Research paperParagenesis of fluids under evaporates in the Volga-Ural Basin

- This study allows to divide the subsalt fluids for nitrogen and methane group.
- The nitrogen group includes Ca-Cl brines, heavy oils, bitumens and N2 gases.
- The methane group includes methane gases and possibly volatile crudes.
- Gas exchange of the groups is accompanied by degassing and mass transfer between them.
- It may be that the groups are products of metamorphism of different times and places.

To unravel the mystery of the relationship between evaporates, Ca-Cl brines and accumulations of oil and N2 in the basins of ancient cratons, their N2, CH4 and He concentration ratios, as well as the isotopic composition (δ15N, δ13C and 3He/4He) were compared within the Volga-Ural basin. The study allowed subsalt fluids from Volga-Ural Basin to divide into two genetic groups. The first one is found within the basin's platform area. It includes Ca-Cl brines, high-viscosity heavy oil, bitumen and N2, which has concentrations higher than that of CH4 and positive values of δ15N. The second one is tied to the edge of the platform, the Ural Foredeep and Peri-Caspian Depression. In this group, only the oil and gas reservoirs, which have more CH4 than N2, and possibly negative values of δ15N, were discovered. Interaction of gas components in compared fluids indicates great role of degassing in the formation of their composition. It is suggested that the fluids of the first group (N2 > CH4) is what remains, and the second group (N2 < CH4) is what is disappears from the rocks during their metamorphism and degassing.