Structural features and formation processes of a complex hydrogeochemical section in the Baikal rift zone

The purpose of the work is to study the effect of organic matter on the formation of ion-salt and gas composition of nitrogen-methane and methane thermal water occurring in the sedimentary rocks of deep horizons of artesian basins. The object of research is the Tunka intermountain artesian basin of the Baikal rift zone and the Tungor gas and oil field of the Okhotsk-Sakhalin basin, in the deep horizons of which soda (inversion) low- and high-mineralized groundwater is common. The study combines the results of the traditional study of the composition of natural solutions and the quantitative research of physical and chemical interactions in the “water – rock” system conducted using the Selector software package according to the degree of the hydrogeochemical process, which was set by the value of the rock/water ratio. Chemically pure water and rocks of medium chemical composition were used in interaction. With the use of physicochemical modeling the formation of thermal water composition in sedimentary rocks depending on the interaction degree between water and rock and the amount of organic matter was unravelled. As a result, it was determined that the organic matter present in the rock has the dominant influence on the intensity of the hydrogeochemical process determining the amount of mineralization, the ratio of components, and the amount of methane, nitrogen, and carbon dioxide produced. The correspondent compositions of the model and natural solutions showed the possibility to form low- and high-mineralized sodium bicarbonate groundwater of different gas-saturation degree in the conditions of deep horizons of sedimentary basins due to the internal reserves of the “water – rock” system not involving any components from external sources.