Low Heat Flow at Shallow Depth Intervals: Case Studies from Belarus
The territory of Belarus belongs to the western part of the Precambrian East European Platform. Its heat flow pattern is representing by alternating low and high heat flow anomalies. An overwhelming majority of heat flow determinations and in general of geothermal observations in Belarus were fulfilled in boreholes finished in the platform cover. Within the Belarusian Anteclise, Orsha Depression, western slope of the Voronezh Anteclise their bottom holes are typically within the zone of active water exchange, where the groundwater circulation sufficiently influences on recorded thermograms. For instance, observed heat flow density for a number of studied boreholes is low and ranges on average from 15–20 until 35–40 mW/m2 within the Orsha Depression. In a number of studied holes in the northern part of the structure, its values are surprisingly low. They are observed within upper horizons of the zone of active water exchange with pronounced groundwater circulation. Permeable rocks within the geologic section comprise the platform cover with a number of freshwater intervals. Their base is spread here up to depths of 150–250 m. The most of heat flow observations within this area were studied in boreholes which depths is only 200–300 m, sometimes less, as deeper wells are seldom within this geologic structure. Groundwater circulation within loose sediments cools them, most of thermograms here have a concaved shape to the depth axis. As a rule, heat flow values are sufficiently lower in a number of intervals in boreholes finished in the freshwater zone, relatively to the heat flow observed within deeper 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 it has a tendency to stabilise only at intervals deeper than 600–800 m. It is the main reason for observed low heat flow zones.
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