GEOCHEMISTRY AND NATURE OF INVERSION IN THE SUBSURFACE HYDROSPHERE OF THE SOLIKAMSK DEPRESSION

Abstract

The geochemistry of brines typical of the Solikamsk Depression (a basin of the Uralian Foredeep) is studied. Strong calcium chloride brines with a salinity of up to 300 g/l are dominant in the Paleozoic sedimentary cover. They originated from magnesium chloride brines in Early Permian evaporite basins, which migrated by density convection into underlying Paleozoic carbonates and were metamorphosed during metasomatic dolomitization of the limestones. Normal hydrogeochemical zoning is characteristic of the Solikamsk Depression. However, a hydrogeochemical anomaly occurs in its deepest northeastern part adjacent to the Urals in the Frasnian-Tournaisian deposits at depths of 1900-2400 m. The anomaly is distinguished by a decrease in salinity to 80-30 g/l, the metamorphic grade (rNa/rCl from 0.39 to 0.9), and bromine concentration (from 1400 to 100 rag/l), and an increase in the sulfate index (rSO4 × 100/rCl from 0.3 to 6.4 of brines). In addition, high concentrations of iodine (up to 50-70 mg/l) and bicarbonate ion (up to 1000-1200 mg/l) were found in the relatively low-saline brines. The formation of the deep-seated low-saline waters by infiltration, dehydration, condensation, or endogenetic activity is discussed. It is shown that the lithologic, hydrogeochemical, and thermobaric conditions and geological structure of the studied anomaly are most consistent with the condensation concept. Deep-seated waters of the Solikamsk Depression (depths of 5000-7000 m) evaporate to gaseous hydrocarbons. The subsequent ascent of the gas-water fluids and interaction of their condensates with connate brines and host rocks determined the geochemical features of the low-saline brines and the association of these brines with condensated gas and gas-oil deposits.