- Email: [email protected]
Experimental study on the pressure dependence of vitrinite maturation
Geochimica
et Cosmochimica
Pergamon
Acta, Vol. 61, No. 14, pp. 2921-2928, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All rights reserved
0016.7037/97 $17.00 + .OO
PI1 SOO16-7037( 97)00104-X
Experimental
study on the pressure
dependence
M. DALLA TORRE, I,* R. FERREIRO MAHLMANN,~ ‘Department
of Geological
and Environmental
‘Mineralogisch-Petrographisches (Received
November
Sciences,
Stanford
University,
of vitrinite
maturation
and W. G. ERNST’ Stanford,
California
94305-2115,
USA
Institut, Base1 University, CH 4056 Base], Switzerland
13, 1996; accepted
in revised form February
28, 1997)
Abstract-We conducted the first systematic experiments over a pressure range of 0.5 to 20.0 kbar, at temperatures of 200 to 35o”C, and run durations of -2, -4, and -7 days to investigate the influence of pressure on vitrinite reflectance. The data indicate that applied pressure lowers vitrinite reflectance; the suppression appears to be stronger under wet than under dry conditions. In addition, high pressure favors liptinite preservation at temperatures 2 3OO”C, whereas at the same temperature range, yet at low pressure, liptinite is decomposed. Our data fit an empirical rate equation which expresses %R, as a function of pressure, temperature, and time. The derived equation satisfactorily describes our experiments and enables extrapolations to geological timescales. Values of % R, calculated for a pressure of 0.5 kbar are remarkably consistant with previous numerical pressure-independent models. A literature review shows that previous experimental data and field observations are inconsistent regarding the effect of pressure on vitrinite maturation-this and some previous studies suggest that vitrinite maturation is suppressed by increased pressure, whereas other workers regarded pressure as having little influence on the rate of coalification. We suggest that original vitrinite composition, experimental design, and natural environment (tectonic regime) play important roles regarding the effects of pressure on vitrinite maturation. Interpretation of geothermal histories of sedimentary rocks based on vitrinite reflectance data must, therefore, include evaluation of the pressure. Otherwise, vitrinite reflectance data from high-pressure terranes should be intemreted with caution if used as a quantitative paleogeothermometer. Copyright 0 1997 Elsevier Scienck Ltd 1. INTRODUCTION
under normal circumstances, stabilizes rapidly within the framework of a geologic timescale-within as little as ten years (Barker, 1991). Estimates of paleotemperature can be calculated directly from empirical regression curves of borehole temperature vs. % R,. Different vitrinite reflectance geothermometer curves need to be used for rapid heating in geothermal fields, as opposed to slow burial in first-cycle sedimentary basins (Barker, 1983, 1988; Barker and Pawlewicz, 1986). Critical comparisons among the various methods have been published recently by Littke ( 1993), Morrow and Issler ( 1993), and Barker and Pawlewicz ( 1994). While it is well established that temperature and time are major factors controlling the attainment of a certain vitrinite reflectance value, the influence of pressure is largely unknown; various studies have suggested that vitrinite reflectance may be an insensitive measure of increasing temperature in vitrinite-bearing strata under high pressures (Goffk and Velde, 1984; Dalla Terre et al., 1994, 1996). The quantitative understanding of modifications of optical properties of vitrinite during the geothermal history is crucial in situations where oil exploration and other scientists rely on vitrinite reflectance as a thermal maturity indicator. Surprisingly, the effect of pressure, important in most geological environments, has not been investigated experimentally in a systematic fashion. This laboratory study measures the effect of pressure on vitrinite reflectance and presents a preliminary empirical rate equation that describes this pressure effect.
Understanding variations of optical properties of vitrinite as a function of temperature, pressure, and time is crucial for the economic and scientific interpretation of the geothermal histories of sedimentary basins and low-temperature metamorphic rocks; hydrocarbon exploration in sedimentary basins relies strongly on vitrinite reflectance as a thermal maturity indicator, and vitrinite reflectance is applied as a paleogeothermometer in regions affected by low-temperature metamorphism (Frey et al., 1980; Kisch, 1987; Teichmiiller, 1987; Underwood et al., 1991). Low-grade metamorphism is used here to indicate the lowest grades of metamorphism, up to the beginning of greenschist facies conditions, covering a temperature range from 150-200 to approximately 35O”C, according to the terminology after Frey and Kisch ( 1987). To interpret the geothermal history or maturity of vitrinitebearing sedimentary rocks, estimates of paleotemperature are obtained from vitrinite reflectance values using different numerical maturity models. Because the maturation of thermodynamically metastable organic matter is an expression of reaction rate, both temperature and effective heating time are important (e.g., Hood et al., 1975; Waples, 1980), and sophisticated models (e.g., EASY%Ro) have been formulated based on multiple-reaction Arrhenius-based chemical kinetics (Bumham and Sweeney, 1989; Sweeney and Bumham, 1990; Hunt et al., 1991). Other authors (e.g., Price, 1983; Barker, 1989) have argued that vitrinite reflectance,
2. EXPERIMENTAL * Present address: Mineralogisch-Petrographisches versitlt Basel, CH 4056 Basel, Switzerland.
Institut,
Uni-
PROCEDURE
We conducted 39 experiments at pressures of 0.5, 1.0, 2.0, 10, and 20 kbar at temperatures of 200, 250, 300, and 350°C involving 2921
2922
M. Dalla Torre, R. F. Mshlmann,
run durations of 2610, 5500, and 9810 min (-2, 4, and 7 days) in order to investigate the effect of pressure on vitrinite reflectance. A charge of 80 mg of dry starting material stored at room temperature and constant humidity in the laboratory was sealed under argon atmosphere conditions into Pt-capsules of 5 and 3 mm diameter. No water was added in these runs. Two experiments were conducted under wet confining conditions-excess purified water was added to the dry starting material before sealing into the Pt-capsules. For simplification, we use the term “dry” experiment when no water was added to the starting material, and “wet” experiment when water was added in the runs. For pressures up to 2 kbar, cold-seal pressure cells in the hydrothermal laboratory at Stanford University were used. The starting material (with and without added water) sealed into Pt-capsules was loaded in pressure vessels connected to a pressure line. Then the system-pressure line and vessel-was flushed with water (confining pressure medium) to purge the air, pressurized to a value well below the final confining pressure, and then heated to the temperature of the run conditions. The pressure increased as a result of the expansion of the water, and was then adjusted to the final desired run pressure by monitoring a pressure gauge connected to the system. Experimental setups and procedures very similar to this have been used previously for artificial organic matter maturation rate studies (e.g., Michels and Landais, 1993; Landais et al., 1994; Michels et al. 1994. 1995a,b). Pressure and temperature uncertainties are on the order of ‘2°C and 225 bar in cold-seal pressure cells. Experiments at pressures above 2.0 kbar were conducted in the high-pressure piston-cylinder laboratory at the United States Geological Survey (USGS) in Menlo Park, California. Dry starting material sealed in Pt-capsules (no water added) were placed in a furnace assembly consisting of salt as pressure medium. The capsules were then loaded to a pressure vessel and pressurized to a value below the final pressure of the run conditions by mechanical compression using a steel-carbide piston. After bringing the temperature to its final value, the pressure was adjusted by monitoring a pressure gauge to match the run conditions. The experimental setup applied in this study is common in experimental petrology to generate ultrahigh pressure conditions (Bohlen, 1984). The precision of pressure and temperature is kO.1 kbar and t2”C, and accuracies are thought to be 20.5 kbar and 2 lO”C, respectively (Bohlen, 1984). The heating (run-up) time was less than 30 min in both types of experiments; we did not investigate the influence of different run-up times on vitrinite reflectance. Heating times in nature are magnitudes slower than the short run-up times of the experiments; therefore, this laboratory study provides an occasion to investigate exclusively the influence of effective heating time on the run products. Quench times at the termination of the experiment were less than 5 min. Before and after each experiment, each capsule was weighed to check for leaks. The starting material and residuals of the experiments embedded in epoxy were polished at Base1 University. After polishing, the samples were dried in the laboratory at room temperature. Random vitrinite reflectance (%R,) was determined in reflected monochromatic light of 546 nm wavelength using a Leitz Orthoplan-photome ter microscope calibrated with zero and a single standard, and an oil-immersion objective with magnification of 125~. The reflected light from the sample surface was introduced into a photomultiplier with an aperture of 2.5 ym’. Standard deviations range from O.Ol%R, at low rank stages to O.O3%R, at high rank stages. A total of 100 measurements were obtained per sample. The starting material, a lignite, gymnosperm wood (huminite), originates from the Frimmersdorf seam in the mining area of the Lower Rhine Basin, open cut Hambach, Germany. Macerals belonging to different maceral groups are easily distinguished from each other and form clarite layers rich in huminite and liptinite. The submacerals of the huminite group differ from one another in morphology: huminite occurs as telinite with an elongated, curledveined, cellular structure and shows a slightly higher reflectance than collinite. The tissue of the gymonosperm huminite is well preserved and the cell cavities are filled with structureless collinite slightly lower in reflectance, and with brownish resinite locally. This texture suggests that the starting material was only slightly affected by the process of compaction, peatification. humification, and gelifi-
and W. G. Ernst
cation. The very beginning of geochemical gelification--the conversion of huminite of peat into vitrinite “vitrinitization’‘-is microscopically well characterized. Throughout this text, we will use the term “vitrinite” for coaly material belonging to the huminite-vitrinite goup. The vitrinite reflectance value of the starting material is 0.197 +- 0.008% R,; the reflectance of maceral varieties ulminite and densinite components of the seam is slightly higher, around 0.30%R,. A petrographical and organic-geochemical description of the microlithotype and the seam was given by Dehmer ( 1988). In artificially matured samples, huminite appears to be entirely homogeneous; the difference in grey scale and reflectance between collinite cell filling and vitrinized cell walls is lost and therefore the cell structure is difficult to recognize. 3. RESULTS
3.1. Vitrinite
Reflectance
Data
The texture and vitrinite reflectance of the starting material reacted differently in response to varying dry experimental conditions. At temperatures of 200 and 25O”C, regardless of pressure, only minor liptinite (resinite, probably partly suberinite) decomposition occurred, whereas at 300 and 35O”C, liptinite generally disappeared at pressures 5 2.0 kbar, leaving behind the empty cell lumens, microfissures, and bedding plane joints. However, at the same temperatures, but at pressures of 10 and 20 kbar, liptinite is generally well preserved and shows only minor change to exsudatinite. Mean values of %R, plotted as a function of pressure, temperature, and time demonstrate that high pressure decreases the reflectance of vitrinite (Figs. I and 2). The re-
‘1.0
kbar}
i$ d
200
250
300
Temperature [“Cl
350
Fig. I. %R,-temperature plot for different pressures, showing that for a given run length and pressure, higher temperatures result in higher vitrinite reflectance values. Curves were calculated using Eqn. 4 for different pressures and run lengths.
2923
Pressure dependence of vitrinite reflectance %R, = k,t”
1.4
(1)
where k. is the rate constant, t is time, and n is a constant. We caution that the above and following equations are not meant to quantitatively describe the chemical and physical reaction mechanisms that lead to a certain vitrinite reflectance value. Equation 1 is regarded only as an empirical rate equation to express %R, as a function of pressure, temperature, and time. Rearranging Eqn. 1 by taking the natural logarithm and plotting In %R, vs. In t reveals a straight line with a slope of n. To illustrate any influence of time in our data on vitrinite reflectance, we used only the data from experiments at a pressure of 1 kbar and temperatures of 300 and 350°C (Fig. 3). Due to sparse vitrinite reflectance data, experiments conducted under conditions other than those were not considered. The mean value of best fits for n for the data used is 0.0714; a similar value (0.078) was found by Huang (1996). The rate constant, ko, was then recalculated, using Eqn. 1 with n = 0.0714. To express % R, or k. as a function of temperature, we used an Arrhenius-type expression:
1.0 0.6 0.2
k. = k, exp k”
0.6 0.2
2.0
0.5 hi:,,
[kbar]
Fig. 2. %I?,-pressure plot for different temperatures, showing that for a given run length and temperature, higher pressure results in lower vitrinite reflectance values. The curves were calculated using Eqn. 4 for different temperatures and run durations.
tarding effect of pressure is most evident at temperatures 2 250°C whereas little or no effect is observed in experiments of 200°C. This is primarily due to the fact that at the lower temperatures and run durations, the maturation rate is generally sluggish and more difficult to measure. Two samples treated under wet conditions at a pressure of 1.0 kbar at temperatures of 250 and 300°C yielded coal ranks of 0.66 and 0.91%R,, respectively. Samples from dry experiments treated at the same temperatures and pressure are 0.75 and 1 .l% R,, and thus yield a higher vitrinite reflectance than those from wet experiments. This suggests that the influence of pressure on maintaining low vitrinite reflectance values is larger under wet than under dry confining conditions (see Figs. 1 and 2). 3.2. Kinetic Analysis In the following kinetic analysis, we used only those data obtained from dry experiments. Our laboratory results show that vitrinite reflectance varies with experimental conditions, i.e., R, is a function of pressure, temperature, and time. To account for these variations, we analyzed the data using an empirical rate equation:
(2)
where k, is a pre-exponential term, E is an apparent activation energy, R is the gas constant (0.0083 14 k.I mole1 K -’), and T is absolute temperature. The term Arrhenius model is apt only in the broadest sense of the word, because the relationship between the complex changes in vitrinite and simple chemical reactions is vague (Burnham and Sweeney, 1989). However, linear regression of In k. vs. 1000/T for different pressure and temperature conditions revealed correlation coefficients (r’) ranging from 0.94 to 0.99 (see Fig.
1.8’ . ’ ”
” ” ” T = 350 “C
e k d
1.4-T
”
. ’ --a-
B A
LO--J-
CI-
T=300”C
-
0.6-
P=lkbar I
02-
2
I
4
1.8’ . ’ ”
* ’ . ’ ‘I ” _ T=350”C,P=lOkbar
1.4__e-l.O- 0
I
6
. ’’ 0
@
0.60.2-
I
2
d&s
6
time Fig. 3. %R,-time plot for different temperatures and pressures, showing that for a given temperature and pressure, longer heating durations result in higher vitrinite reflectance values. The data acquired at 300 and 350°C for a pressure of 1 kbar were used to fit the value of n in Eqn. 1.
2924
M. Dalla Torre, R. F. Mahlmann,
and W. G. Ernst
ki
3 1.6 3 0 1.2 -a 2 0.8 - ?? 20.0kbar -2.50 1.6 1.8 1.4
g 2.0
c?i 0.0 0.0
2.2
l/T x 1000 (“K) Fig. 4. Straight lines are obtained when plotting In (k,) versus 1 lT for the rate of vitrinite maturation. In all cases, the lines are parallel within uncertainty of the experiments. Higher pressure shifts the lines toward smaller In (k,) values.
4 for three
of the five experimental
datasets).
The regression
lines are parallel within uncertainty, indicating that E does conditions. not differ greatly with a range of experimental E values obtained from this procedure range from 23.40 to 27.78 kJ mol _I, with a mean at 24.99 kJ mol-’ (Table 1) Similar small apparent activation energies were also found by Huang ( 1996). We repeat that these apparent activation energies are only vaguely related to any chemical reaction activation energy. Regression lines of high-pressure experiments are shifted to smaller In k. values, implying a pressure dependence of the pre-exponential factor, k, , in Eqn. 2 (Fig. 4). To account for this variation, we used the relationship:
k, = k,P-“’
0.4 0.8
1.2
1.6
2.0
Rr[%] measured Fig. 5. A satisfactoty correlation is obtained when measured vitrinite reflectance data are compared with those calculated using Eqn. 4.
lated %R, data were compared using different numerical/ graphical methods (Fig. 6). Values for % R, calculated for a pressure of 0.5 kbar and a heating time of 1 m.y. are remarkably consistent with the time-dependent methods employed by Sweeney and Burnham ( 1990), and at values of %R, 2 4 and 55 with that by Huang ( 1996), and the timeindependent model by Barker (1988). For a heating time of 10 m.y., Eqn. 4 gives results that are consistent with the model of Sweeney and Bumham ( 1990). Calculations for a pressure of 10 kbar shift vitrinite maturation to higher temperatures (Fig. 6).
(3)
where k2 is a pre-exponential term, P is pressure in bars, and m is a constant. A linear regression of In k, versus In P revealed 195.195 bar s-’ for k2 and 0.131 form as best fits. To summarize, the equation obtained by the above procedure is: % R, = k2 P Y”
0.4
exp k:
-
Bumham (1990)
(4)
with k2 = 195.195 bar s-‘, m = 0.131, n = 0.0714, and E = 24.99 kJ mol-’ . Using the fitted data for n, m, E, and kz and Eqn. 4 results in a good correlation between % R, calculated by the equation used to generate our fitting curves and measured %R, data (Fig. 5). To place the obtained rate equation in perspective, calcu-
Table
1. Best fits for Eqn. 2. In k. = In k, - fT
for T = 200,
250, 300, and 350°C. The decrease in In k, with increasing pressure (except for the 2.0 kbar series) indicates the influence of pressure on vitrinite reflectance. P (kbar) 0.5
1.0 2.0 10.0 20.0
E (k.I mall’)
In k,
n
r2
23.55 26.17 24.06 23.40 27.78
5.036 4.080 4.498 3.927 3.541
0.07 14 0.0714 0.0714 0.0714 0.0714
0.985 0.960 0.939 0.956 0.996
50
150
250
Temperature [“Cl
350
Fig. 6. Comparison of calculated vitrinite reflectance data using different pressure-independent numerical models and Eqn. 4. Temperature conditions for the Diablo Range rocks from the Franciscan Complex are after Dalla Torre et al. ( 1996). For discussion, see text.
Pressure
dependence
4. DISCUSSION
4.1. Pressure
Effect
Pressure has been widely regarded by many geologists as having little influence on the coalification process, at least when compared to the controls of temperature and heating time (Teichmtiller and Teichmtiller, 1979; Stach et al., 1982; Murchison et al., 1985). However, the results of several field studies have suggested that vitrinite reflectance is occasionally an insensitive measure of increasing temperature in vitrinite-bearing strata under high pressures. Such sedimentary rocks typically experienced low-temperature (as defined in the introduction), but high-pressure metamorphism characterized by very low geothermal gradients ( - lo-20”C/km) and a typical mineral assemblage including, e.g., Mg-carpholite, lawsonite, or glaucophane (Bucher and Frey, 1994). Low coal ranks, for example, have been reported for the high-pressure (Fe-Mg-carpholite-bearing) schists from Vanoise, the Brianconnais zone of the French Alps. Vitrinite reflectance values are around 1.0 to lS%R, (Gaffe and Velde, 1984) and temperatures of 300°C at 6 kbar were computed for the metamorphic peak in this area (Gaffe, 1982). Different authors also reported low vitrinite reflectance values for the Diablo Range accretionary prism of the Franciscan subduction zone, covering, depending on the locality, a range from 1.2 to 2S%R, (Dalla Torre et al., 1996) and I .l-2.1%R, (Bostick, 1970, 1974). Phase equilibria indicate metamorphic temperatures around 300°C and pressures of 210 kbar (Dalla Torre et al., 1996) -similar values were reported by Patrick and Day ( 1989), whereas slightly lower temperatures and pressures were estimated by Ernst ( 1993). For the high-pressure lawsonite-bearing rocks of New Caledonia (Diessel et al., 1978; Kisch, 1987) coal ranks are between 2.86 and 4.0% R,, and minimum temperatures of 255°C at pressures of 3 kbar were estimated based on phase equilibria. For the high-pressure Fe-glaucophanebearing units of the same area, vitrinite reflectance values range from 4.77 to 6.53%R,; minimum temperatures are 300°C at pressures of 6.5 kbar (Diessel et al., 1978; Kisch, 1987). These field data suggest that in the high-pressure zones of New Caledonia, metamorphic conditions derived from vitrinite reflectance are in line with physical conditions estimated based on mineral stabilities. In contrast, for highpressure rocks from the Vanoise and the Diablo Range, metamorphic conditions derived from organic matter are too low if compared to temperatures obtained from phase assemblages, and it has been suggested that high pressure suppresses vitrinite maturation in these areas (Dalla Torre et al., 1994, 1996). Conclusions similar to these were obtained by Carr ( 199 1) , based on the examination of datasets from the Central and Viking Grabens in the North Sea. Recent studies have shed some light on chemical properties of kerogen as a function of experimental conditions. Price and Wenger ( 1992) conducted aqueous-pyrolysis experiments using kerogen from the Retort Phosphatic Shale from the Lower Permian Phosphoria Formation and found that high pressure retards all aspects of organic matter metamorphism, including hydrocarbon generation, maturation, and thermal destruction. Michels et al. ( 1994, 1995a,b) pyrolized extracted kerogen from Woodford Shale in sealed gold
of vitrinite
reflectance
2925
tubes at pressures ranging from 0.3 to 1.3 kbar and temperatures of 260 to 400°C in the presence and absence of water. Elemental analyses and rock-Eva1 pyrolysis of residuals suggested that various organic species react differently in response to both pressure and nature of the pressurizing medium; therefore, Michels et al. (1994) emphazised that a pressure effect cannot simply be related to a global organic maturation retardation or acceleration. Michels et al. ( 1995a) found that wet confining pressure lowers the total yields of bitumen and reduces the consumption of oil potential in the extracted solid residue more than relative to dry confining conditions. Experiments similar to those above were performed by Monthioux et al. ( 1985), who heated powders of concentrated organic matter in sealed gold tubes at temperatures ranging from 250 to 550°C and pressures of 0.5 to 4.0 kbar, with and without water. Their reaction products, in contrast, did not change appreciably in response to increasing pressure in agreement with experiments performed later by Landais et al. ( 1994). Although, knowledge of the chemical variation of organic matter with experimental conditions is crucial to a further understanding of reactions during organic matter maturation, the influence of compositional variations on optical properties of vitrinite are difficult to evaluate. Controlled laboratory experiments to investigate the effect of pressure on vitrinite reflectance have been rather limited, and many of the results are inconsistent. Chandra (1965) exposed coal powders to maximum pressures of -6 kbar (88,200 psi) at temperatures of 325 and 350°C. His results were erratic and showed only modest changes in reflectance anisotropy with increasing pressure. Maximum reflectance actually increased at higher pressures, and the pressure-induced effects were greater at higher temperature. Hryckowian et al. (1967) tested anthracite coals in hydrothermal vessels over a temperature range of 400 to 900°C at pressures of only 0.14-1.4 kbar (2,000-20,000 psi). Their results show that changes in reflectance anisotropy are definitely related to both applied pressure and temperature, but that temperature is the more important factor; on the other hand, there were no definite trends in the variation of mean reflectance with total pressure. Goodarzi ( 1985) investigated optical properties of dry (unspecified) vitrinite matured at pressures ranging from 1.O to 3.0 kbar and high temperatures from 350 to 600°C. At 550 and 600°C he found slightly larger vitrinite reflectance values for samples treated at 1.0 kbar than for those that matured at pressures above 1.5 kbar. However, at a value exceeding 1.0 kbar but at the same temperature, vitrinite reflectance was insensitive to relative pressure differences. In addition, at the lower of his high temperatures (350-450°C)) the run products did not show any effect of pressure on vitrinite reflectance. Data such as those presented by Goodarzi ( 1985) are difficult to interpret and extrapolate to geological temperatures because there is no obvious reason why chemical reactions that occur at high temperature also occur at temperatures typical for sedimentary basins and low-temperature metamorphic regions. Systematic experiments on pressure dependence of vitrinite reflectance involving changes in temperature, static load pressure up to 2.5 kbar, and vapor pressure were conducted by Sajgo et al. ( 1986). High load pressures in both open and quasi-closed systems suppressed vitrinite maturation to a
2926
considerable
M. Dalla Torre, R. F. Mlhlmann,
extent,
whereas
the process
was accelerated
in
only 0.06 kbar of vapor pressure. Dry confined pyrolysis on a starting coal with R. of 0.5% by Hill et al. (1994) at temperatures of 300 and 340°C first showed an increase in vitrinite reflectance up to a pressure of 0.6 kbar, and then a decrease from 0.6 to 2.0 kbar. These authors concluded that suppression of vitrinite reflectance in response to pressure is secondary; however, neglecting it could lead to errors in the interpretation of maturity based on vitrinite reflectance. The most recent systematic experiments on optical properties of vitrinite as a function of pressure, temperature, and time were conducted by Huang ( 1996) using lignite as starting material. Run durations were as long as 305 days, whereas the highest pressures applied were only 2.0 kbar. Huang ( 1996) concluded that temperature, rather than time, is the major factor controlling vitrinite reflectance. However, in contrast to the results obtained by Hill et al. ( 1994), samples treated at 2.0 kbar did not show any significant difference in vitrinite reflectance compared to those matured at 0.1 and 0.5 kbar under wet conditions. The results obtained in the present study show that high pressure suppresses the maturation rate of the starting material used here under both dry and wet conditions, and are generally in line with those of Sajgo et al. ( 1986), Michels et al. (1995a), Carr (1991), Price and Wenger (1992), and Hill et al. (.1994), but in marked contrast with data presented by Huang ( 1996), Landais et al. ( 1994), and Monthioux et al. (1985). This short literature review shows that the results of previous studies are inconsistent and indicates that the effect of pressure on vitrinite maturation is not well understood. Dalla Torre et al. ( 1996) argued that if vitrinite maturation were suppressed by high pressure, one might expect that a pressure effect would apply equally to all the high pressure/lowtemperature metamorphic terranes, which is not the case. We believe that the observed inconsistances can be explained by a number of factors. Durand et al. (1986) and Huang (1996), for example, proposed that the original hydrogen content in the starting material is crucial for the evolution of vitrinite reflectance. Alternatively, different reaction mechanisms may occur under different natural and experimental conditions (see also Huang, 1996). High shear stresses have also been shown to further complicate the evolution of vitrinite reflectance in natural rocks-field studies showed an increase in the anisotropy of vitrinite reflectance in shear zones (e.g., Stach et al., 1982; Teichmtiller, 1987). Moreover, Teichmtiller ( 1987) argued that rock porosity and the reorientation of vitrinite particles owing to deformation may effect maturation. All of these factors are assumed to influence the evolution of vitrinite reflectance; however, available experimental and held data show that, under certain circumstances, high pressure suppresses vitrinite maturation measurably. This implies that a pressure-dependent rate equation must be used to derive estimates of paleotemperatures for regions affected by high pressure (Carr, 199 1). Calculated % R, data obtained from the simple rate equation presented here using high pressure compare favorably with data from high-pressure rocks from the Diablo Range (Fig. 6). Pressure-independent methods for the calculation of % R, after Sweeney and Buma quasi-closed
system
with
and W. G. Ernst
ham ( 1990), assuming a heating time of 1 m.y., and the time-independent method after Barker ( 1988) yield 230 and 260°C. Using our preliminary model with 10 kbar yields a temperature of 290°C: this estimate appears to be more consistent with recent temperature estimates by Dalla Torre et al. ( 1996). On the other hand, our rate equation cannot explain the low vitrinite reflectance data from the high-pressure schists of the Vanoise (Gaffe and Velde, 1984) and from schists of the pumpellyite-actinolite facies in the Alpine nappes of Grisons (see Ferreiro-Mahlmann, 1995, 1996), suggesting that organic matter from these areas reacted differently-possibly due to the reasons stated above-from those used in our experiments. Definitive work on pressure retardation of vitrinite reflectance, therefore, must involve experiments conducted under dry and wet conditions using starting materials of different composition. Despite the differences in opinion about the influence of pressure on vitrinite reflectance, the influence of time in the laboratory was found to be rather similar (Huang, 1996; this study). Although the influence of time on vitrinite reflectance is less well documented in our experiments than in those by Huang ( 1996), similar values for y1were obtained (see above). In addition, the apparent activation energies in both studies are similarly low: a value of 2,578 was given by Huang (1996; m in his Eqn. 4), which is equivalent to E/R in Eqns. 2 and 4 of this study, resulting in values ranging from 2,815 to 3,341 for different pressure conditions. 5. CONCLUSIONS Organic material with an initial R, value of 0.19% was experimentally matured (no water added) over a pressure range of 0.5 to 20.0 kbar at temperatures of 200 to 350°C and run durations of -2, -4, and -7 days; the results show that applied pressure suppresses vitrinite reflectance. The addition of water to the starting material appears to further enhance the pressure effect. Our data suggest that a definitive kinetic model of vitrinite maturation should involve not only temperature and time, but should also account for pressure in order to allow accurate interpretation of geothermal histories of deep sedimentary basins and low-temperature/highpressure metamorphosed regions. Acknowledgments-We thank W. Tschudin and T. Fischer for the sample preparation, M. Wolf for providing the starting material, B. Hankins and C. Jove for help and introduction to the cold-seal and piston-cylinder laboratories, and I. Stebbins for the use of his glove box. The manuscript benefitted greatly from comments by N. Bostick and an anonymous reviewer. Comments on an earlier version by B. R. Hacker, J. Dahl, E. Gnos, and A. K. Burnham are greatly appreciated. This study was financially supported by the Swiss National Science Foundation, fellowship grant 81BS-44414 to M.D.T., as well as by Stanford and Base1 Universities, and the USGS.
REFERENCES Barker C. E. ( 1983 ) Influence of time on metamorphism of sedimentary organic matter in liquid-dominated geothermal systems, western North America. Geology 11, 384-388. Barker C. E. (1988) Geothermics of petroleum systems: Implications of the stabilization of kerogen thermal maturation after a geologically brief heating duration at peak temperature. In Petro-
Pressure
dependence
leum Systems of the United States (ed. L. B. Magoon); United States Geol. Bull. 1870, 26-29. Barker C. E. ( 1989) Temperature and time in the thermal maturation of sedimentary organic matter. In Thermal History of Sedimentary Basins (ed. N. D. Naeser and T. H. McCulloh), pp. 73-98. Springer. Barker C. E. (1991) Implications for organic maturation studies of evidence for a geologically rapid increase and stabilization of vitrinite reflectance at peak temperature: Cerro Prieto geothermal system, Mexico. AAPG Bull. 75, 1852-1863. Barker C. E. and Pawlewicz M. _I.( 1986) The correlation of vitrinite reflectance with maximum temperature in humic organic matter. In Paleogeothermics (ed. G. Buntebarth and L. Stegena), pp. 7993. Springer. Barker C. E. and Pawlewicz M. J. (1994) Calculation of vitrinite reflectance from thermal histories and peak temperatures. In Vitrinite Reflectance as a Maturity Parameter, Applications and Limitations (ed. P. K. Mukhopadhyay and W. G. Dow); Amer. Chem. Sot. Symp. Ser. 570, pp. 216-229. Bohlen S. R. ( 1984) Equilibria for precise pressure calibration and frictionless furnace assembly for the piston-cylinder apparatus. Neues Jhrbk Mineral. Monatsh. 9, 404-412. Bostick N. H. ( 1970) Thermal alteration of clastc organic particles (phytoclasts) as an indicator of contact and burial metamorphism in sedimentary rocks. Ph.D. thesis, Stanford University. Bostick N. H. ( 1974) Phytoclasts as indicators of thermal metamorphism, Franciscan assemblage and Great Vallay sequence (Upper Mezocoic), California. Geol. Sot. Amer. Spec. Pap. 153, 1-17. Bostick N. H., Cashman S., McColloh T. H., and Wade11 C. T. ( 1978) Gradients of vitrinite reflectance and and present temperature in the Los Angeles and Ventura Basins, California. In Symposium in Geochemistry: Low Temperature Metamorphism of Kerogen and Clay Minerals (ed. D. Oltz), pp. 65-95. Sot. Econo. Paleontol. Mineral. Pacific Sec. Bucher K. and Frey M. ( 1994) Metamorphic grade. In Petrogrogenesis of MetamorphicRocks (ed. K. Bucher and M. Frey), pp. 99144. Springer Verlag. Burnham A. K. and Sweeney J. J. (1989) A chemical kinetic model of vitrinite maturation and reflectance. Geochim. Cosmochim. Acta 53,2649-2657. Carr A. D. ( 1991) A pressure dependent kinetic model for vitrinite reflectance. In Organic Geochemistry: Advances and Applications in Energy and the Natural Environment (ed. D. Manning), pp. 285-287. Europ. Assoc. Org. Geochim. Chandra D. ( 1965) Reflectance of coals carbonized under pressure. Econ. Geol. 60,621-629. Dalla Terre M., DeCapitani C., Frey M., and Underwood M. B. ( 1994) Very low-grade metamorphism of shales from the Diablo Range, Franciscan Complex, California. Eos 75, 699. Dalla Terre M., De Capitani C., Frey M., Underwood M. B., Mullis J., and Cox R. ( 1996) Very low-grade metamorphism of shales from the Diablo Range, Franciscan Complex, California, USA: New constraints on the exhumation path. Geol. Sot. Amer. Bull. 108, 578-601. Dehmer J. ( 1988) Petrographische und organisch-geochemische Untersuchungen an rezenten Torfen und terti%ren Braunkohlen-Ein Beitrag zur Fazies und Genese gebanderter gebanderter Braunkohlen. Ph.D. dissertation, Univ. Aachen. Diessel C. F. K., Brothers R. N., and Black P. M. (1978) Coalification and graphitisation in high-pressure schists in New Caledonia. Contrib. Mineral. Petrol. 68, 63-78. Durand B., Alpern B., Pittion J. L. and Pradier B. (1986) Reflectance of vitrinite as a control of thermal history of sediments. In Thermal Modeling in Sedimentary Basins (ed. J. Burrus), pp. 441-414. 1st IFP Expl. Res. Conf. Carcans, France, Ed. Tech, Paris. Ernst W. G. (1993) Metamorphism of Franciscan tectonostratigraphic assemblage, Pacheco Pass area, east-central Diablo Range, California Coast Ranges. Geol. Sot. Amer. Bull. 105,618-636. Ferreiro-Mlhlmann R. ( 1995) Das Diagenese-Metamorphose-Muster von Vitrinitreflexion und Illit-“Kristallinitat” in Mittelbttnden und im Oberhalbstein. Teil 1. Beztige zur Stockwerktektonik. Schweiz. Mineral. Petrograph. Mitt. 75/l, 85- 122.
of vitrinite
reflectance
2927
Ferreiro-Mahlmann R. ( 1996) Das Diagenese-Metamorphose-Muster von Vitrinitreflexion und Illit-“Kristallinitlt” in Mittelbtinden und im Oberhalbstein. Teil 2. Korrelation kohlenpetrographischer und mineralogischer Parameter. Schweiz. Mineral. Petrograph. Mitt. 76/l, 23-46. Frey M. and Kisch H. J. (1987) Scope of the Subject. In Low Temperature Metamorphism (ed. M. Frey), pp. l-8. Blackie. Frey M. et al. ( 1980) Very low grade metamorphism in external parts of the Central Alps: Illite crystallinity, coal rank, and fluid inclusion data. Eclogae Geol. Helvet. 73/l, 173-203. Gaffe B. (1982) Definition du facies a FeMg carpholite-chloritoide, un marqueur de mttamorphisme de HP-BT dans les mttasediments alumineux. Ph.D. dissertation, Univ. P. et M. Curie. Gaffe B. and Velde B. ( 1984) Contrasted metamorphic evolutions of thrusted cover units of the BrianGonnais zone (French Alps): A model for the conservation of HP-LT metamorphic mineral assemblages. Earth Planet. Sci. Lett. 68, 351-360. Goodarzi F. (1985) Optical properties of vitrinite carbonized at different pressures. Fuel 64, 156- 162. Hill R. J., Jenden P. D., Tang Y. C., Teerman S. C., and Kaplan I. R. (1994) Influence of pressure on pyrolysis of coal. In Vitrmite Reflectance as a Maturity Papameter: Applications and Limitations (ed. P. K. Mukhopadhyay and W. G. Dow); Amer. Chem. Sac. Symp. Ser. 570, pp. 161-193. Hood A., Gutjahr C. C. M., and Heacock R. L. (1975) Organic metamorphism and the generation of petroleum. AAPG Bull. 59,986996. Hryckowian E., Dutcher R. R., and Dachille F. ( 1967) Experimental studies of anthracite coals at high pressures and temperatures. Econ. Geol. 62, 517-539. Huang Wuu-Liang. ( 1996) Experimental study of vitrinite maturation: Effects of temperature, time, pressure, water, and hydrogen index. Org. Geochem. 24, 233-241. Hunt J. M., Lewan M. D., and Hennet R. J-C. (1991) Modeling oil generation with time-temperature index graphs based on the Arrhenius equation. AAPG Bull. 75, 795-807. Kisch H. J. (1987) Correlation between indicators of very low-grade metamorphism. In Low Temperature Metamorphism (ed. M. Frey), pp. 226-300. Blackie. Landais P., Michels R., and Elie M. ( 1994) Are time and temperature the only constraints to the simulation of organic matter maturation? Adv. Org. Geochem. 22, 617-630. Littke R. ( 1993) Deposition, Diagenesis and Weathering of Organic Matter-rich Sediments. Springer. Michels R. and Landais R. (1993) Artificial coalification: Comparison of confined pyrolysis and hydrous pyrolysis. Energy Fuels 73, 1691-1696. Michels R., Landais P., Philp R. P., and Torkelson B. E. (1994) Effects of pressure on organic matter matuartion during confined pyrolysis of Woodford kerogen. Energy Fuels 8, 741-754. Michels R., Landais P., Torkelson B. E., and Philp R. P. (1995a) Effects of effluents and water pressure on oil generation during confined pyrolysis and high-pressure hydrous pyrolysis. Geochim. Cosmochim. Acta 59, 204-215. Michels R., Landais P., Philp R. P., and Torkelson B. E. (1995b) Influence of pressure and the presence of water on the evolution of the residual kerogen during confined, hydrous, and high-pressure hydrous pyrolysis of woodford shale. Energy Fuels 9, 204-215. Monthioux M., Landais P., and Monin J-C. (1985) Comparison between natural and artificial maturation series of humic coals from the Mahakam delta, Indonesia. Org. Geochem. 8, 275-292. Morrow D. W. and Issler D. R. (1993) Calculation of vitrinite reflectance from thermal histories: A comparison of some methods. AAPG Bull. 77, 610-624. Murchison D. G., Cook A. C., and Raymond A. C. (1985) Optical properties of organic matter in relation to thermal gradients and structural deformation. Phil. Trans. Roy. Sot. London 315, 157186. Patrick B. E. and Day H. W. (1989) Controls on the first appearance of jadeite, northern Diablo Range, California. J. Metamorph. Geol. 7, 629-639. Price L. C. ( 1983) Geologic time as a parameter in organic metamor-
2928
M. Dalla Torre. R. F. Mahlmann,
phism and vitrinite reflectance as an absolute paleogeothermometer. J. Petrol. Geol. 6, 5-38. Price L. C. and Wenger L. M. (1992) The influence of pressure on petroleum generation and maturation as suggested by aqueous pyrolysis. Org. Geochim. 19, 141-159. Sajgo C., McEvoy J., Wolff G. A., and Horvath Z. A. ( 1986) Influence of temperature and pressure on maturation processes-I. Preliminary report. Org. Geochem. 10, 33 1-337. Stach E., Mackowsky M., Teichmtiller M., Taylor G. H., Chandra D., and Teichmtiller R. ( 1982) Stach’s Textbook of Coal Petrology, 3rd. ed. Gebrtider Borntraeger. Sweeney J. J. and Burnham A. K. (1990) Evaluation of a simple mode1 of vitrinite reflectance based on chemical kinetics. AAPG Bull. 74, 1559-1570. Teichmiiller M. ( 1987) Organic material and very low-grade meta-
and W. G. Ernst
(ed. M. Frey), morphism. In Low Temperature Metamorphism pp. l14- 161. Blackie. Teichmtiller M.. and Teichmtiller R. (1979) Diagenesis of coal (coalification). In Dia~enesis in Sediments und Sedimentuq Rocks (ed. G. Larsen and G. V. Chilingar), pp. 207-246. Elsevier. Tissot B. P. and Welte D. H. ( 1984) Petroleum Formation and Occurrence. Springer. Underwood M. B. et al. ( 1991) Correlation among paleotemperature indicators within erogenic belts: Examples from pelitic rocks of the Franciscan Complex (California), the Shimanto Belt (Japan), and the Kandik Basin (Alaska). (abstr.) Eos 72, 549. Waples D. W. ( 1980) Time and temperature in petroleum formation: Application of Lopatin’s method to petroleum exploration. AAPG Bull. 64, 936-926.
et Cosmochimica
Pergamon
Acta, Vol. 61, No. 14, pp. 2921-2928, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All rights reserved
0016.7037/97 $17.00 + .OO
PI1 SOO16-7037( 97)00104-X
Experimental
study on the pressure
dependence
M. DALLA TORRE, I,* R. FERREIRO MAHLMANN,~ ‘Department
of Geological
and Environmental
‘Mineralogisch-Petrographisches (Received
November
Sciences,
Stanford
University,
of vitrinite
maturation
and W. G. ERNST’ Stanford,
California
94305-2115,
USA
Institut, Base1 University, CH 4056 Base], Switzerland
13, 1996; accepted
in revised form February
28, 1997)
Abstract-We conducted the first systematic experiments over a pressure range of 0.5 to 20.0 kbar, at temperatures of 200 to 35o”C, and run durations of -2, -4, and -7 days to investigate the influence of pressure on vitrinite reflectance. The data indicate that applied pressure lowers vitrinite reflectance; the suppression appears to be stronger under wet than under dry conditions. In addition, high pressure favors liptinite preservation at temperatures 2 3OO”C, whereas at the same temperature range, yet at low pressure, liptinite is decomposed. Our data fit an empirical rate equation which expresses %R, as a function of pressure, temperature, and time. The derived equation satisfactorily describes our experiments and enables extrapolations to geological timescales. Values of % R, calculated for a pressure of 0.5 kbar are remarkably consistant with previous numerical pressure-independent models. A literature review shows that previous experimental data and field observations are inconsistent regarding the effect of pressure on vitrinite maturation-this and some previous studies suggest that vitrinite maturation is suppressed by increased pressure, whereas other workers regarded pressure as having little influence on the rate of coalification. We suggest that original vitrinite composition, experimental design, and natural environment (tectonic regime) play important roles regarding the effects of pressure on vitrinite maturation. Interpretation of geothermal histories of sedimentary rocks based on vitrinite reflectance data must, therefore, include evaluation of the pressure. Otherwise, vitrinite reflectance data from high-pressure terranes should be intemreted with caution if used as a quantitative paleogeothermometer. Copyright 0 1997 Elsevier Scienck Ltd 1. INTRODUCTION
under normal circumstances, stabilizes rapidly within the framework of a geologic timescale-within as little as ten years (Barker, 1991). Estimates of paleotemperature can be calculated directly from empirical regression curves of borehole temperature vs. % R,. Different vitrinite reflectance geothermometer curves need to be used for rapid heating in geothermal fields, as opposed to slow burial in first-cycle sedimentary basins (Barker, 1983, 1988; Barker and Pawlewicz, 1986). Critical comparisons among the various methods have been published recently by Littke ( 1993), Morrow and Issler ( 1993), and Barker and Pawlewicz ( 1994). While it is well established that temperature and time are major factors controlling the attainment of a certain vitrinite reflectance value, the influence of pressure is largely unknown; various studies have suggested that vitrinite reflectance may be an insensitive measure of increasing temperature in vitrinite-bearing strata under high pressures (Goffk and Velde, 1984; Dalla Terre et al., 1994, 1996). The quantitative understanding of modifications of optical properties of vitrinite during the geothermal history is crucial in situations where oil exploration and other scientists rely on vitrinite reflectance as a thermal maturity indicator. Surprisingly, the effect of pressure, important in most geological environments, has not been investigated experimentally in a systematic fashion. This laboratory study measures the effect of pressure on vitrinite reflectance and presents a preliminary empirical rate equation that describes this pressure effect.
Understanding variations of optical properties of vitrinite as a function of temperature, pressure, and time is crucial for the economic and scientific interpretation of the geothermal histories of sedimentary basins and low-temperature metamorphic rocks; hydrocarbon exploration in sedimentary basins relies strongly on vitrinite reflectance as a thermal maturity indicator, and vitrinite reflectance is applied as a paleogeothermometer in regions affected by low-temperature metamorphism (Frey et al., 1980; Kisch, 1987; Teichmiiller, 1987; Underwood et al., 1991). Low-grade metamorphism is used here to indicate the lowest grades of metamorphism, up to the beginning of greenschist facies conditions, covering a temperature range from 150-200 to approximately 35O”C, according to the terminology after Frey and Kisch ( 1987). To interpret the geothermal history or maturity of vitrinitebearing sedimentary rocks, estimates of paleotemperature are obtained from vitrinite reflectance values using different numerical maturity models. Because the maturation of thermodynamically metastable organic matter is an expression of reaction rate, both temperature and effective heating time are important (e.g., Hood et al., 1975; Waples, 1980), and sophisticated models (e.g., EASY%Ro) have been formulated based on multiple-reaction Arrhenius-based chemical kinetics (Bumham and Sweeney, 1989; Sweeney and Bumham, 1990; Hunt et al., 1991). Other authors (e.g., Price, 1983; Barker, 1989) have argued that vitrinite reflectance,
2. EXPERIMENTAL * Present address: Mineralogisch-Petrographisches versitlt Basel, CH 4056 Basel, Switzerland.
Institut,
Uni-
PROCEDURE
We conducted 39 experiments at pressures of 0.5, 1.0, 2.0, 10, and 20 kbar at temperatures of 200, 250, 300, and 350°C involving 2921
2922
M. Dalla Torre, R. F. Mshlmann,
run durations of 2610, 5500, and 9810 min (-2, 4, and 7 days) in order to investigate the effect of pressure on vitrinite reflectance. A charge of 80 mg of dry starting material stored at room temperature and constant humidity in the laboratory was sealed under argon atmosphere conditions into Pt-capsules of 5 and 3 mm diameter. No water was added in these runs. Two experiments were conducted under wet confining conditions-excess purified water was added to the dry starting material before sealing into the Pt-capsules. For simplification, we use the term “dry” experiment when no water was added to the starting material, and “wet” experiment when water was added in the runs. For pressures up to 2 kbar, cold-seal pressure cells in the hydrothermal laboratory at Stanford University were used. The starting material (with and without added water) sealed into Pt-capsules was loaded in pressure vessels connected to a pressure line. Then the system-pressure line and vessel-was flushed with water (confining pressure medium) to purge the air, pressurized to a value well below the final confining pressure, and then heated to the temperature of the run conditions. The pressure increased as a result of the expansion of the water, and was then adjusted to the final desired run pressure by monitoring a pressure gauge connected to the system. Experimental setups and procedures very similar to this have been used previously for artificial organic matter maturation rate studies (e.g., Michels and Landais, 1993; Landais et al., 1994; Michels et al. 1994. 1995a,b). Pressure and temperature uncertainties are on the order of ‘2°C and 225 bar in cold-seal pressure cells. Experiments at pressures above 2.0 kbar were conducted in the high-pressure piston-cylinder laboratory at the United States Geological Survey (USGS) in Menlo Park, California. Dry starting material sealed in Pt-capsules (no water added) were placed in a furnace assembly consisting of salt as pressure medium. The capsules were then loaded to a pressure vessel and pressurized to a value below the final pressure of the run conditions by mechanical compression using a steel-carbide piston. After bringing the temperature to its final value, the pressure was adjusted by monitoring a pressure gauge to match the run conditions. The experimental setup applied in this study is common in experimental petrology to generate ultrahigh pressure conditions (Bohlen, 1984). The precision of pressure and temperature is kO.1 kbar and t2”C, and accuracies are thought to be 20.5 kbar and 2 lO”C, respectively (Bohlen, 1984). The heating (run-up) time was less than 30 min in both types of experiments; we did not investigate the influence of different run-up times on vitrinite reflectance. Heating times in nature are magnitudes slower than the short run-up times of the experiments; therefore, this laboratory study provides an occasion to investigate exclusively the influence of effective heating time on the run products. Quench times at the termination of the experiment were less than 5 min. Before and after each experiment, each capsule was weighed to check for leaks. The starting material and residuals of the experiments embedded in epoxy were polished at Base1 University. After polishing, the samples were dried in the laboratory at room temperature. Random vitrinite reflectance (%R,) was determined in reflected monochromatic light of 546 nm wavelength using a Leitz Orthoplan-photome ter microscope calibrated with zero and a single standard, and an oil-immersion objective with magnification of 125~. The reflected light from the sample surface was introduced into a photomultiplier with an aperture of 2.5 ym’. Standard deviations range from O.Ol%R, at low rank stages to O.O3%R, at high rank stages. A total of 100 measurements were obtained per sample. The starting material, a lignite, gymnosperm wood (huminite), originates from the Frimmersdorf seam in the mining area of the Lower Rhine Basin, open cut Hambach, Germany. Macerals belonging to different maceral groups are easily distinguished from each other and form clarite layers rich in huminite and liptinite. The submacerals of the huminite group differ from one another in morphology: huminite occurs as telinite with an elongated, curledveined, cellular structure and shows a slightly higher reflectance than collinite. The tissue of the gymonosperm huminite is well preserved and the cell cavities are filled with structureless collinite slightly lower in reflectance, and with brownish resinite locally. This texture suggests that the starting material was only slightly affected by the process of compaction, peatification. humification, and gelifi-
and W. G. Ernst
cation. The very beginning of geochemical gelification--the conversion of huminite of peat into vitrinite “vitrinitization’‘-is microscopically well characterized. Throughout this text, we will use the term “vitrinite” for coaly material belonging to the huminite-vitrinite goup. The vitrinite reflectance value of the starting material is 0.197 +- 0.008% R,; the reflectance of maceral varieties ulminite and densinite components of the seam is slightly higher, around 0.30%R,. A petrographical and organic-geochemical description of the microlithotype and the seam was given by Dehmer ( 1988). In artificially matured samples, huminite appears to be entirely homogeneous; the difference in grey scale and reflectance between collinite cell filling and vitrinized cell walls is lost and therefore the cell structure is difficult to recognize. 3. RESULTS
3.1. Vitrinite
Reflectance
Data
The texture and vitrinite reflectance of the starting material reacted differently in response to varying dry experimental conditions. At temperatures of 200 and 25O”C, regardless of pressure, only minor liptinite (resinite, probably partly suberinite) decomposition occurred, whereas at 300 and 35O”C, liptinite generally disappeared at pressures 5 2.0 kbar, leaving behind the empty cell lumens, microfissures, and bedding plane joints. However, at the same temperatures, but at pressures of 10 and 20 kbar, liptinite is generally well preserved and shows only minor change to exsudatinite. Mean values of %R, plotted as a function of pressure, temperature, and time demonstrate that high pressure decreases the reflectance of vitrinite (Figs. I and 2). The re-
‘1.0
kbar}
i$ d
200
250
300
Temperature [“Cl
350
Fig. I. %R,-temperature plot for different pressures, showing that for a given run length and pressure, higher temperatures result in higher vitrinite reflectance values. Curves were calculated using Eqn. 4 for different pressures and run lengths.
2923
Pressure dependence of vitrinite reflectance %R, = k,t”
1.4
(1)
where k. is the rate constant, t is time, and n is a constant. We caution that the above and following equations are not meant to quantitatively describe the chemical and physical reaction mechanisms that lead to a certain vitrinite reflectance value. Equation 1 is regarded only as an empirical rate equation to express %R, as a function of pressure, temperature, and time. Rearranging Eqn. 1 by taking the natural logarithm and plotting In %R, vs. In t reveals a straight line with a slope of n. To illustrate any influence of time in our data on vitrinite reflectance, we used only the data from experiments at a pressure of 1 kbar and temperatures of 300 and 350°C (Fig. 3). Due to sparse vitrinite reflectance data, experiments conducted under conditions other than those were not considered. The mean value of best fits for n for the data used is 0.0714; a similar value (0.078) was found by Huang (1996). The rate constant, ko, was then recalculated, using Eqn. 1 with n = 0.0714. To express % R, or k. as a function of temperature, we used an Arrhenius-type expression:
1.0 0.6 0.2
k. = k, exp k”
0.6 0.2
2.0
0.5 hi:,,
[kbar]
Fig. 2. %I?,-pressure plot for different temperatures, showing that for a given run length and temperature, higher pressure results in lower vitrinite reflectance values. The curves were calculated using Eqn. 4 for different temperatures and run durations.
tarding effect of pressure is most evident at temperatures 2 250°C whereas little or no effect is observed in experiments of 200°C. This is primarily due to the fact that at the lower temperatures and run durations, the maturation rate is generally sluggish and more difficult to measure. Two samples treated under wet conditions at a pressure of 1.0 kbar at temperatures of 250 and 300°C yielded coal ranks of 0.66 and 0.91%R,, respectively. Samples from dry experiments treated at the same temperatures and pressure are 0.75 and 1 .l% R,, and thus yield a higher vitrinite reflectance than those from wet experiments. This suggests that the influence of pressure on maintaining low vitrinite reflectance values is larger under wet than under dry confining conditions (see Figs. 1 and 2). 3.2. Kinetic Analysis In the following kinetic analysis, we used only those data obtained from dry experiments. Our laboratory results show that vitrinite reflectance varies with experimental conditions, i.e., R, is a function of pressure, temperature, and time. To account for these variations, we analyzed the data using an empirical rate equation:
(2)
where k, is a pre-exponential term, E is an apparent activation energy, R is the gas constant (0.0083 14 k.I mole1 K -’), and T is absolute temperature. The term Arrhenius model is apt only in the broadest sense of the word, because the relationship between the complex changes in vitrinite and simple chemical reactions is vague (Burnham and Sweeney, 1989). However, linear regression of In k. vs. 1000/T for different pressure and temperature conditions revealed correlation coefficients (r’) ranging from 0.94 to 0.99 (see Fig.
1.8’ . ’ ”
” ” ” T = 350 “C
e k d
1.4-T
”
. ’ --a-
B A
LO--J-
CI-
T=300”C
-
0.6-
P=lkbar I
02-
2
I
4
1.8’ . ’ ”
* ’ . ’ ‘I ” _ T=350”C,P=lOkbar
1.4__e-l.O- 0
I
6
. ’’ 0
@
0.60.2-
I
2
d&s
6
time Fig. 3. %R,-time plot for different temperatures and pressures, showing that for a given temperature and pressure, longer heating durations result in higher vitrinite reflectance values. The data acquired at 300 and 350°C for a pressure of 1 kbar were used to fit the value of n in Eqn. 1.
2924
M. Dalla Torre, R. F. Mahlmann,
and W. G. Ernst
ki
3 1.6 3 0 1.2 -a 2 0.8 - ?? 20.0kbar -2.50 1.6 1.8 1.4
g 2.0
c?i 0.0 0.0
2.2
l/T x 1000 (“K) Fig. 4. Straight lines are obtained when plotting In (k,) versus 1 lT for the rate of vitrinite maturation. In all cases, the lines are parallel within uncertainty of the experiments. Higher pressure shifts the lines toward smaller In (k,) values.
4 for three
of the five experimental
datasets).
The regression
lines are parallel within uncertainty, indicating that E does conditions. not differ greatly with a range of experimental E values obtained from this procedure range from 23.40 to 27.78 kJ mol _I, with a mean at 24.99 kJ mol-’ (Table 1) Similar small apparent activation energies were also found by Huang ( 1996). We repeat that these apparent activation energies are only vaguely related to any chemical reaction activation energy. Regression lines of high-pressure experiments are shifted to smaller In k. values, implying a pressure dependence of the pre-exponential factor, k, , in Eqn. 2 (Fig. 4). To account for this variation, we used the relationship:
k, = k,P-“’
0.4 0.8
1.2
1.6
2.0
Rr[%] measured Fig. 5. A satisfactoty correlation is obtained when measured vitrinite reflectance data are compared with those calculated using Eqn. 4.
lated %R, data were compared using different numerical/ graphical methods (Fig. 6). Values for % R, calculated for a pressure of 0.5 kbar and a heating time of 1 m.y. are remarkably consistent with the time-dependent methods employed by Sweeney and Burnham ( 1990), and at values of %R, 2 4 and 55 with that by Huang ( 1996), and the timeindependent model by Barker (1988). For a heating time of 10 m.y., Eqn. 4 gives results that are consistent with the model of Sweeney and Bumham ( 1990). Calculations for a pressure of 10 kbar shift vitrinite maturation to higher temperatures (Fig. 6).
(3)
where k2 is a pre-exponential term, P is pressure in bars, and m is a constant. A linear regression of In k, versus In P revealed 195.195 bar s-’ for k2 and 0.131 form as best fits. To summarize, the equation obtained by the above procedure is: % R, = k2 P Y”
0.4
exp k:
-
Bumham (1990)
(4)
with k2 = 195.195 bar s-‘, m = 0.131, n = 0.0714, and E = 24.99 kJ mol-’ . Using the fitted data for n, m, E, and kz and Eqn. 4 results in a good correlation between % R, calculated by the equation used to generate our fitting curves and measured %R, data (Fig. 5). To place the obtained rate equation in perspective, calcu-
Table
1. Best fits for Eqn. 2. In k. = In k, - fT
for T = 200,
250, 300, and 350°C. The decrease in In k, with increasing pressure (except for the 2.0 kbar series) indicates the influence of pressure on vitrinite reflectance. P (kbar) 0.5
1.0 2.0 10.0 20.0
E (k.I mall’)
In k,
n
r2
23.55 26.17 24.06 23.40 27.78
5.036 4.080 4.498 3.927 3.541
0.07 14 0.0714 0.0714 0.0714 0.0714
0.985 0.960 0.939 0.956 0.996
50
150
250
Temperature [“Cl
350
Fig. 6. Comparison of calculated vitrinite reflectance data using different pressure-independent numerical models and Eqn. 4. Temperature conditions for the Diablo Range rocks from the Franciscan Complex are after Dalla Torre et al. ( 1996). For discussion, see text.
Pressure
dependence
4. DISCUSSION
4.1. Pressure
Effect
Pressure has been widely regarded by many geologists as having little influence on the coalification process, at least when compared to the controls of temperature and heating time (Teichmtiller and Teichmtiller, 1979; Stach et al., 1982; Murchison et al., 1985). However, the results of several field studies have suggested that vitrinite reflectance is occasionally an insensitive measure of increasing temperature in vitrinite-bearing strata under high pressures. Such sedimentary rocks typically experienced low-temperature (as defined in the introduction), but high-pressure metamorphism characterized by very low geothermal gradients ( - lo-20”C/km) and a typical mineral assemblage including, e.g., Mg-carpholite, lawsonite, or glaucophane (Bucher and Frey, 1994). Low coal ranks, for example, have been reported for the high-pressure (Fe-Mg-carpholite-bearing) schists from Vanoise, the Brianconnais zone of the French Alps. Vitrinite reflectance values are around 1.0 to lS%R, (Gaffe and Velde, 1984) and temperatures of 300°C at 6 kbar were computed for the metamorphic peak in this area (Gaffe, 1982). Different authors also reported low vitrinite reflectance values for the Diablo Range accretionary prism of the Franciscan subduction zone, covering, depending on the locality, a range from 1.2 to 2S%R, (Dalla Torre et al., 1996) and I .l-2.1%R, (Bostick, 1970, 1974). Phase equilibria indicate metamorphic temperatures around 300°C and pressures of 210 kbar (Dalla Torre et al., 1996) -similar values were reported by Patrick and Day ( 1989), whereas slightly lower temperatures and pressures were estimated by Ernst ( 1993). For the high-pressure lawsonite-bearing rocks of New Caledonia (Diessel et al., 1978; Kisch, 1987) coal ranks are between 2.86 and 4.0% R,, and minimum temperatures of 255°C at pressures of 3 kbar were estimated based on phase equilibria. For the high-pressure Fe-glaucophanebearing units of the same area, vitrinite reflectance values range from 4.77 to 6.53%R,; minimum temperatures are 300°C at pressures of 6.5 kbar (Diessel et al., 1978; Kisch, 1987). These field data suggest that in the high-pressure zones of New Caledonia, metamorphic conditions derived from vitrinite reflectance are in line with physical conditions estimated based on mineral stabilities. In contrast, for highpressure rocks from the Vanoise and the Diablo Range, metamorphic conditions derived from organic matter are too low if compared to temperatures obtained from phase assemblages, and it has been suggested that high pressure suppresses vitrinite maturation in these areas (Dalla Torre et al., 1994, 1996). Conclusions similar to these were obtained by Carr ( 199 1) , based on the examination of datasets from the Central and Viking Grabens in the North Sea. Recent studies have shed some light on chemical properties of kerogen as a function of experimental conditions. Price and Wenger ( 1992) conducted aqueous-pyrolysis experiments using kerogen from the Retort Phosphatic Shale from the Lower Permian Phosphoria Formation and found that high pressure retards all aspects of organic matter metamorphism, including hydrocarbon generation, maturation, and thermal destruction. Michels et al. ( 1994, 1995a,b) pyrolized extracted kerogen from Woodford Shale in sealed gold
of vitrinite
reflectance
2925
tubes at pressures ranging from 0.3 to 1.3 kbar and temperatures of 260 to 400°C in the presence and absence of water. Elemental analyses and rock-Eva1 pyrolysis of residuals suggested that various organic species react differently in response to both pressure and nature of the pressurizing medium; therefore, Michels et al. (1994) emphazised that a pressure effect cannot simply be related to a global organic maturation retardation or acceleration. Michels et al. ( 1995a) found that wet confining pressure lowers the total yields of bitumen and reduces the consumption of oil potential in the extracted solid residue more than relative to dry confining conditions. Experiments similar to those above were performed by Monthioux et al. ( 1985), who heated powders of concentrated organic matter in sealed gold tubes at temperatures ranging from 250 to 550°C and pressures of 0.5 to 4.0 kbar, with and without water. Their reaction products, in contrast, did not change appreciably in response to increasing pressure in agreement with experiments performed later by Landais et al. ( 1994). Although, knowledge of the chemical variation of organic matter with experimental conditions is crucial to a further understanding of reactions during organic matter maturation, the influence of compositional variations on optical properties of vitrinite are difficult to evaluate. Controlled laboratory experiments to investigate the effect of pressure on vitrinite reflectance have been rather limited, and many of the results are inconsistent. Chandra (1965) exposed coal powders to maximum pressures of -6 kbar (88,200 psi) at temperatures of 325 and 350°C. His results were erratic and showed only modest changes in reflectance anisotropy with increasing pressure. Maximum reflectance actually increased at higher pressures, and the pressure-induced effects were greater at higher temperature. Hryckowian et al. (1967) tested anthracite coals in hydrothermal vessels over a temperature range of 400 to 900°C at pressures of only 0.14-1.4 kbar (2,000-20,000 psi). Their results show that changes in reflectance anisotropy are definitely related to both applied pressure and temperature, but that temperature is the more important factor; on the other hand, there were no definite trends in the variation of mean reflectance with total pressure. Goodarzi ( 1985) investigated optical properties of dry (unspecified) vitrinite matured at pressures ranging from 1.O to 3.0 kbar and high temperatures from 350 to 600°C. At 550 and 600°C he found slightly larger vitrinite reflectance values for samples treated at 1.0 kbar than for those that matured at pressures above 1.5 kbar. However, at a value exceeding 1.0 kbar but at the same temperature, vitrinite reflectance was insensitive to relative pressure differences. In addition, at the lower of his high temperatures (350-450°C)) the run products did not show any effect of pressure on vitrinite reflectance. Data such as those presented by Goodarzi ( 1985) are difficult to interpret and extrapolate to geological temperatures because there is no obvious reason why chemical reactions that occur at high temperature also occur at temperatures typical for sedimentary basins and low-temperature metamorphic regions. Systematic experiments on pressure dependence of vitrinite reflectance involving changes in temperature, static load pressure up to 2.5 kbar, and vapor pressure were conducted by Sajgo et al. ( 1986). High load pressures in both open and quasi-closed systems suppressed vitrinite maturation to a
2926
considerable
M. Dalla Torre, R. F. Mlhlmann,
extent,
whereas
the process
was accelerated
in
only 0.06 kbar of vapor pressure. Dry confined pyrolysis on a starting coal with R. of 0.5% by Hill et al. (1994) at temperatures of 300 and 340°C first showed an increase in vitrinite reflectance up to a pressure of 0.6 kbar, and then a decrease from 0.6 to 2.0 kbar. These authors concluded that suppression of vitrinite reflectance in response to pressure is secondary; however, neglecting it could lead to errors in the interpretation of maturity based on vitrinite reflectance. The most recent systematic experiments on optical properties of vitrinite as a function of pressure, temperature, and time were conducted by Huang ( 1996) using lignite as starting material. Run durations were as long as 305 days, whereas the highest pressures applied were only 2.0 kbar. Huang ( 1996) concluded that temperature, rather than time, is the major factor controlling vitrinite reflectance. However, in contrast to the results obtained by Hill et al. ( 1994), samples treated at 2.0 kbar did not show any significant difference in vitrinite reflectance compared to those matured at 0.1 and 0.5 kbar under wet conditions. The results obtained in the present study show that high pressure suppresses the maturation rate of the starting material used here under both dry and wet conditions, and are generally in line with those of Sajgo et al. ( 1986), Michels et al. (1995a), Carr (1991), Price and Wenger (1992), and Hill et al. (.1994), but in marked contrast with data presented by Huang ( 1996), Landais et al. ( 1994), and Monthioux et al. (1985). This short literature review shows that the results of previous studies are inconsistent and indicates that the effect of pressure on vitrinite maturation is not well understood. Dalla Torre et al. ( 1996) argued that if vitrinite maturation were suppressed by high pressure, one might expect that a pressure effect would apply equally to all the high pressure/lowtemperature metamorphic terranes, which is not the case. We believe that the observed inconsistances can be explained by a number of factors. Durand et al. (1986) and Huang (1996), for example, proposed that the original hydrogen content in the starting material is crucial for the evolution of vitrinite reflectance. Alternatively, different reaction mechanisms may occur under different natural and experimental conditions (see also Huang, 1996). High shear stresses have also been shown to further complicate the evolution of vitrinite reflectance in natural rocks-field studies showed an increase in the anisotropy of vitrinite reflectance in shear zones (e.g., Stach et al., 1982; Teichmtiller, 1987). Moreover, Teichmtiller ( 1987) argued that rock porosity and the reorientation of vitrinite particles owing to deformation may effect maturation. All of these factors are assumed to influence the evolution of vitrinite reflectance; however, available experimental and held data show that, under certain circumstances, high pressure suppresses vitrinite maturation measurably. This implies that a pressure-dependent rate equation must be used to derive estimates of paleotemperatures for regions affected by high pressure (Carr, 199 1). Calculated % R, data obtained from the simple rate equation presented here using high pressure compare favorably with data from high-pressure rocks from the Diablo Range (Fig. 6). Pressure-independent methods for the calculation of % R, after Sweeney and Buma quasi-closed
system
with
and W. G. Ernst
ham ( 1990), assuming a heating time of 1 m.y., and the time-independent method after Barker ( 1988) yield 230 and 260°C. Using our preliminary model with 10 kbar yields a temperature of 290°C: this estimate appears to be more consistent with recent temperature estimates by Dalla Torre et al. ( 1996). On the other hand, our rate equation cannot explain the low vitrinite reflectance data from the high-pressure schists of the Vanoise (Gaffe and Velde, 1984) and from schists of the pumpellyite-actinolite facies in the Alpine nappes of Grisons (see Ferreiro-Mahlmann, 1995, 1996), suggesting that organic matter from these areas reacted differently-possibly due to the reasons stated above-from those used in our experiments. Definitive work on pressure retardation of vitrinite reflectance, therefore, must involve experiments conducted under dry and wet conditions using starting materials of different composition. Despite the differences in opinion about the influence of pressure on vitrinite reflectance, the influence of time in the laboratory was found to be rather similar (Huang, 1996; this study). Although the influence of time on vitrinite reflectance is less well documented in our experiments than in those by Huang ( 1996), similar values for y1were obtained (see above). In addition, the apparent activation energies in both studies are similarly low: a value of 2,578 was given by Huang (1996; m in his Eqn. 4), which is equivalent to E/R in Eqns. 2 and 4 of this study, resulting in values ranging from 2,815 to 3,341 for different pressure conditions. 5. CONCLUSIONS Organic material with an initial R, value of 0.19% was experimentally matured (no water added) over a pressure range of 0.5 to 20.0 kbar at temperatures of 200 to 350°C and run durations of -2, -4, and -7 days; the results show that applied pressure suppresses vitrinite reflectance. The addition of water to the starting material appears to further enhance the pressure effect. Our data suggest that a definitive kinetic model of vitrinite maturation should involve not only temperature and time, but should also account for pressure in order to allow accurate interpretation of geothermal histories of deep sedimentary basins and low-temperature/highpressure metamorphosed regions. Acknowledgments-We thank W. Tschudin and T. Fischer for the sample preparation, M. Wolf for providing the starting material, B. Hankins and C. Jove for help and introduction to the cold-seal and piston-cylinder laboratories, and I. Stebbins for the use of his glove box. The manuscript benefitted greatly from comments by N. Bostick and an anonymous reviewer. Comments on an earlier version by B. R. Hacker, J. Dahl, E. Gnos, and A. K. Burnham are greatly appreciated. This study was financially supported by the Swiss National Science Foundation, fellowship grant 81BS-44414 to M.D.T., as well as by Stanford and Base1 Universities, and the USGS.
REFERENCES Barker C. E. ( 1983 ) Influence of time on metamorphism of sedimentary organic matter in liquid-dominated geothermal systems, western North America. Geology 11, 384-388. Barker C. E. (1988) Geothermics of petroleum systems: Implications of the stabilization of kerogen thermal maturation after a geologically brief heating duration at peak temperature. In Petro-
Pressure
dependence
leum Systems of the United States (ed. L. B. Magoon); United States Geol. Bull. 1870, 26-29. Barker C. E. ( 1989) Temperature and time in the thermal maturation of sedimentary organic matter. In Thermal History of Sedimentary Basins (ed. N. D. Naeser and T. H. McCulloh), pp. 73-98. Springer. Barker C. E. (1991) Implications for organic maturation studies of evidence for a geologically rapid increase and stabilization of vitrinite reflectance at peak temperature: Cerro Prieto geothermal system, Mexico. AAPG Bull. 75, 1852-1863. Barker C. E. and Pawlewicz M. _I.( 1986) The correlation of vitrinite reflectance with maximum temperature in humic organic matter. In Paleogeothermics (ed. G. Buntebarth and L. Stegena), pp. 7993. Springer. Barker C. E. and Pawlewicz M. J. (1994) Calculation of vitrinite reflectance from thermal histories and peak temperatures. In Vitrinite Reflectance as a Maturity Parameter, Applications and Limitations (ed. P. K. Mukhopadhyay and W. G. Dow); Amer. Chem. Sot. Symp. Ser. 570, pp. 216-229. Bohlen S. R. ( 1984) Equilibria for precise pressure calibration and frictionless furnace assembly for the piston-cylinder apparatus. Neues Jhrbk Mineral. Monatsh. 9, 404-412. Bostick N. H. ( 1970) Thermal alteration of clastc organic particles (phytoclasts) as an indicator of contact and burial metamorphism in sedimentary rocks. Ph.D. thesis, Stanford University. Bostick N. H. ( 1974) Phytoclasts as indicators of thermal metamorphism, Franciscan assemblage and Great Vallay sequence (Upper Mezocoic), California. Geol. Sot. Amer. Spec. Pap. 153, 1-17. Bostick N. H., Cashman S., McColloh T. H., and Wade11 C. T. ( 1978) Gradients of vitrinite reflectance and and present temperature in the Los Angeles and Ventura Basins, California. In Symposium in Geochemistry: Low Temperature Metamorphism of Kerogen and Clay Minerals (ed. D. Oltz), pp. 65-95. Sot. Econo. Paleontol. Mineral. Pacific Sec. Bucher K. and Frey M. ( 1994) Metamorphic grade. In Petrogrogenesis of MetamorphicRocks (ed. K. Bucher and M. Frey), pp. 99144. Springer Verlag. Burnham A. K. and Sweeney J. J. (1989) A chemical kinetic model of vitrinite maturation and reflectance. Geochim. Cosmochim. Acta 53,2649-2657. Carr A. D. ( 1991) A pressure dependent kinetic model for vitrinite reflectance. In Organic Geochemistry: Advances and Applications in Energy and the Natural Environment (ed. D. Manning), pp. 285-287. Europ. Assoc. Org. Geochim. Chandra D. ( 1965) Reflectance of coals carbonized under pressure. Econ. Geol. 60,621-629. Dalla Terre M., DeCapitani C., Frey M., and Underwood M. B. ( 1994) Very low-grade metamorphism of shales from the Diablo Range, Franciscan Complex, California. Eos 75, 699. Dalla Terre M., De Capitani C., Frey M., Underwood M. B., Mullis J., and Cox R. ( 1996) Very low-grade metamorphism of shales from the Diablo Range, Franciscan Complex, California, USA: New constraints on the exhumation path. Geol. Sot. Amer. Bull. 108, 578-601. Dehmer J. ( 1988) Petrographische und organisch-geochemische Untersuchungen an rezenten Torfen und terti%ren Braunkohlen-Ein Beitrag zur Fazies und Genese gebanderter gebanderter Braunkohlen. Ph.D. dissertation, Univ. Aachen. Diessel C. F. K., Brothers R. N., and Black P. M. (1978) Coalification and graphitisation in high-pressure schists in New Caledonia. Contrib. Mineral. Petrol. 68, 63-78. Durand B., Alpern B., Pittion J. L. and Pradier B. (1986) Reflectance of vitrinite as a control of thermal history of sediments. In Thermal Modeling in Sedimentary Basins (ed. J. Burrus), pp. 441-414. 1st IFP Expl. Res. Conf. Carcans, France, Ed. Tech, Paris. Ernst W. G. (1993) Metamorphism of Franciscan tectonostratigraphic assemblage, Pacheco Pass area, east-central Diablo Range, California Coast Ranges. Geol. Sot. Amer. Bull. 105,618-636. Ferreiro-Mlhlmann R. ( 1995) Das Diagenese-Metamorphose-Muster von Vitrinitreflexion und Illit-“Kristallinitat” in Mittelbttnden und im Oberhalbstein. Teil 1. Beztige zur Stockwerktektonik. Schweiz. Mineral. Petrograph. Mitt. 75/l, 85- 122.
of vitrinite
reflectance
2927
Ferreiro-Mahlmann R. ( 1996) Das Diagenese-Metamorphose-Muster von Vitrinitreflexion und Illit-“Kristallinitlt” in Mittelbtinden und im Oberhalbstein. Teil 2. Korrelation kohlenpetrographischer und mineralogischer Parameter. Schweiz. Mineral. Petrograph. Mitt. 76/l, 23-46. Frey M. and Kisch H. J. (1987) Scope of the Subject. In Low Temperature Metamorphism (ed. M. Frey), pp. l-8. Blackie. Frey M. et al. ( 1980) Very low grade metamorphism in external parts of the Central Alps: Illite crystallinity, coal rank, and fluid inclusion data. Eclogae Geol. Helvet. 73/l, 173-203. Gaffe B. (1982) Definition du facies a FeMg carpholite-chloritoide, un marqueur de mttamorphisme de HP-BT dans les mttasediments alumineux. Ph.D. dissertation, Univ. P. et M. Curie. Gaffe B. and Velde B. ( 1984) Contrasted metamorphic evolutions of thrusted cover units of the BrianGonnais zone (French Alps): A model for the conservation of HP-LT metamorphic mineral assemblages. Earth Planet. Sci. Lett. 68, 351-360. Goodarzi F. (1985) Optical properties of vitrinite carbonized at different pressures. Fuel 64, 156- 162. Hill R. J., Jenden P. D., Tang Y. C., Teerman S. C., and Kaplan I. R. (1994) Influence of pressure on pyrolysis of coal. In Vitrmite Reflectance as a Maturity Papameter: Applications and Limitations (ed. P. K. Mukhopadhyay and W. G. Dow); Amer. Chem. Sac. Symp. Ser. 570, pp. 161-193. Hood A., Gutjahr C. C. M., and Heacock R. L. (1975) Organic metamorphism and the generation of petroleum. AAPG Bull. 59,986996. Hryckowian E., Dutcher R. R., and Dachille F. ( 1967) Experimental studies of anthracite coals at high pressures and temperatures. Econ. Geol. 62, 517-539. Huang Wuu-Liang. ( 1996) Experimental study of vitrinite maturation: Effects of temperature, time, pressure, water, and hydrogen index. Org. Geochem. 24, 233-241. Hunt J. M., Lewan M. D., and Hennet R. J-C. (1991) Modeling oil generation with time-temperature index graphs based on the Arrhenius equation. AAPG Bull. 75, 795-807. Kisch H. J. (1987) Correlation between indicators of very low-grade metamorphism. In Low Temperature Metamorphism (ed. M. Frey), pp. 226-300. Blackie. Landais P., Michels R., and Elie M. ( 1994) Are time and temperature the only constraints to the simulation of organic matter maturation? Adv. Org. Geochem. 22, 617-630. Littke R. ( 1993) Deposition, Diagenesis and Weathering of Organic Matter-rich Sediments. Springer. Michels R. and Landais R. (1993) Artificial coalification: Comparison of confined pyrolysis and hydrous pyrolysis. Energy Fuels 73, 1691-1696. Michels R., Landais P., Philp R. P., and Torkelson B. E. (1994) Effects of pressure on organic matter matuartion during confined pyrolysis of Woodford kerogen. Energy Fuels 8, 741-754. Michels R., Landais P., Torkelson B. E., and Philp R. P. (1995a) Effects of effluents and water pressure on oil generation during confined pyrolysis and high-pressure hydrous pyrolysis. Geochim. Cosmochim. Acta 59, 204-215. Michels R., Landais P., Philp R. P., and Torkelson B. E. (1995b) Influence of pressure and the presence of water on the evolution of the residual kerogen during confined, hydrous, and high-pressure hydrous pyrolysis of woodford shale. Energy Fuels 9, 204-215. Monthioux M., Landais P., and Monin J-C. (1985) Comparison between natural and artificial maturation series of humic coals from the Mahakam delta, Indonesia. Org. Geochem. 8, 275-292. Morrow D. W. and Issler D. R. (1993) Calculation of vitrinite reflectance from thermal histories: A comparison of some methods. AAPG Bull. 77, 610-624. Murchison D. G., Cook A. C., and Raymond A. C. (1985) Optical properties of organic matter in relation to thermal gradients and structural deformation. Phil. Trans. Roy. Sot. London 315, 157186. Patrick B. E. and Day H. W. (1989) Controls on the first appearance of jadeite, northern Diablo Range, California. J. Metamorph. Geol. 7, 629-639. Price L. C. ( 1983) Geologic time as a parameter in organic metamor-
2928
M. Dalla Torre. R. F. Mahlmann,
phism and vitrinite reflectance as an absolute paleogeothermometer. J. Petrol. Geol. 6, 5-38. Price L. C. and Wenger L. M. (1992) The influence of pressure on petroleum generation and maturation as suggested by aqueous pyrolysis. Org. Geochim. 19, 141-159. Sajgo C., McEvoy J., Wolff G. A., and Horvath Z. A. ( 1986) Influence of temperature and pressure on maturation processes-I. Preliminary report. Org. Geochem. 10, 33 1-337. Stach E., Mackowsky M., Teichmtiller M., Taylor G. H., Chandra D., and Teichmtiller R. ( 1982) Stach’s Textbook of Coal Petrology, 3rd. ed. Gebrtider Borntraeger. Sweeney J. J. and Burnham A. K. (1990) Evaluation of a simple mode1 of vitrinite reflectance based on chemical kinetics. AAPG Bull. 74, 1559-1570. Teichmiiller M. ( 1987) Organic material and very low-grade meta-
and W. G. Ernst
(ed. M. Frey), morphism. In Low Temperature Metamorphism pp. l14- 161. Blackie. Teichmtiller M.. and Teichmtiller R. (1979) Diagenesis of coal (coalification). In Dia~enesis in Sediments und Sedimentuq Rocks (ed. G. Larsen and G. V. Chilingar), pp. 207-246. Elsevier. Tissot B. P. and Welte D. H. ( 1984) Petroleum Formation and Occurrence. Springer. Underwood M. B. et al. ( 1991) Correlation among paleotemperature indicators within erogenic belts: Examples from pelitic rocks of the Franciscan Complex (California), the Shimanto Belt (Japan), and the Kandik Basin (Alaska). (abstr.) Eos 72, 549. Waples D. W. ( 1980) Time and temperature in petroleum formation: Application of Lopatin’s method to petroleum exploration. AAPG Bull. 64, 936-926.