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Tectonic influence on vitrinite reflectance
International Journal of Coal Geology, 16 (1990) 235-237
235
Elsevier Science Publishers B.V., Amsterdam
Tectonic influence on vitrinite reflectance M.F. Middleton Geological Survey of Western Australia, I00 Plain Street, Perth, W.A. 6000, Australia (Received April 27, 1990)
ABSTRACT
Vitrinite reflectance is a popular indicator of coal rank and index to thermal maturation. It has been used by numerous workers to model numerically the thermal development of sedimentary basins (Buntebarth and Stegena, 1986 ). Vitrinite reflectance is generally acknowledged by most workers to increase with temperature and time. The thermal variation of vitrinite reflectance is reasonably well explained as a response to a first-order chemical reaction described by the Arrhenius equation. A hypothesis has not been advanced to describe its variation with time. Most workers assume that pressure has little influence on vitrinite reflectance, despite observations of Hower and Davis ( 1981 ) that ( 1 ) pressure increases the anisotropy of vitrinite reflectance and (2) pressure suppresses m a x i m u m vitrinite reflectance at low pressures (less than about 200 to 300 MPa; Hower and Davis, 198 l, fig. 18 ). The aim of this paper is to propose a hypothesis of vitrinite reflectance variation that is dependent on temperature, pressure and time, and is fully explained by known physical processes. Vitrinite reflectance can be considered as a measure of the number of photons that have been vertically reflected from locations in the vitrinite molecular structure. The n u m b e r of these vertical reflection locations (VRLs) increase with temperature, and the work of Hower and Davis ( 1981 ) suggests that the number of VRLs decrease with pressure under normal burial conditions is slight and is masked by the strong dependence of vitrinite reflectance on burial temperature. The suppression of VRLs with increasing pressure (at low pressure) is possibly due to both intracrystalline/molecular plastic slip and rotation of rigid inequant grains/molecules as discussed by Levine and Davis (1984). Further, I propose that these suppressed VRLs slowly reappear according to either a plastic creep or diffusion law. I shall assume a diffusion law for the present model. For pressures greater than about 300 MPa, vitrinite reflectance increases with increasing pressure and this behavior is probably controlled by nucleation and growth of favorably oriented grains/ molecules as suggested by Levine and Davis (1984). Therefore, a numerical O166-5162/90/$03.50
© 1990 - - Elsevier Science Publishers B.V.
236
M.F. MIDDLETON
model of vitrinite reflectance behavior with temperature, pressure and time can be proposed on the above principles. Empirical observations lead to the conclusion that the vitrinite reflectance ( 1 ) increases with temperature, (2) decreases with pressure (low pressure), (3) increases with high pressure, and (4) increases with time. I assume the vitrinite reflectance (R) is described by the relationship: R=temperature factor X (nucleation factor + diffusion factor). The temperature factor is described by the Arrhenius equation, the nucleation factor is assumed to be a constant at low pressures, and the diffusion factor is the suppression effect plus its slow recovery with time. The quantitative statement of this relation is: R = e x p ( - a / T ) [ A - B s i n ( 3 . 1 4 P/S) e x p ( - t / c ) ], where T is temperature in degrees Celsius, P is pressure in MPa, t is time, A, a, B, and c are constants, and S is the pressure where the diffusion factor becomes negligible (about 300 MPa). The above equation is valid for pressure in the range to S. The equation has been calibrated with the empirical observations of Hower and Davis ( 198 l, fig. 18) and various other workers (Buntebarth and Stegena, 1986), and the values of the constants are assumed to a = 2 3 2 Ma, A = 7.0, B=0.6A, c = 2 0 0 Ma, and S = 3 0 0 MPa. This model provides a logical and physically meaningful dependence of vitrinite reflectance on time. Table 1 shows vitrinite reflectance versus time as calculated using models proposed by Hood et al. and Middleton and Falvey (see Buntebarth and Stegena, 1986), Wood (1988) and the present hypothesis. Calculations were made assuming a rapid burial to a temperature of 110 degrees Celsius and a pressure of 75 MPa (about 3 km). The values of vitrinite reflectance determined by the present hypothesis compare favorably with those determined by other methods. In conclusion, the present hypothesis for the influence of tectonic processes (temperature and pressure) on vitrinite reflectance includes the empirical observation of reflectance suppression at low pressures, and assumes the process is governed by a diffusion law. The diffusion law provides a physical basis for the observed time dependence of vitrinite reflectance. The present model is only a preliminary and simplistic formulation, and further work is required to elucidate details such as the temperature dependence of the diffusion term which appears in the empirical observations of Hower and Davis (1981, fig. 18).
REFERENCES Buntebarth, G. and Stegena, L., 1986. Methods in paleogeothermics. In: G. Buntebarth and L. Stegena (Editors), Paleogeothermics. Springer-Verlag, pp. 5-39. Hower, J.C. and Davis, A., 1981. Application of vitrinite reflectance anisotropy in the evaluation of coal metamorphism. Geol. Soc. Am. Bull., 92: 350-366.
TECTONICINFLUENCEON VITRINITEREFLECTANCE
237
Levine, J.R. and Davis, A., 1984. Optical anisotropy of coals as an indicator of tectonic deformation, Broad Top coal field, Pennsylvania. Geol. Soc. Am. Bull., 95: 100-108. Wood, D.A., 1988. Relationship between thermal maturity indices calculated using Arrhenius equation and Lopatin method: implications for petroleum exploration. Am. Assoc. Pet. Geol. Bull., 72:115-134. DISCUSSION
D. Welte (KFA-Jiilich, F.R. Germany) Comment: Many times the main problem in maturity predictions or calculations is the application of the idea of"stable geothermal gradients"; however, there are no stable geothermal gradients. There is a continuous flow of thermal energy through the rock column of a basin and its changes with changing boundary conditions, i.e., heat input at the base over time (e.g., rifting phases, volcanism, intrusions, etc. ) erosion on the surface, climatic changes and surface temperatures, water/sediment interface temperatures, and last, but not least, tectonic rearrangements of the layer-cake arrangement of the sedimentary sequences. The consequence is that we must work more intensely with "reconstructed temperature histories", rather with non-existent geothermal gradients. Such temperature reconstructions are possible with modern modeling packages. One of these packages is the IES 1-D PC-version of the "Pre-Drilling Intelligence" (PDI) software package. P.C. Lyons (U.S. Geological Survey) Comment (in response to a c o m m e n t by A. Boucot, Univ. Oregon): Tectonic stress has not been ignored in coalification studies because it has been reported that meta-anthracites (e.g., in the Narragansett basin of New England) require shearing in order to be formed. However, there are data indicating that pressure (lithostatic and shear pressure) has had no effects on coalification. W. Langenberg (Alberta Geological Survey) Question: Would you be able to construct a new Karweil curve that could be read in time-depth burial curves?
Response: A set of curves similar to the Karweil curves could be easily constructed. Such a set of curves would have the benefit that they describe vitrinite reflectance from time = 0 to large geological times. This would be an improvement on the Karweil curves, which do not describe vitrinite reflectance behaviour for small times (t ~ 0, rapid heating).
235
Elsevier Science Publishers B.V., Amsterdam
Tectonic influence on vitrinite reflectance M.F. Middleton Geological Survey of Western Australia, I00 Plain Street, Perth, W.A. 6000, Australia (Received April 27, 1990)
ABSTRACT
Vitrinite reflectance is a popular indicator of coal rank and index to thermal maturation. It has been used by numerous workers to model numerically the thermal development of sedimentary basins (Buntebarth and Stegena, 1986 ). Vitrinite reflectance is generally acknowledged by most workers to increase with temperature and time. The thermal variation of vitrinite reflectance is reasonably well explained as a response to a first-order chemical reaction described by the Arrhenius equation. A hypothesis has not been advanced to describe its variation with time. Most workers assume that pressure has little influence on vitrinite reflectance, despite observations of Hower and Davis ( 1981 ) that ( 1 ) pressure increases the anisotropy of vitrinite reflectance and (2) pressure suppresses m a x i m u m vitrinite reflectance at low pressures (less than about 200 to 300 MPa; Hower and Davis, 198 l, fig. 18 ). The aim of this paper is to propose a hypothesis of vitrinite reflectance variation that is dependent on temperature, pressure and time, and is fully explained by known physical processes. Vitrinite reflectance can be considered as a measure of the number of photons that have been vertically reflected from locations in the vitrinite molecular structure. The n u m b e r of these vertical reflection locations (VRLs) increase with temperature, and the work of Hower and Davis ( 1981 ) suggests that the number of VRLs decrease with pressure under normal burial conditions is slight and is masked by the strong dependence of vitrinite reflectance on burial temperature. The suppression of VRLs with increasing pressure (at low pressure) is possibly due to both intracrystalline/molecular plastic slip and rotation of rigid inequant grains/molecules as discussed by Levine and Davis (1984). Further, I propose that these suppressed VRLs slowly reappear according to either a plastic creep or diffusion law. I shall assume a diffusion law for the present model. For pressures greater than about 300 MPa, vitrinite reflectance increases with increasing pressure and this behavior is probably controlled by nucleation and growth of favorably oriented grains/ molecules as suggested by Levine and Davis (1984). Therefore, a numerical O166-5162/90/$03.50
© 1990 - - Elsevier Science Publishers B.V.
236
M.F. MIDDLETON
model of vitrinite reflectance behavior with temperature, pressure and time can be proposed on the above principles. Empirical observations lead to the conclusion that the vitrinite reflectance ( 1 ) increases with temperature, (2) decreases with pressure (low pressure), (3) increases with high pressure, and (4) increases with time. I assume the vitrinite reflectance (R) is described by the relationship: R=temperature factor X (nucleation factor + diffusion factor). The temperature factor is described by the Arrhenius equation, the nucleation factor is assumed to be a constant at low pressures, and the diffusion factor is the suppression effect plus its slow recovery with time. The quantitative statement of this relation is: R = e x p ( - a / T ) [ A - B s i n ( 3 . 1 4 P/S) e x p ( - t / c ) ], where T is temperature in degrees Celsius, P is pressure in MPa, t is time, A, a, B, and c are constants, and S is the pressure where the diffusion factor becomes negligible (about 300 MPa). The above equation is valid for pressure in the range to S. The equation has been calibrated with the empirical observations of Hower and Davis ( 198 l, fig. 18) and various other workers (Buntebarth and Stegena, 1986), and the values of the constants are assumed to a = 2 3 2 Ma, A = 7.0, B=0.6A, c = 2 0 0 Ma, and S = 3 0 0 MPa. This model provides a logical and physically meaningful dependence of vitrinite reflectance on time. Table 1 shows vitrinite reflectance versus time as calculated using models proposed by Hood et al. and Middleton and Falvey (see Buntebarth and Stegena, 1986), Wood (1988) and the present hypothesis. Calculations were made assuming a rapid burial to a temperature of 110 degrees Celsius and a pressure of 75 MPa (about 3 km). The values of vitrinite reflectance determined by the present hypothesis compare favorably with those determined by other methods. In conclusion, the present hypothesis for the influence of tectonic processes (temperature and pressure) on vitrinite reflectance includes the empirical observation of reflectance suppression at low pressures, and assumes the process is governed by a diffusion law. The diffusion law provides a physical basis for the observed time dependence of vitrinite reflectance. The present model is only a preliminary and simplistic formulation, and further work is required to elucidate details such as the temperature dependence of the diffusion term which appears in the empirical observations of Hower and Davis (1981, fig. 18).
REFERENCES Buntebarth, G. and Stegena, L., 1986. Methods in paleogeothermics. In: G. Buntebarth and L. Stegena (Editors), Paleogeothermics. Springer-Verlag, pp. 5-39. Hower, J.C. and Davis, A., 1981. Application of vitrinite reflectance anisotropy in the evaluation of coal metamorphism. Geol. Soc. Am. Bull., 92: 350-366.
TECTONICINFLUENCEON VITRINITEREFLECTANCE
237
Levine, J.R. and Davis, A., 1984. Optical anisotropy of coals as an indicator of tectonic deformation, Broad Top coal field, Pennsylvania. Geol. Soc. Am. Bull., 95: 100-108. Wood, D.A., 1988. Relationship between thermal maturity indices calculated using Arrhenius equation and Lopatin method: implications for petroleum exploration. Am. Assoc. Pet. Geol. Bull., 72:115-134. DISCUSSION
D. Welte (KFA-Jiilich, F.R. Germany) Comment: Many times the main problem in maturity predictions or calculations is the application of the idea of"stable geothermal gradients"; however, there are no stable geothermal gradients. There is a continuous flow of thermal energy through the rock column of a basin and its changes with changing boundary conditions, i.e., heat input at the base over time (e.g., rifting phases, volcanism, intrusions, etc. ) erosion on the surface, climatic changes and surface temperatures, water/sediment interface temperatures, and last, but not least, tectonic rearrangements of the layer-cake arrangement of the sedimentary sequences. The consequence is that we must work more intensely with "reconstructed temperature histories", rather with non-existent geothermal gradients. Such temperature reconstructions are possible with modern modeling packages. One of these packages is the IES 1-D PC-version of the "Pre-Drilling Intelligence" (PDI) software package. P.C. Lyons (U.S. Geological Survey) Comment (in response to a c o m m e n t by A. Boucot, Univ. Oregon): Tectonic stress has not been ignored in coalification studies because it has been reported that meta-anthracites (e.g., in the Narragansett basin of New England) require shearing in order to be formed. However, there are data indicating that pressure (lithostatic and shear pressure) has had no effects on coalification. W. Langenberg (Alberta Geological Survey) Question: Would you be able to construct a new Karweil curve that could be read in time-depth burial curves?
Response: A set of curves similar to the Karweil curves could be easily constructed. Such a set of curves would have the benefit that they describe vitrinite reflectance from time = 0 to large geological times. This would be an improvement on the Karweil curves, which do not describe vitrinite reflectance behaviour for small times (t ~ 0, rapid heating).