Applicability Limits of the Maturity Concept in Organic Geochemistry

Abstract

To find out how various maturity criteria reflect how much OM as a whole approaches the state of chemical equilibrium, correlation analysis of relationships between 27 composition parameters was carried out. The OM was obtained from carbonate, siliceous–carbonate, carbonate–siliceous, and siliceous rocks from the northern and central regions of the Volga–Urals area (more than 100 samples). The data were processed using the apparatus of nonparametric statistics. We analyzed interrelations only between maturity indicators that are based on the same type of reactions (for example, reactions breaking C–C bonds). It has been established that none of the 85 correlation coefficients corresponds to the values characteristic of the functional dependence. The largest absolute value is 0.87. Therefore, maturity can be a factor determining the value only of one of all the parameters. For the rest, two options are possible. First, even for reactions of the same type, it is impossible to judge whether chemical equilibrium is generally approached. Second, the values of almost all the parameters are affected not only by reactions of approaching equilibrium but also by several other comparable and unknown factors. It is shown that, if the difference between the samples is up to five-fold values of any of the parameters, it is impossible to determine whether the OM of one of the samples is more mature than in any other. Thus, the concept of maturity is applicable at best only to roughly subdivide OM samples, with an increment in any parameter corresponding to at least a ten-fold change in it. For a detailed description, the term maturity can be used only with specifying the parameter by which it is determined (for example, maturity based on the Ts/Tm ratio). In this case, several such indicators based on reactions of different type should be used.