Modeling Maturation History and Hydrocarbon Generation

These equations are applied to determine hydrocarbon yield as a function of time, and the rates of hydrocarbon output (Sj curve) during both natural and experimental cracking of kerogen. 

The results of open-system pyrolysis (Rock-Eval II) have been used to specify the kinetic parameters controlling maturation. Hydrocarbon yield rates as determined by these experiments are shown in Fig. 6.9a. Both nonlinear optimization technique (Levenberg-Marquardt method Press et al. 1986 Issler and Snowdon 1990) and linear methods are used to determine the values of the reaction parameters Aj, Ej, andX, . This technique minimizes an error function by comparing the hydrocaibon release rates, Sj, calculated by Eq. 6.9 and those rates measured in open-system pyrolysis. An example of the spectrum of activation energies obtained from this analysis is shown in Fig. 6.9b. 

Our algorithm for fitting the kinetic parameters E,-, Aj, and X,- uses the total time-temperature history of the source rock sample inferred from the level of organic maturation (solid line in Fig. 6.8b) and from open-system pyrolysis in which temperatures change linearly at rates of 5,15, and 30 C mm, ranging from 300 to 600 °C. For this purpose, the time integrals m Eqs. 6.8 and 6.9 are divided into two parts  

Determining the activation energy spectrum from Rock-Eval pyrolysis data is an inverse task of mathematical statistics having multiple solutions. Some problems are related to the restoration of chemical-kinetic parameters of effective reactions for organic matter maturation in source rocks. For example, reactions with activation energies of less than 50 Kcal moD do not contribute to the Rock-Eval pyrolysis Sj curve because these reactions can occur during the burial stage and would not contribute to S.  

Another problem relates to the use of variable values of Aj, There is no physical reason why all reactions in energy spectrum must have the same frequency factor as is usually assumed in chemical-kinetics modeling (Tissot et al. 1987 Espitalie et al. 1988 Welte and Yalcin 1988 lingerer et al. 1990). Spectra with variable values of A,- have a more irregular shape than spectra with a single-frequency factor, but the first spectra allow us to achieve a closer coincidence of the observed and calculated curves for in the fitting of kinetics parameters. 

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