-tems. Energy Fuels 9, 507±512.
Tang, Y., Stauer, M., 1995. Formation of pristene, pristane and phytane: kinetic study by laboratory pyrolysis of Mon-terey source rock. Organic Geochemistry 23, 451±460. Tissot, B., 1969. PremieÁre donneÂes sur les meÂchanismes et la
cineÂtique de la formation du peÂtrole dans les seÂdiments. Simulation d'un scheÂma reÂactionnel sur ordinateur. Rev. Inst. Fr. PeÂt. 24, 470±501.
Tissot, B., Califet-Debyser, Y., Deroo, G., Oudin, J.L., 1971. Origin and evolution of hydrocarbons in early Toarcian Shales, Paris Basin, France. Bull. Am. Ass. Pet. Geol. 55 (12), 2177±2193.
Ungerer, P., 1990. State of art of research in kinetic modelling of oil formation and expulsion. In: Durand, B., Behar, F. (Eds.), Advances in Organic Geochemistry. Pergamon, Oxford, pp. 1±25.
Ungerer, P., Pelet, R., 1987. Extrapolation of the kinetics of oil and gas generation from laboratory experiments to sedimen-tary basins. Nature 327, 52±54.
