Equilibrium temperatures of hydrocarbon gas formation in sedimentary strata of the Western oil and gas region of Ukraine (according to thermodynamic modelling)

The article reviews the question of interpretation of natural gases component composition from the aspect of their evolution. The parameters available for study, which show a strong correlation with the conditions of formation, migration and accumulation of natural hydrocarbon gases, are selected. Among these parameters, the ratio of the butanes isomeric form (i-C4/n-C4) was selected for thermodynamic analysis as a dependable indicator of the kerogen degradations temperature regime. It is shown that the dependence of the i-C4/n-C4 ratio on the normalized methane content shows the trend of increasing kerogen maturity, and deviations from this trend indicate a distant migration of hydrocarbon fluid from the formation zone to the current deposit. Analysis of the residence and thermodynamic conditions of hydrocarbons in the deposits of the Western oil and gas region showed that kerogen/gas systems are in a state close to equilibrium, in terms of thermodynamics. The composition of the gas/kerogen equilibrium system in the conditions of sedimentary thickness for two heat fluxes – 75 and 100 mW/m2 was calculated by the method based on Jaynes’s formalism. Among hydrocarbons in gases, the content of isomeric forms of butane and pentane, as well as methane, ethane and propane was calculated. The results of the calculations revealed a monotonic dependence of the equilibrium temperature and depth of formation on the ratio of isobutane to n-butane. It was found that the results of thermodynamic calculations coincide with experiments on kerosene pyrolysis and correlate with studies of the composition and residence of natural gases. Equilibrium formation temperatures were determined for 59 gas, oil and gas condensate fields of the Western oil and gas region, the information on which contained data on the i-C4/n-C4 ratio. Based on the results of calculations, maps of these temperatures distribution within the region were constructed. The analysis of the maps showed the presence of two distinct temperature maxima, which are concentrated in the Boryslav-Pokuttya oil and gas region and are located at the intersection of regional faults. It has been suggested that the hydrocarbon source is significantly distant from modern deposits for the study region. The results of the calculation are compared with the data obtained using the model of fossil hydrocarbons inorganic origin.