Geothermal Trends of Denmark

Abstract

In the first phase of the EC programme all temperature measurements reported from Danish land borings drilled to depths greater than 500 m were compiled. From 50 holes 194 measurements were available, almost exclusively bottom hole temperatures (BHT) from oil and gas exploration boreholes. Most data originate from North Jylland. Due to the great inhomogeneity of data, only generalized geoisotherms were mapped. The temperature trends show the highest values in areas with the thickest sequences of sedimentary rocks. From the BHT, heat flow was estimated at six sites. Values range from, 43 to 66 mW m −2. Climatic corrections may increase these values by at least 10 mW m−2. In the second phase of the programme the work has been aimed at measuring accurate temperatures and heat flow. Four heat flow boreholes are drilled to depths of 150 m to 250 m, and measurements are carried out in shallow boreholes and a more than 3 km deep borehole drilled by other agencies. New temperature logging equipment has been built, needle probe and divided bar thermal conductivity equipment and data interpretation techniques have been improved. Heat production and in particular a great number of thermal conductivities have been measured on core materials. New heat flow values are determined as the holes reach thermal equilibrium. We conclude that so far only in central North Jylland subsurface temperatures are known with reasonable accuracy. More heat flow and temperature data are needed from the other parts of the country. When borehole sites are carefully selected, and the thermal conductivities are carefully measured on core materials, heat flux can be measured to a sufficient precision at shallow (~200 m) to intermediate (~500 m) depths. When compared to values measured at greater depth and with palaeoclimatic corrections applied, such measurements can be used for the subsurface temperature modelling.