Modeling the transformation of І and ІІІ types kerogen by the method of entropy maximization

Abstract

Purpose, methods and research methodology. The aim of the work is to calculate and compare the trends of transformation of organic matter of I-A and III-A type kerogen, which is in contact with organic and inorganic gases in the process of immersion of organ-containing rocks. The calculations were performed for I and III type kerogen and a mixture of organic and inorganic gases within depths of 1-20 km and heat flows from 40 to 100 mW / m2. Results, scientific novelty and practical significance of research. A comparison and analysis of changes in the total entropy of the system was performed for I and III type kerogen, which showed the complex nature of the total entropy functional dependence on depth. It was revealed that the entropy has two reversible sections, the maxima of which are at a depth of 6 and 12 km. The analysis of changes in the Gibbs energy during the immersion of the geochemical system unambiguously indicates the presence of a stability zone for the hydrocarbon component. The maximum of this zone corresponds to the minimum value of the Gibbs energy, depends on the kerogen type and heat flow, is in the range of 4-7 km and indicates the area of stability, or "oil window". The complex nature of the balance between constitutional water and kerogen, depending on the heat flow and depth, has been established. To analyze this equilibrium, a simple dehydration equilibrium constant (Kd) was proposed, which generalizes the transformations of water in the kerogen matrix. Thermodynamic methods were used to calculate and compare the gas-generating capacity of I and III type kerogen for all heat flows, which showed that I type kerogen is the most productive with gas-generating potential, and III type is the least productive. To estimate the proportional composition of hydrocarbon gases in equilibrium with kerogen, the fat content coefficient of the gas generated by I and III type kerogen was calculated. It is shown that with immersion, the fat content coefficient first increases rapidly, which indicates an increase in the proportional content of alkanes heavier than methane. This growth reaches a maximum within 2-3 km for all considered heat flows, after which the fat content coefficient decreases. The equilibrium constant of the Kolbe-Schmitt reaction is calculated, which showed that regardless of the heat flow, the rate of kerosene decarboxylation decreases with increasing depth due to the shift of equilibrium to the left, and the contribution of this reaction to kerogen conversion is insignificant.