Vitrinite maturation

Huang W.-L. (1996) Experimental study of vitrinite maturation effects of temperature, time, pressure, water, and hydrogen index. Org. Geochem. 24, 233—41. 

Temperatures within the sedimentary section calculated as a fimction of time are used to estimate the maturation level of the organic matter. The kinetic model of vitrinite maturation (Sweeney and Burnham 1990) is the primary method of maturity estimation. Comparison of measured vitrinite reflectance Rg) with the calculated values and present-day temperature profile (Fig. 6.6) is used to control modeling parameters. The time-temperature index (TTI) (Lopatin 1971 Waples 1980) is also computed as a simple method to describe the paleothermal regime. 

Burnham A. K. and Sweeney J. J. (1989) A chemical kinetic model of vitrinite maturation and reflectance. Geochim. Cosmochim. Acta 53, 2649-2657. 

The experimentally determined alteration rate is defined by the expression %R = where Kq is a function of temperature, and t is in days. The overall activation energy describing the kinetics of complex devolatilization reactions responsible for increased VR measured in our laboratory experiments is 21.8 0.3 kJ mol. Combined with earlier rate studies by Dalla Torre et cd. in 1997, we conclude that the rate of vitrinite maturation apparently is imaffected by oxidation state and nature of the starting Ugnitic material (conifer or hardwood). Elevated total aqueous fluid pressure very slightly retards the rate of VR increase. Non-systematic trends are observed for the resinite exudatinite bituminite present in the run products. Our new experimental data confirm that VR is chiefly a function of temperature and time. In support of earlier field, theoretical and laboratory studies, the crucial variable that determines VR over geologic time intervals is host-rock temperature. 

Fig. 1. Vitrinite reflectance measured on lignite B as a function of experimental run duration at 200°C, 2.0 kbar aqueous fluid pressure, with t02 defined by the hematite-magnetite and magnetite + quartz-fayalite solid buffers (data from Table 1 and Appendix A). The laboratory rate curve is visually estimated the approximate quasi-threshold value for %R is 0.54. Also illustrated in the computed EASY%Ro model for vitrinite maturation at 200°C. EASY%Rq rate model curves agree with experimentally determined rates for the 250 and 300°C isotherms, but are low for the 200°C and slightly high for the 400 0 runs (J. J. Sweeney, personal communication, 01/10/03).

Table 1. Kinetic study of vitrinite maturation, 2.0 kbar fluid pressure, 200 0...

At constant temperature, rates of vitrinite alteration under contrasting oxidation/reduction conditions (HM and FMQ oxygen buffers) are indistinguishable within the limits of error for individual experiments. Provided buffer and charge equilibrated through reaction with H2, oxidation state within this f02 range evidently does not play an important role in the kinetics of vitrinite maturation. 

Within measurement uncertainties, both the pair of initially dry and the two wet experiments of Dalla Torre et al. (1997) are compatible with our wet experiments. The relative abundance of H2O per se imperceptibly retards the rate of vitrinite maturation. 

Dalla Torre M., Ferreiro mahlmann R. and Ernst W. G. (1997) Experimental study on the pressure dependence of vitrinite maturation. Geochim. Cosmochim. Acta 61, 2921 -2928. 

Seewald j. S., Eglinton L. B. and Ong Y. L. (2000) An experimental study of OTganic-inorganic interactions during vitrinite maturation. Geochim. Cosmochim. Acta 64, 1577-1591. 

Burnham AK, Sweeney JJ (1989) A chemical kinetic model of vitrinite maturation and reflectance. Geochem Cosmochim Acta 53 2649-2657 Carothers WW, Kharaka YK (1978) Aliphatic acid anions in oil-field waters - implications for the origin of natural gas. Am Assoc Pet Geol Bull 62 2441-2453 Constantinides G, Arich G (1967) Non-hydrocarbon compounds in petroleum. In Nagy B, Colombo U (eds) Fundamental aspects of petroleum geochemistry. Elsevier, New York, pp 77-108... 

To assess potential yield and maturity of source rocks and classify those according to their vitrinite reflectance . 

The Australian Permian coals vary widely in rank (maturity) and type (vitrinite content) from the Oaklands (N.S.W.) coal at 72% (dry ash-free basis) carbon, a hard brown coal (6), containing 17% vitrinite, at one extreme - through high volatile bituminous coals such as Galilee (Queensland) coal at 77% carbon, 16% vitrinite Blair Athol (Queensland) coal at 82% carbon, 28% vitrinite, Liddell (N.S.W.) coal at 82% carbon, and >70% vitrinite - to low volatile bituminous such as Peak Downs (Queensland) at 89% carbon, 71% vitrinite, and Bulli seam (N.S.W.) 89% carbon, 45% vitrinite. 

Hatcher P. G., Wenzel K. A., and Cody G. D. (1994) The coalification reactions of vitrinite derived from coalified wood transformations to the rank of bituminous coal. In Vitrinite Reflections as a Maturity Parameter (eds. P. K. Mukhopadhyay and W. G. Dow). American Chemical Society, Washington, DC, pp. 112-135. 

Determination of maturity levels is critical to the success of any exploration program. Recovery of immature, but organic-rich, source rocks would indicate good source potential for such rocks if buried more deeply in other parts of the basin. At the other extreme, an overmature source rock would indicate a mature part of the basin not capable of generating additional liquid hydrocarbons, but possibly gas. There are several indicators available that can be used to estimate the relative maturity of a source rock. The traditional method is measuring the maturity of vitrinite. The chemical composition of the maceral... 

The second important point is that not all samples contain vitrinite. Pre-Devonian samples do not contain vitrinite since higher plants did not evolve until around this time and vitrinite is derived from higher plant remains. However, there are a number of alternative maturity scales that can be used. For example, the thermal alteration index (Staplin, 1969) and spore coloration index (Burgess, 1974) have been used... 

Figure 3 Vitrinite reflectance is used to provide an indication of the maturity of a source rock or potential source rock. The approximate values for the onset of oil generation, peak oil generation, and onset of gas generation are also shown in this diagram. It should he noted that there is some disagreement as to the precise values for these ditferent processes (Hunt, 1996) (reproduced hy permission of Freeman from Petroleum Geochemistry and...

The initial result from Rock Eval pyrolysis is a chromatogram with the two major peaks described above. Si and S2 (Figure 4), along with the S3 peak. As the maturity of a sample increases, the temperature at which the Si peak appears remains relatively constant however, the temperature at which S2 maximizes increases. S3 is not used directly for maturity determinations. The increase in the temperature at which S2 maximizes results from the fact that what is being measured here is the temperature at which the residual material in the rock breaks down. As the maturity level of the rock increases, the temperature required to degrade the residual material also increases. It is important to note that a direct correlation between Tmax vitrinite reflectance is not necessary. 

The data described above can be used to predict the location of better source rocks in vertically drained basins, especially in deltaic-type environments with relatively young source rocks. With long-distance vertical migration, some of the biomarker parameters may become skewed. A number of factors must first be considered before applying this approach first, some of the parameters vary with maturity second, C30 steranes are not present in lacustrine samples and so the approach will not work in that situation and finally, it will not work where the oils were deposited prior to land plant evolution, since no vitrinite was present at that time. Oils from mixed source rocks also complicate the issue. The ability to predict source rock properties on the basis of biomarker distributions in cmde oils is a very interesting concept, since most exploration efforts try to discover oil and not source rocks. 

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