Application of geochemical parameters to assess the thermal maturity of the initial organic matter and reservoir fluids of the Astrakhan gas condensate field

<jats:p>The paper considers the application of a set of geochemical indicators to assess the thermal maturity of the initial organic matter (OM) and reservoir fluids of the Astrakhan gas condensate field (AGCF). The relevance of the topic is due to the need to reliably determine the conditions for the formation of deposits and predict the migration routes of hydrocarbons. To assess thermal maturity, geochemical indicators are used, including biomarker indices and coefficients reflecting changes in the composition of hydrocarbons during diagenesis and catagenesis. Vitrinite reflectivity, biomarker indices (pristane and phytane), and the Szymanski metamorphism coefficient are analyzed as the main indicators of maturity. It is emphasized that using only one indicator is not enough: the values of the coefficients are influenced by the type of initial OM, biodegradation, migration and retrograde processes, as well as the nonlinearity of the index-temperature relationship. Special attention is paid to the interpretation of these indicators, taking into account the specifics of carbonate reservoirs and high hydrogen sulfide aggression on AGCF. It has been established that AGCF is characterized by a high degree of thermal maturity. The reflectivity of vitrinite corresponds to the gradations of the MK3-AK4 scale of catagenesis. The high content of acid gases confirms the profound transformation of the system. The analysis of the Szymanski metamorphism coefficient showed values in the range of 0.52-0.9, which indicates that the system has reached thermodynamic equilibrium at high temperatures. The Kennon – Kessow diagram indicates the post-mature stage of OM. It is concluded that AGCF is a secondary (transformed) deposit with an extremely high degree of thermal maturity of OM. The deposit was formed due to the migration of high-temperature fluids from adjacent generation centers. The maturity assessment makes it possible to predict the migration routes of hydrocarbons and the prospects for the region's oil and gas potential.</jats:p>