Low Heat Flow in the Orsha Depression, Belarus

Abstract

The Precambrian Orsha Depression occupies the northeastern part of Belarus. Observed heat flow density for studied boreholes is low and ranges on average from 20 until 35-40 mW/m2. At this background within a number studied holes within the northern part of the structure its values are surprizinglyly low.They are observed within upper horizons of the zone of active water exchange with pronounced groundwater circulation, where permeable rocks within the geologic section comprise the platform cover with fresh water intervals. Their base is spread here up to depths of 200 250 m. The most of heat flow observations within these parts of the Orsha Depression were studied in boreholes which depths is only 200 300 m, as deeper wells are seldom within this geologic structure. An active circulation of groundwater within loose sediments has its imprint on recorded thermograms, where most of them have a concaved shape to the depth axis. Heat flow within the zone of active water exchange is considerably subjected to the influence of the downward filtration of underground waters. As a rule, heat flow values are sufficiently lower in a number of interval in boreholes finished in the fresh water zone, relatively to the heat flow observed within deep horizons of the platform cover. In some of studied boreholes the observed heat flow is as low as 5-15 mW/m2. In most cases the observed heat flow has a tendency to stabilise only at intervals deeper than 600-800 m. It requires the detailed analysis when compiling heat flow maps. Such aspects are discussed in the presentation.