Pub. Id: A113 (1988)

First Page: 297

Last Page: 309

Book Title: M 45: The Pannonian Basin: A Study in Basin Evolution

Article/Chapter: An Organic Maturation Study of the Hod-I Borehole (Pannonian Basin): Chapter 21

Subject Group: Geologic History and Areal Geology

Spec. Pub. Type: Memoir

Pub. Year: 1988

Author(s): Cs. Sajgo, Z. A. Horvath, J. Lefler

Abstract:

Several maturation parameters were determined for a 5800-m-thick Neogene sequence in the Pannonian basin of southeastern Hungary. The oil window was found to start at 3450 m corresponding to a vitrinite reflectance of about 0.7% R0. This is typical of a mixture of Type II-Type III kerogen such as occurs in the U.S. Gulf Coast. The carbon preference index (CPI) of n-alkanes in the rock extracts also approaches 1.0 at this depth. Hydrocarbon generation appeared to continue to the bottom hole depth of 5800 m with an oil generation minimum or gap occurring in the 5000-5400 m interval that might be due to a change in the nature of the organic matter. Two isomerizations of biological marker compounds, the shift of steranes from 20(R) to 20(S) and of hopanes from 22(R to 22(S) were found to reach equilibrium at 4000 and 3000 m, respectively, just prior to the threshold of intense oil generation. Also, the C29-monoaromatic steranes were converted almost 100% to C28-triaromatic steranes prior to this depth.

Text:

INTRODUCTION

Investigation of the maturity of organic matter in sedimentary rocks has received much attention in the last two decades and numerous methods have been developed to detect zones of oil and gas formation (see, for example, Philippi, 1965; Vassoyevich et al., 1970; Lopatin, 1971, 1976; Laplante, 1974; Hood et al., 1975; Dow, 1978; and Espitalie et al., 1977). Several thorough and comprehensive studies have been published in the last few years (Tissot and Welte, 1978; Hunt, 1979 and Heroux et al., 1979). In several instances, various maturation indices have been successfully applied to hydrocarbon prospecting, but a maturation index that is applicable to all geological situations has not been found. Vitrinite reflectance is the most frequently used maturation index, but is limited to sed mentary formations that contain vitrinite. Even when vitrinite-bearing rocks are present, difficulties in interpretation occur. Estimation of maturity from the color of spores is also not straightforward (e.g., see Raynaud and Robert, 1976). Analysis of changes in the composition of soluble organic matter is often complicated by the addition of migrated components.

Organic maturation does not occur by a single type of reaction: without an exact knowledge of the chemical changes that take place during maturation, maturity parameters can only be related to one another from field studies. For example, the model of oil generation outlined by Tissot and Welte (1978) is straightforward, but not universally applicable. The vitrinite reflectance and temperature ranges they assign to the oil generation zone disagree with some observations (Sajgo, 1980a,b; Price, 1982; Price et al., 1979, 1981; and Saxby, 1982).

In this chapter we discuss the variation in a number of maturation parameters in samples from the borehole Hodmezovasarhely-I (Hod-I) in the Pannonian basin.

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GEOLOGIC SETTING OF HOD-I

Hod-I is located in southeast Hungary (about 25 km east of Szeged) in a Neogene sedimentary trough. Drilling by the National Oil and Gas Trust (OKGT) terminated at 5842.5 m depth in Badenian (middle Miocene) sedimentary rocks. Continuous Miocene to present sedimentation is assumed although the Sarmatian cannot be dated paleontologically (Mucsi, 1973; Mucsi and Revesz, 1975; and Szentgyorgyi, 1975). The pre-Pannonian Miocene rocks consist of pelitic lime marl between 5100 and 5450 m; below 5450 m the sediments consist primarily of coarse-grained siltstones. The Pannonian (s. l.) strata were mainly deposited in nearshore delta and lacustrine environments and consist of alternating sandy, clayey, and marly layers 0.02 to 10.0 m thick and with a carbonate.

Several organic geochemical analyses have been reported on samples from this well. These are summarized in Sajgo (1980a,b) and Sajgo et al. (1987). In general they show the threshold of intense petroleum generation starting at 3450 m at a present sediment temperature of 142 °C.

THERMAL MATURITY OF DISPERSED ORGANIC MATERIAL USING VITRINITE DATA

Problems with Measurement

The vitrinite reflectance method adapted from coal petrology is based on the observation that the optical parameters of vitrinite change gradually as a function of progressive coalification (maturation). The reflectance of light by vitrinite grains in a polished section prepared from the sedimentary rock or from a separated kerogen concentrate, and covered with oil, can be used as a measure of the coal rank. For pure coals, the different macerals are found together and can be fairly well distinguished. About 80% of sedimentary rocks contain vitrinite, but organic rich rocks and carbonates may contain little or none. Identification of vitrinite in the latter rock types is sometimes difficult. Thus, the reflectance of other microcomponents (semifusinite, bitumen, pseudovitrinite) is occ sionally measured by mistake instead of vitrinite.

Allochthonous or reworked vitrinite may also be present in the sedimentary rocks. This results in a bimodal population. Consequently, in addition to the average reflectance values, the reflectance histogram should also be presented so that anyone can reevaluate the vitrinite data. The measurements reported here were carried out with a precision of 0.01% and the histogram for each sample is constructed from at least 50 points. For coals the precision of the reflectance histogram is 0.05%; for dispersed organic matter, it is 0.10% because of the uncertainties mentioned above.

Forty core samples between 2050 and 5815 m depth were used for vitrinite measurements (Figure 1) (Horvath, 1980). Ten of the samples were also measured by J. R. Castano at Shell Development Company. Graphite was present in all the samples studied indicating a metamorphic source area. In sample No. 1/3 of early Pannonian (s. l.) age, no allochthonous (reworked) vitrinite occurred in addition to graphite. The reflectance histogram shows small dispersion. Most of the upper Pannonian samples contained reworked vitrinite. Because of greater dispersion, the limits of the autochthonous vitrinites could only be determined with difficulty. Nevertheless, for samples No. 4/1 to 27/6, Castano's measurements yielded systematically lower values than measurements we conducted on the same sa ples. For samples deeper than No. 27/6, Castano's measurements produced higher average values. When comparing our reflectance histograms to those of Castano, two phenomena are apparent. First, in measuring, many more grains of lower reflectance were identified: the dispersion is higher. Second, when calculating the average reflectance, Castano excluded grains of higher and lower reflectance. According to Castano, samples 35/1 and 39/9 contain considerable amounts of bitumen but no vitrinite. Shell assumes that in this maturity range (R0 > 1.6) the reflectance of bitumen is the same as that of vitrinite and bitumen was used to obtain vitrinite reflectance values. For sample 45/9, the agreement between the reflectance of bitumen and vitrinite is good. However, in our opinion, the reflectance of bitumen above 1.6 often exceeds that of vitrinite.

Because no depositional hiatus or erosion could be determined for the formations of the borehole Hod-I, it may be assumed that the present temperature of each sample site is its maximum temperature (see also Dovenyi et al., 1983). Ammosov et al. (1977) collected all the available vitrinite reflectance data from basins in the Soviet Union where the sedimentation has been continuous. Based on these data, they constructed a scale that shows the minimum rock temperature required to reach a given reflectance value (Figure 2). Except for two samples, the average reflectance values follow the relationship determined by Ammosov et al. (1977) within the error range of ± 10% quoted previously.

Bitumen Analyses of Solvent Extract

The qualitative and quantitative changes of soluble organic matter during maturation have been studied by many authors (Brooks and Smith, 1967; Albrecht et al., 1976; Tissot et al., 1971, 1974, 1977; Allan and Douglas, 1977; and Radke et al., 1980). In the Hod-I borehole, Sajgo (1980a,b) proposed two oil generation zones from a plot of the chloroform soluble extract (in milligrams) divided by the total Corg (in grams) as a function of depth. Figure 3 shows more or less the same phenomenon for ^sum CH mg/Corg g. Oil generation starts at about 3450 m depth. At about 5000 m depth, no significant oil generation could be detected, although there may be gas generation. Below 5450 m depth, a second oil generation zone may be observed that extends to the deepest sample. he first maximum is well-known to petroleum geologists as the main phase of oil generation. In the borehole Hod-I this appears to occur within a temperature range of 140-200°C, which is higher than that observed elsewhere either because of the very short time that these sediments have been at high temperatures (due to rapid sedimentation) or because oil generation usually requires temperatures that are this high (Thompson, 1983). In this oil generation zone the vitrinite reflectance value varies between 0.69 and 1.5%.

We propose that a second oil generation zone starts at 218°C and continues at least to the 233°C measured at the well bottom. In this range the vitrinite reflectance value (Figure 2) varies

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between 1.6 and 2.15% (Sajgo, 1980b, later determined these values to be 1.41 to 1.57%).

Similar examples of a gap in generation in wells from other areas have been summarized by Sajgo (1980b). Other examples have been found by Yakovets et al. (1976) and Sajgo, unpublished work. We infer that our lower zone of oil generation is a similar phenomena to the deep zone of oil generation observed by Price et al. (1979) and Price (1982). Some of these apparent gaps may represent gas generation in preference to oil. In the Hod-I borehole, the H/C ratio of the kerogen in the gap drops to 0.76-0.78 (Figure 2), which is too low to sustain oil generation. The ratio increases to the 0.9 range in the deeper oil generating zone.

Figure 4 shows the distributions of n-alkanes and the relative quantities of three isoprenoid hydrocarbons (norpristane, pristane and phytane). The variety of alkane spectra may reflect the variety of organic matter types. In the deeper samples the relative quantity of lighter hydrocarbons (C-16 to C-20) increases at the expense of the heavier ones (> C-20), perhaps due to cracking of the heavier hydrocarbons at increased temperatures. The distribution may also be affected by migration, maturation and different types of organic matter.

The ratio pristane/phytane varies with the type of depositional environment (oxidizing versus reducing) and goes through a maximum with increasing maturity (Brooks et al., 1969; Leythaeuser, 1975; Flekken, 1978; and Radke et al., 1980). In Figure 5, this change with maturation is clearly shown, with a maximum around 3600 m.

All of the geochemical data shown in Figures 3 to 6 are sensitive to changes in the type of organic matter and to migration (both to migration out and migration in of oil). The lithology of the rock matrix should also be taken into account. The ratio ^Sgr CH/NSO compounds also indicates the lack of oil generation between 5000 and 5400 m (Figure 6). However, this ratio is also dependent on the type of organic material and on the migration processes, as well as on the maturity.

The ratio of saturated hydrocarbons to saturated esters (E1470/E1740) (infrared extinction ratio) in the asphaltene fraction

Fig. 1. Reflectograms of the Hod-I samples. The reflectograms marked "A" were produced by Z. A. Horvath at LGR. Those marked "B" were produced by J. R. Castano at Shell by measuring vitrite reflectance. Those marked "C" were produced by Castano by measuring reflectance of bitumen.

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shows a decrease in the hydrocarbon content of asphaltene below 4000 m due to asphaltene disproportionation (Figure 7). Below 4700 m the scatter in (E1470/E1740) is low, and the ratio ^Sgr CH/NSO is usually below 4700 m as well (Figure 6). The low scatter in E1470/E1740 thus implies that hydrocarbons in this depth range were not generated from asphaltane, and must therefore come from kerogen.

APPLICATIONS OF THE TRANSFORMATION OF BIOLOGICAL COMPOUNDS DURING HEATING

The coalification of plant material in sediments is a complicated process, so that one should not expect that a simple relationship can explain the coal rank of coals with different geological histories. For this reason, and because the sedimentation and thermal histories are often complicated, relationships between temperature, geologic age, and coal rank that work well in some basin cannot, in our opinion, be applied without restrictions to other regions (see, for example, Table 1). In our opinion, the ideal geochemical maturity parameter should be applicable to both oil and rock samples, independent of organic matter type and irreversible; or, if reversible, it eventually must attain an equilibrium state. The pressure dependence of most geochemical reactions is not well established it is better, therefore, if no change in the volume occurs during the reaction. It is also important that the physical fractionation processes occurring during migration should not affect the maturity parameter. To the best of our knowledge the following three reactions more or less satisfy these requirements.

These reactions involve biological marker compounds (Speers and Whitehead, 1969). Biological marker compounds are organic compounds whose structure can be related to a biological precursor because of only minor alteration during sedimentation and diagenesis. In this chapter, the concentrations of four such compounds (steranes, hopanes, mono- and triaromatic steroid hydrocarbons) will be discussed as a function of depth. The concentrations of the starting material and of the products of the three reactions (Figure 8) were determined with a computerized gas chromatograph-mass spectrometer system (Mackenzie et al., 1980, 1981; and Sajgo and Lefler, 1986).

Reaction Kinetics

Steranoid and hopanoid structures experience several different types of chemical reactions during maturation. These reactions can be subdivided into: (1) isomerization reactions, (2) aromatization reactions, and (3) decomposition reactions. The third of these reactions accompanies generation of oil and gas, and the products of this reaction have not been identified. The first and second reaction types are intramolecular reactions whose products can be recovered and quantified. Thus the progress of the reaction can be followed by component ratios.

Reaction kinetics describe the progress of the chemical reactions with time. The rate at which the chemical reactions proceed

Fig. 2. The average reflectance versus depth plot of the Hod-I borehole. The open squares correspond to the minimum temperature values needed to achieve a given vitrinite reflectance as determined for basins with no uplift in the Soviet Union (Ammosov et al., 1977).

Fig. 3. Variation of the amount of chloroform extract relative to total organic carbon as a function of depth.

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is a function of the concentrations of the compounds involved in the reaction:

[EQUATION (1)]

where ^tgr denotes the reaction rate, ci the concentration of the ith compound, and ^tgr the time.

The order of the chemical reaction is the same as the order of the differential equation. For first-order reactions, Equation (1) may be of the following form:

[EQUATION (2)]

We assume that for intramolecular transformations the reaction rate described by Equation (2). This expression, however, is not correct if the surrounding rock material also contributes to the intramolecular rearrangement (catalytic). The following equation:

[EQUATION (3)]

would describe the reaction progress for second-order reactions. If the rock concentration (c2) is constant in time, and is very high compared to the compounds studied, its effect can be included in the reaction rate constant k and Equation (2) applies. This is known as pseudo-first-order kinetics.

The reactions studied are of two kinds: (1) reversible reactions leading to equilibrium, for example, isomerization; and (2) irreversible reactions, for example, aromatization. The three reactions are shown in Figure 8. For a detailed explanation of them, see Mackenzie et al., 1980, 1981). The two isomerization reactions can be modeled as follows:

[EQUATION (4)]

When equilibrium is reached, the reaction rate equals zero, or:

[EQUATION (5)]

The equilibrium constant is given by:

[EQUATION (6)]

where CR* and CS* are the equilibrium concentrations of the R and S isomers. When the concentrations of R and S are far from equilibrium, the macroscopic reaction rate (the rate of transformation of CS) can be written:

[EQUATION (7)]

By means of the C-GC-MS system the relative concentrations of CR and CS can be measured, thus:

[EQUATION (8)]

Applying the same transformation in the relationships (6), (7), and (8), it follows that:

[EQUATION (9)]

where ß = 1 + 1/K and K is the equilibrium constant. Rearranging Equation (9) and integrating with respect to cS gives:

[EQUATION (10)]

There is no problem with the integration of the left side of Equation (10) (the concentration-dependent part). For the right side of Equation (1) (the time-dependent part):

[EQUATION (11)]

where A is the preexponential factor, ^DgrH* is the activation energy of the reaction, R is the universal gas constant, and T is the absolute temperature.

The reaction rate coefficient k, is not constant but depends on temperature. If the chemical reaction proceeds at constant temperature, k will be independent of time. If so, then in the right side of the Equation (10), k could be taken outside the integral. In our case this cannot be done because during basin evolution rocks are buried progressively deeper so that they pass through zones of different temperatures. This problem usually has been eliminated by introducing EHT (the effective heating time; Hood et al., 1975). When studying geological samples Hood et al. (1975) came to the conclusion that in the chemical reactions of vitrinite it is sufficient to take into account the time spent within 15 °C of the maximum temperature.

Another solution is to use the method of absolute times. Assuming that the functions describing the basin subsidence and the change of the geothermal gradient through time are known, we can write for each individual sedimentary layer:

[EQUATION (12)]

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Fig. 4. The relative distributions of n-alkanes (C15-C40) and isoprenoids (C13-C20) in chloroform extracts.

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Fig. 4. Continued.

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Substituting the relationships (11) and (12) into Equation (10) we get:

[EQUATION (13)]

The relationship given by Equation (13) forms the basis of the method of absolute times. Unfortunately, for most realistic temperature histories as described by Equation (12), the integral on the right of Equation (13) cannot be written in closed form and one of two numerical techniques must be used: (1) Assume a thermal history for Equation (12), substitute this relationship into Equation (13) and integrate. The expression on the left of Equation (13) can be calculated from measurements (CS, CR, CS* and CR*) and Equations (8) and (9). For aromatization reactions the mono- and triaromatic concentrations are used instead of CS and CR. This calculation is made step-by-step (between the adjacent measured points) so that th preexponential factor, A, can be determined as a function of depth or temperature. If this function is constant within the limits of measurement error and its fluctuation is random, the assumed thermal history is consistent with the maturity data. If the fluctuation of A is greater than expected or changes as a function of the number of samples, the assumed thermal history should be replaced by another model. In this way, the thermal history of a basin can be reconstructed. (2) A theoretically more satisfying method can be applied when the data for several different reactions (for example aromatization, hopane isomerization or sterane isomerization) are also available from the basin in question. In this case a system of equations can be established similar to Equation (13). The function g(^tgr) can be described as:

[EQUATION (14)]

where h(^tgr) is the burial depth of the sample and g^prime(^tgr) is the geothermal gradient as a function of time.

By solving the system of equations, the functions h(^tgr) and g^prime(^tgr) can be determined. Although theoretically simple, this procedure is complicated by difficulties in solving the integral in Equation (13). For this reason, it is more convenient to apply a differential method as follows. Equations (10) and (11) can be transformed to derive the more general relationships:

[EQUATION (15)]

Fig. 5. Variation of the pristane to phytane ratio (pr/ph) in chloroform extract as a function of depth.

Fig. 6. The ratio of hydrocarbons (HC) to nonhydrocarbons (NSO) in chloroform extracts as a function of depth.

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and

[EQUATION (16)]

Taking the finite differences in Equation (16) and differentiating by h instead of r on the basis of Equation (14), and substituting Equations (12) and (14) into Equation (11), the thermal history can be obtained (a detailed discussion can be found in the papers of Sajgo and Lefler, 1986 and Lefler and Sajgo, 1986). The accuracy of this method is limited by the density of the available samples.

Either of these two methods (EHT and AT) are suitable for the reconstruction of thermal history using the six reactants and products of the three reactions discussed previously. If during a given time, the reaction (either an equilibrium or an irreversible transformation) reaches a point where changes in concentration cannot be measured, nothing further can be deduced about the thermal history from that reaction.

In Equations (9), (10), and (13), ß depends on the temperature, and thus also on time. In case of isomerization reactions, the temperature-dependent part of ß, K, is given by Equations(6) and (11). Based on both theoretical considerations and on measurement data, the temperature dependence of the two reaction rate constants for isomerization are equal, thus the temperature and time dependence of K can be neglected. In the aromatization reaction, the equilibrium is in practice displaced towards the triaromatic product, thus ß = 1.

Calculations of Reactions of the Biological Marker Compounds

Using the results of the reactions and measurements shown in Figures 9-11, the reaction kinetic parameters of the sterane and hopane isomerization and of the aromatization reactions were calculated both by the EHT and by the AT methods. For the EHT method, the time which each sample spent within 15 °C of its maximum temperature was calculated using biostratigraphic data and temperature measurements within the borehole

Fig. 7. Variation of the E1470/E1740 (infrared extinction ratio) in asphaltene as a function of depth.

Fig. 8. Reactions studied: (A) Configurational isomerization of the 5^agr(H), 14^agr(H), 17^agr(H), 20(R)-C29 steranes to 20(S) steranes. (B) Configurational isomerization of the 17^agr(H), 21ß(H), 22(R)-C22 to 22(S) hopanes. (C) Aromatization of the 5^agr(H) and 5ß(H) isomers of C29 Cring-monoaromatic steroid hydrocarbons to C28-triaromatic steroid hydrocarbons.

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assuming a constant thermal gradient through time. For the AT method, the burial history was also determined from bio-stratigraphic data, and subsidence was considered to be constant between biostratigraphic markers. The activation energy of the reaction was calculated by the differential method as a first approximation, then refined by successive iterations.

Sterane Isomerization

The EHT Method. From measurements, K = 1.38 and ß = 1.724. By means of the EHT method:

[EQUATION]

Taking the logarithm of Equation (11):

[EQUATION (17)]

Substituting the measurement data into Equation (17):

[EQUATION]

Based on the correlation coefficient (k.e.), the measurements plotted onto a straight line. The activation energy was calculated

Fig. 9. Increase in the extent of geochemical isomerization at C-20 of 5^agr(H), 14^agr(H), 17^agr(H), 20(R)-C29 steranes as a function of depth/temperature.

Fig. 10. Increase in the extent of geochemical isomerization at C22 of 17^agr(H), 21ß(H), 22(R)-C31, and C32 hopanes (mean value) as a function of depth/temperature.

Table 1. Estimated values of vitrinite reflectance (R0)

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to be 97.84 kJ/mol, the preexponential factor, A, 5.43 ^times 1012 m.y.-1.

The AT Method. In the relationship (ln k - 1/T), the values of k determined from the differential method produced a straight line corresponding to Equation (17):

[EQUATION]

The corresponding activation energy, ^DgrH is 96.64 kJ/mol, preexponential factor, A is 2.548 ^times 1011 m.y.-1. Using ^DgrH* values as starting data obtained from the differential and EHT methods, successive iterations by the AT method gives

[EQUATION]

for the preexponential factor and 92.32 kJ/mol for the activation energy.

Hopane Isomerization

The EHT Method. From measurements, K = 1.326 ^rarr ß = 1.754. By the EHT method,

[EQUATION]

Substituting the measurement data into Equation (17) gives

[EQUATION]

This fit is good. The value of the activation energy was calculated to be ^DgrH* = 96.205 kJ/mol, the preexponential factor was A = 1.101 ^times 1013 m.y.-1.

The AT Method. The values determined by the differential method also plot along a straight line on a graph of k versus 1/T. The relationship that corresponds to Equation (17) is

[EQUATION]

and the fit is good. The activation energy was calculated to be ^DgrH* = 96.62 kJ/mol, the preexponential factor:

[EQUATION]

Using these values as starting data obtained by the differential and the EHT methods, successive iterations by the AT method gives

[EQUATION]

and

[EQUATION]

Aromatization Reaction

The EHT Method. From measurements,

[EQUATION]

With the EHT method: Teff = 15° ^rarr ^tgrEHT = 1.16 m.y. Substituting this data into Equation (17) gives

[EQUATION]

In this case the fit is poorer than for the isomerization reactions. The corresponding activation energy is 119.8 kJ/mol and the preexponential factor is 9.68 ^times 106 m.y.-1.

The AT Method. For this reaction, the differential method produced scattered results because of the small number of measurements and their uneven distribution.

Successive iteration using the AT method gives

[EQUATION]

and

[EQUATION]

From the data in this paper, McKenzie et al. (1983) obtained the following: for sterane isomerization, 91 kJ/mol for activation energy and 1.89 ^times 1011 m.y.-1 for the preexponential factor; for aromatization of the monoaromatic steroid hydrocarbon, 200 kJ/mol for activation energy and 5.68 ^times 1028 m.y.-1 for the preexponential factor. The agreement is satisfactory for isomerization,

Fig. 11. Increase in the extent of geochemical aromatization of C29 C-ring monoaromatic steroid hydrocarbons to C28-triaromatic steroid hydrocarbons as a function of depth/temperature.

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but not for aromatization. Our activation energy for aromatization is one-half the value of McKenzie et al., and our frequency factor is at least ten orders of magnitude smaller.

Effective Heating Times

Using the values of activation energy and preexponential factor that were obtained above by the method of absolute times, we recalculated the effective heating time for each of the three reactions. We obtained effective heating times of 0.78 m.y. for sterane isomerization, 0.73 m.y. for hopane isomerization, and 0.567 m.y. for aromatization reaction. This indicates that these reactions reached equilibrium or completion within less than one million years in the well Hod-I.

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Acknowledgments:

We thank the National Oil and Gas Trust of Hungary for providing samples and permission to publish. We would also like to thank Shell Development Company for their generosity in providing us with their vitrinite reflectance measurements for Hod-I. The computerized gas chromatograph-mass spectrometer measurements have been carried out at Bristol during the tenure of a fellowship of the Scientific Exchange Agreement by Cs.S. who is grateful to Professor G. Guiochon (Ecole Polytechnique Paris) and Professor G. Eglinton (University of Bristol) for his fellowship. Technical assistance from Mrs. A. Marot and Mrs. M. Heltay in Budapest, and Mrs. A. Gowar in Bristol, is gratefully acknowledged.

Copyright 1997 American Association of Petroleum Geologists