Hydrogeochemical features of terrigenous Devonian play in Middle Urals

The authors discuss hydrogeochemical features of terrigenous Devonian Play in Middle Urals, which is the main play of the Middle Urals. Characteristics of its geofiltration structure are presented, fractured-sedimentary and porous-sedimentary types of geofiltration media are identified. Distribution of geofiltration settings in the study area is demonstrated. Ranges of hydrochemical indicators are determined using 195 representative samples of formation brines. Salinity, metamorphization, calcium and bromine content, sulphate content and subsurface isolation are used as the most significant characteristics. The paper presents the hydrogeochemical maps illustrative of main common factors of spatial distribution of the indicators. Cluster analysis was used to reveal the special hydrochemical group in terrigenous Devonian series of Palaeozoic section, namely, “highly metamorphized connate brines” that form a vast areal in the southern part of the region. The northern part mainly contains the “background platform-type brines”. As a result of the work, two hydrogeochemical zones (southern and northern) are delineated. The southern zone corresponds to the environment of the separate basin with poor lateral water transmissibility. This zone contains highly metamorphized brines with high calcium and bromine content and low sulphate content; the zone clearly conforms with the present-day areal of oil and gas occurrence. The northern zone is considered to be a less promising since conditions for more active water exchange existed here