Vitrinite Reflectance as a Maturity Parameter - ACS Publications

Chapter 14

Calculation of Vitrinite Reflectance from Thermal Histories and Peak Temperatures A Comparison of Methods 1,2

Charles E . Barker Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: November 9, 1994 | doi: 10.1021/bk-1994-0570.ch014

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and M a r k J . Pawlewicz

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U.S. Geological Survey, Box 25046, M S 971, Denver, C O 80225 University of Adelaide, South Australia 5005, Australia 2

One purpose of vitrinite reflectance, a thermal maturation parameter, is to characterize the degree of heating in sedimentary rocks. This use of vitrinite reflectance makes its calibration to peak temperature (T ) important because T constitutes an absolute thermal maturation parameter. This paper compares the two major methods available for predicting thermal maturation through vitrinite reflectance evolution: thermal history (kinetic models) and T (vitrinite reflectance geothermometers, VRG). V R G applied to the published cases of burial heating show that the mean random vitrinite reflectance (R ), predicted from kinetic models closely agrees with the measured R when a realistic or measured T value is used in setting the thermal history. Similarly, a published comparison of these two types of V R G and kinetic models in an active hydrothermal metamorphism (geothermal) system suggests that V R G give predictions that are a better match to present day temperatures. peak

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The purpose of thermal maturation parameters, such as vitrinite reflectance, is to measure the degree of heating. This use of vitrinite reflectance makes its calibration to peak temperature (Tp k) important because T k constitutes an absolute thermal maturation parameter. T p k is also a key inflection point in burial history reconstruction and is a physically significant point with which to compare the organic geochemistry of samples. Thermal maturation parameters are widely used to correlate with other physical parameters. One of these physical parameters, Tp j^, is a fundamental physical value rather than a thermal maturation parameter defined by an analytical method, such as mean random vitrinite reflectance (R _ ). Studies using temperature-based comparisons rather than thermal maturation parameters have an obvious advantage in that standards and instrumentation are readily available worldwide and its connection to other physical sciences is quantitative. ea

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This chapter not subject to U.S. copyright Published 1994 American Chemical Society

In Vitrinite Reflectance as a Maturity Parameter; Mukhopadhyay, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Calculation of Vitrinite Reflectance

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Tpeak * limited in its application to sedimentary basins because direct measurements are possible only in strata that can be demonstrated to be at the highest temperature attained in their geologic history. In rocks that have cooled, can be determined by using fluid inclusions, apatite fission track analysis (AFTA) or measurements on equilibrium mineral assemblages. AFTA studies are limited in that tracks are not generally retained above about 100°C. Studies of equilibrium mineral assemblages are also limited in that during burial heating, equilibration can be difficult to attain or may not be recorded by mineral precipitation. These problems make it advantageous to measure other more commonly available and easily measured thermal maturation parameters. Thus, T p k is presently not considered a replacement, but supplementary, to vitrinite reflectance, AFTA, or other thermal maturation parameters. The reason that other thermal maturation parameters are needed is because R _ is subjective in its measurement and its accuracy should be assessed in each case. Subjectivity in R _ measurements results from a microscope operator selecting the vitrinite particle that has its reflectance measured quantitatively. Thus, the scatter in R _ data is often attributable to differences in laboratory technique and problems with finding and measuring the same vitrinite group maceral between samples. These operational and methodological factors restrict the precision to one decimal place even though the precision of the physical operation (i.e. microphotometry) is 0.01%. Other physical and chemical changes such as reflectance suppression and pressure can also locally affect vitrinite reflectance evolution. Such problems, although limiting the effectiveness of R - in some cases, do not invalidate the general use of such data. Two major methods are available for predicting thermal maturation through vitrinite reflectance evolution: thermal history (kinetic models) and T p ^ (vitrinite reflectance geothermometers, VRG). The thermal history method sums, over the entire burial history of a rock package, the extent of R . evolution reactions that are modeled using kinetic equations. The T p k method considers that chemical reactions up to peak temperature are dominant in setting the cumulative R . evolution. In other words, kinetic models allow time as an unlimited factor and assume that time can be effective in R _ evolution after marked temperature decreases or after hundreds of millions of years at constant temperature. V R G consider time to be a factor limited in effectiveness to the time as T p k is approached. We think the most effective way to confirm and compare these methods is by using cases formulated by other scientists. For this reason, this paper focuses on calibration of V R G and their comparison to published kinetic models for determining vitrinite reflectance evolution. The examples used in this comparison are the hypothetical and actual geological case studies of burial heating systems presented by Morrow and Issler (1) and Bray et al. (2). For geothermal systems, which are considered as cases of active hydrothermal metamorphism, we use data from Whelan et al. (3) for comparison. ea

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: November 9, 1994 | doi: 10.1021/bk-1994-0570.ch014

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Vitrinite Reflectance Geothermometry Previous Calibrations. Previous calibrations of V R G by us were made using published data, in which T p ^ was estimated from geologic reconstruction and by grouping burial heating and hydrothermal metamorphism environments together. This grouping of data produced a scattered data set (4) but one that had a surprisingly high coefficient of e a


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VITRINITE REFLECTANCE AS A MATURITY PARAMETER

correlation. The scatter in the calibration of (4) obscured any indication that different burial heating and hydrothermal metamorphism environments may follow different ther mal maturation paths. However, the point of (4) was that there was a significant corre lation between T p k and R _ and that time must have a reduced effect. Much of the scatter in this calibration is now attributed to: 1) differences in R . measurements be tween laboratories (5); 2) grouping equilibrium temperature log data from geothermal systems with uncorrected temperature log data from burial heating cases; 3) the rela tively crude estimates of Τ ^ from burial history reconstruction methods; and 4) dif ferences in heating rate and pressure (6). However, this scatter is not due to using ran dom reflectance ( R . ) rather than maximum reflectance at high thermal maturity, be cause R _ is strongly correlated with maximum reflectance (7) over a wide reflectance range. Barker and Goldstein (8) also grouped burial heating and hydrothermal metamor phism paths together, even though two subtle trends in the data were present, because of the perception that no difference in thermal maturation at a given T k should be present regardless of the thermal maturation path. The main point of (8) was to demonstrate that fluid inclusions can reequilibrate and approach T p k and, as support for this argu ment, demonstrate a strong relationship with R . which is also thought to be controlled ea

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ty'TpeakThe grouping of data from hydrothermal metamorphism and burial heating environ ments in all of these calibrations produces a curve that predicts temperatures somewhat high for burial heating in the upper temperature ranges. New insights on the effect of heating rate and pressure on vitrinite reflectance evolution suggest it is best to develop regression equations for each environment to produce an optimal calibration for the more accurate prediction (as done in Barker (9) and expanded in Aizawa (10) and Barkerfii) rather than force Tp j^ and R . data from diverse environments into a single curve as has been done in the past. The different thermal maturation paths are thought to be due to physical differences in each environment, such as pressure and the ability of the environment to allow es cape or removal of reaction products generated during thermal maturation (6). Differ ences in heating duration and heating rate are seemingly of reduced importance because the systems used in calibrating these paths span a wide range of elapsed time and heat ing rate. Heating duration is thought to be effective only over a short term reaction period in which the stabilization of organic matter thermal maturation occurs (11,12). ea

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Present Calibration. Calibration of the present V R G is based on a direct estimate of Tpeak- This estimate can be made by determining the upper mode in a distribution of reequilibrated fluid-inclusion homogenization temperatures (T^) (8). A minimum esti mate of Tp fc is also possible using the upper limit of the T^ range measured in second ary fluid inclusions. T p k estimated from fluid inclusion data, is more precise because it is a direct temperature measurement made using rocks from the system and is inde pendent of the uncertainty present in burial history reconstruction. The improved resolution of T p ^ from fluid inclusions data indicates that \dtrinite reflectance evolution during burial heating, hydrothermal metamorphism and contact metamorphism by dikes follow different paths. Hydrothermal metamorphism includes extinct systems, which often formed hydrothermal ore deposits, and active geothermal ea

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systems. Contact metamorphism by dikes is discussed by Barker et al. (13), but not further here, as research on this issue is still in progress. Tpeak and R _ data from an updated compilation based mostly on Barker (9), Aizawa (10), and Barker and Goldstein (8) and unpublished data are used to calibrate a geothermometer for the burial heating and hydrothermal metamorphism paths. We also group the results based on gross heating rate given by burial heating and hydrothermal metamorphism thermal maturation paths. Reduced major axis regression (Fig. 1; Table I) suggests a calibration curve of : v

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Data scatter about these different thermal maturation paths is attributed to the sec ond order effects of heating rate, pressure and system closure within each environment, measurement error in both variables, and so forth. Comparison of Kinetic Models and Vitrinite Reflectance Geothermometry Burial heating. Morrow and Issler (1) provided a thorough review of the theoretical basis of kinetic models and such a discussion is not repeated here. Note that in the following discussion the % Ro symbol used in ( 1 ) for vitrinite reflectance represents the mean random vitrinite reflectance measured in oil (R _ ,as described above). Figure 2 shows R _ calculated by Morrow and Issler (1, see their figure 2) for selected kinetic models for a constant temperature persisting over a 200 m.y. reaction duration. Note that V R G predictions are comparable near an elapsed time of about 10 m.y. The V R G predictions are about 10 to 20% lower for an elapsed time of 200 m.y than the predictions of the more conservative kinetic models presented by (1). The low R _ predictions at very large elapsed times for time-limited models are expected be cause V R G do not consider increasing R _ after T p k is reached. This stabilization of vitrinite reflectance evolution is in contrast to kinetic models that theoretically can al low vitrinite reflectance to increase forever. Figure 3 shows the idealized temperature histories presented by Morrow and Issler (1; see theirfigure5) along with R _ values estimated from the burial heating V R G presented in this paper. In both cases, the R . estimates based on the V R G show close agreement with those of the kinetic models. Figure 4 shows the R _ values computed from an estimate of peak paleotemperature of 117°C based on apatite fission track analysis (AFTA) on samples from a depth of 805 m in the Esso Bissette well (1; see their Fig. 6). The bar through the R _ predictions shows the computed range based on the acceptable Τ ρ ^ values quoted by (1). These V R G predictions overlap with the predictions from the kinetic models and the mea sured R _ Note that Morrow and Issler (1) conclude that none of the estimates from kinetic models fit the data very well in this reconstruction. Perhaps suppression of \dtrinite reflectance is a possible explanation for the poor fit (19,20). v

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Figure 2. Comparison of predicted R _ based on Figure 2 of Morrow and Issler (1). Solid lines indicate constant-temperature plots of vitrinite reflectance evolution for the various models presented by Morrow and issler (1 ) and the V R G for burial heating. R . estimates made from Morrow and Issler (1) = Ri; Middleton (14) = Rmd; Waples (15) = Rw; Sweeney and Burnham (16) = Re; MATOIL (17) = Rm; Horvath et al. (18) = Rh; V R G = Burial heating calibration, this paper. Note that some of the more recently published kinetic models predict little significant change in R _ as time increases after about 20-30 m.y. suggesting stabilization of vitrinite reflectance evolution. v

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Figure 3. Plots of calculated R . in response to idealized temperature histories in tectonic settings with moderate to rapid heating rates. Plot (A) models the temperature history of a foreland basin and plot (B) models temperature history of a wrench fault basin. The lines represent R _ predictions for models presented by Morrow and Issler (1, see their Figure 5) and the V R G for burial heating (this paper). The lines represent (—) Waples (15); ( ) Matoil (17); (····) Easy%R (16); (—) Issler (1) and Middleton (14); and ( ······) V R G , burial heating, this paper. v

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Figure 4. Two burial history scenarios taken from Figure 6 of Morrow and Issler (1) for the Esso Bissette well. R _ data (solid squares) are compared to the calculated R . value (circle and range bar) from V R G calibration for burial heating (this paper). The R . is calculated using a Τ ^ of 117°C estimated from apatite fis sion track analysis. The range bar through the R _ predictions shows the computed R _ range based on the range of acceptable T p k quoted by Morrow and Issler (1 \ The lines represent (—) Waples (15); ( ) Matoil (17); (....) Easy%Ro (16); (—) Issler (1) and Middleton (14); and ( · · · · · · ) V R G , burial heating, this paper. v

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The last comparison in burial heating systems uses Bray et al. (2) who present vitrinite reflectance and paleotemperature data from the 47/29a-1 well drilled into the East M i d lands Shelf, United Kingdom. The 47/29a-1 well is substituted for the Gilbert F-53 well case study that Morrow and Issler present because T p ^ in (1) is only estimated from geologic models of burial and is not as well known as their other natural examples. Figure 5 shows a comparison of various model predictions to the measured R _ from the 47/29a-l well (2, see theirfigures4 and 5). V R G predictions of R . are in good agreement with the predictions from the Sweeney and Burnham (16) kinetic model and paleotemperatures estimated from AFTA. v


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Hydrothermal Metamorphism in Geothermal Systems. V R G for hydrothermal metamorphism is a well documented and accepted technique (21,22,23). A case study by Whelan et al. (3) compared the predictions of V R G of (9) and kinetic models to oxygen isotope geothermometry and measured present-day temperatures. These measured temperatures are thought to be near T p j . Whelan et al. (3) found that predictions from V R G for hydrothermal metamorphism (Fig. 6) are in good agreement with the present temperatures and oxygen isotope geothermometry. They concluded that the predictions from the kinetic model of Sweeney and Burnham (16) did not give as good of a fit to present day temperatures as the V R G . Further, the heating rate of 5°C/ m.y. input into the kinetic model of (16) gave the best fit to the present day temperatures. This heating rate does not fit well with the known age of these ocean bottom sediments that are estimated at being heated to near 280°C for most of the past 125,000 years (3). e a

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Discussion These comparisons show that, based on the hypothetical and natural examples presented by Morrow and Issler (1 ), as well as those of Bray et al. (2) and Whelan et al. (3), V R G predictions agree favorably with predictions from kinetic models where Tp j^ is well known. Thus, Morrow and Issler (1) were in error when they arbitrarily excluded V R G from consideration by asserting such models have been contradicted by time-temperature modeling of Burnham and Sweeney (24). In fact, Burnham and Sweeney (25) and Sweeney and Burnham (16,25) found that predictions from V R G and kinetic models compare favorably. Similar results have been found by (26, 27) and Laughland, this volume, and many others who have direct measurements of T p k to make the V R G and kinetic model predictions. The kinetic model of (16) also suggests negligible changes in R _ of less than 0.1% after some 20 to 30 m.y. at a constant (peak) temperature (Fig. 2). This is a negligible change in R _ because microscopist and laboratory bias and other possible errors in reflectance measurements lead to the general observation that reflectance values are limited in precision to one decimal place (28). Further, Barker and Crysdale (29) found that R _ value predicted from the Sweeney and Burnham ( 16) kinetic model after adjusting the thermal history to fit Tp fc closely agrees with the measured R _ . Thus, the kinetic model of (16) supports die concept of stabilization after about 20 to 30 m.y. This stabilization of vitrinite reflectance evolution is the theoretical basis of V R G . We also point out that Morrow and Issler (1) misstate the theoretical basis of V R G by mingling the observations of Price (30) on the importance of higher order reactions with the observations of Barker and Pawlewicz (4) that R _ is strongly correlated with T p ^ . These two observations are separate and are not linked by us. ea

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Figure 5. A comparison of measured geothermal gradient and T p k estimates in the 47/29a-l well, East Midlands Shelf, United Kingdom. Modified from Bray et al. (2, see their figures 4 and 5). The measured geothermal gradient suggests the well has cooled from Τ ρ ^ . Note V R G predictions (solid triangles) are in good agreement with the paleotemperature predictions from the Sweeney and Burnham (76) kinetic model (dots) and AFTA (solid squares or a horizontal line which indi cates the estimated paleotemperature range). e a

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Figure 6. Whelan et al. (3) comparison of various geothermometry and predictions from the Sweeney and Burnham (16) kinetic model for various heating rates. The solid (y=x) line represents perfect agreement between temperature estimate and measurement. A l l computations, present temperature measurements and paleotemperature estimates from Whelan et al. (3), except for the geothermometry for hydrothermal metamorphism (from this paper). For the purposes of plotting, the data values of (3) marked less than or greater than were rounded down or up by 5°C, respectively. The data symbols: (o) O-isotopes geothermometry (3); (A) Hy drothermal metamorphism this paper, (Δ) Geothermal (9); (•) 1 °C/1,000 yr (16); ( · ) 10 °C/yr (16); and (•) 5 °C/my (16). In Vitrinite Reflectance as a Maturity Parameter; Mukhopadhyay, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Barker and Pawlewicz (4) empirical observation infers that reaction time does not have a continuing effect on R . and thus, vitrinite reflectance evolution must stabilize at Tpeak after some limited reaction time period. Barker (11) suggested that stabilization of vitrinite reflectance evolution in burial heating systems requires up to 10 m.y. — thisis not a geologically insignificant period as Morrow and Issler (1) state. The R _ pre dicted by the more conservative of the kinetic models that Morrow and Issler present (Fig. 2) show that, in practical terms, there is little discernible change in vitrinite reflec tance past 20-30 m.y. This is a small difference in the estimated stabilization time given the simplifications inherent in both methods. v

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Concluding Remarks V R G have proven accurate or the only applicable method in: 1) accretionary or orogenic tectonic regimes where the burial history or heating is complex or reconstructible only with considerable uncertainty (27, 31, among others); 2) in cases where direct measures of T k are available (26, 32, 33); and 3), in geothermal systems (21, 22, 23). We acknowledge that in geological systems where the burial history is well known, kinetic methods are accurate especially if heating rate can be measured and applied to the proper kinetic model to refine the T ^ ^ prediction. We recommend that the method used should reflect the quality of the data that is available in a geologic system. If the thermal history and(or) burial history is poorly known, the use of V R G seems appropri ate because it needs only a R _ measurement to make an estimate of Τ ρ ^ . We reiter ate, however, that in our studies where the burial history is well known (as in 30), we have found that if the burial history is modified to consider a T p k measured in that system, comparable vitrinite reflectance predictions can be made with both V R G and kinetic methods. p e a

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Acknowledgments This paper was much improved and focused by the reviewers. We thank B. J. Cardott of the Oklahoma Geological Survey, and J. L . Clayton, M . A . Henry, D.K. Higley, J. D . King and M.D. Lewan of the U.S. Geological Survey. Literature Cited 1. Morrow, D.W.; Issler, D.R., Am. Assoc. Pet. Geol. 1993, 77, 610-624. 2. Bray, R.J., Green, P.F., and Duddy, I.R., In Exploration Britain: Geological Insights for the Next Decade; Hardman, R.F., Ed., Special Publication no. 67; Geo logical Society: London, 1992, 3-25. 3. Whelan, J.K., Seewald, J., Eglinton, L. and Miknis, F. Initial Reports, Ocean Drill ing Program. in press. 4. Barker, C.E., and Pawlewicz, M.J. In Paleogeothermics; Buntebarth, G., and Stegena, L., Ed.; Springer-Verlag New York, 1986, pp. 79-93. 5. Dembicki, H., Jr. Geochim. Cosmochim., 1984, 48, 2641-2649. 6. Teichmüller, M . , In Coal and Coal-bearing Strata: Recent Advances; Scott, A.C., Ed., Special Publication 32; Geological Society: London, 1987, 127-169. 7. Kilby, W.E. Int. J. Coal Geol., 1991, 19, 201-218. 8. Barker, C.E., and Goldstein, R.H. Geology, 1990, 18, 1003-1006.



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9. Barker, C.E. Geology, 1983, 11, 384-388. 10. Aizawa, J. In Proceedings; International Conference on Coal Science, 1989, 1, 93-96. 11. Barker, C.E. In Thermal History of Sedimentary Basins; Naeser, N.D., and McCulloh, T.H., Eds.: Springer-Verlag, New York, 1989; pp. 73-98. 12. Barker, C.E. Am. Assoc. Pet. Geol. Bull., 1991, 75, 1852-1863. 13. Barker, C.E., Bone, Y., Duddy, I.R., Marshallsea, S.J., and Green, P.F. Abstracts Soc. Org. Petrol., 1992, 9, 102-104. 14. Middleton, M.F. Geophys. J. Royal Astro. Soc. 1982, 68, 121-132. 15. Waples, D. Am. Assoc. Pet. Geol. Bull., 1980, 64, 916-926. 16. Sweeney, J.J., and Burnham, A.K.Am.Assoc.Pet.Geol.Bull., 1990, 74, 15591570. 17. BEICIP, MATOIL, release no. 2 : Bureau d'Etudes industrielles et de Cooperation de l'Institut Français du Petrole, Paris, 1990. 18. Horváth, F., Dövényi, P., Szalay, Á., and Royden, L . H . In The Pannonian Basin— a Study in Basin Evolution; Royden, L.H., and Horváth, F., Eds., Memoir 45, A m Assoc Pet. Geol., 1988, 355-372. 19. Wilkins, R.W.T., Wilmshurst, J.R., Russell, N.J., Hladky, G., Ellacott, M.V., and Buckingham, C. Org. Geochem., 1992, 18, 629-640. 20. Lewan, M.D. Abstracts Soc. Org. Petrol., 1993, 10, 1-3. 21. Taguchi, K., Mitani, M . , Inaba, T. and Shimada, I. In Proceedings, Fourth International Symposium on Water-Rock interaction: 1983, 455-458. 22. Gonsalez, R.C. Vitrinite Reflectance of cores and cuttingsfromthe Ngatamariki well NM-2; Report 85.10; New Zealand Geothermal Institute: Auckland, 1985, 60p. 23. Struckmeyer, H., and Browne, P.R.L., In Proceedings; Tenth New Zealand Geothermal Workshop. 1988, 251-255. 24. Burnham, A.K., and Sweeney, J.J. Geochim. Cosmochim, 1989 53, 2649-2657. 25. Sweeney, J.J., and Burnham, A . K . Am. Assoc. Pet. Geol. Bull., 1993, 77, p. 665667. 26. Tilley, B.J., Nesbitt, B.E., and Longstaffe, F.J. Am. Assoc. Pet. Geol. Bull., 1989,73, 1206-1222. 27. Laughland, M . M . Organic metamorphism and thermal history of selected portions of the Franciscan accretionary complex of Coastal California; unpublished Ph.D. thesis, University of Missouri: Columbia, M O , 1991, 318 p. 28. Robert, P. Organic Metamorphism and Geothermal History; D. Reidel Publishing Co.: Boston. M A , 1988, 311p. 29. Barker, C.E., and Crysdale, B.L. In 50 field conference guidebook; Stroock, B . and Andrew, S., Eds.; Wyoming Geological Association: Casper, WY, 1993; 235258. 30. Price, L.C. J. Pet Geol., 1983, 6, 5-38. 31. Yang, C., and Hesse, R. Org. Geochem., 1993, 20, 381-403. 32. Bone, Y., and Russell, N.J. Aust. J. Earth Sci., 1988, 35, 567-570. 33. Diessel, C.F.K., Coal-Bearing Depositional Systems; Springer-Verlag: New York, NY, 1992; 661p. th

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Vitrinite Reflectance as a Maturity Parameter - ACS Publications

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