Molecular nitrogen in salts and subsalt fluids in the Volga-Ural Basin

Analysis of the distribution and genesis of nitrogen in salts and subsalt formation fluids in the Volga-Ural Basin makes it possible to distinguish two genetically distinct groups of subsalt fluids and corresponding two distinct sources of nitrogen. One of them includes calcium chloride bridnes, nitrogen-saturated oils, and nitrogen-bearing formation gases. The fluids of this group have δ15N > 0‰ and occur closer to the surface in the platform part of the basin. The other group comprises only oils saturated with CH4 and methane formation gases, and the fluids of the latter group likely have δ15N < 0‰ and were found near the margin of the platform, in the Ural Foredeep and near the Caspian depression. The first group with isotopically heavy nitrogen is supposedly formed by fluids remaining after the loss of volatile components during ancient metamorphism of the rocks, whereas the second group was likely generated by the volatiles released during the degassing of the same rocks.