Rock thermal conductivity of Mesozoic geothermal aquifers in the Northeast German Basin

This study reports laboratory-measured thermal-conductivity values of Mesozoic sandstones from eight wells (predominantly geothermal boreholes) of the Northeast German Basin (NEGB). The measurements were made on drill core using the optical scanning method. Bulk thermal conductivities of sandstones corrected for in situ thermal conditions range between 2.1 and 3.9 W/m/K. In general, the Mesozoic sandstones show a large effective porosity typically ranging between 16% and 30%. Matrix thermal conductivity ranges from 3.4 to 7.4 W/m/K. The higher values reflect the large quartz content in sandstone. Based on the in situ thermal conductivity and corresponding interval temperature gradient, obtained from high-precision temperature logs measured under thermal borehole equilibrium, interval heat-flow values were computed in the Middle Buntsandstein section (between 1400 and 1500 m) of two boreholes located in the Stralsund area. The heat flow averages to 74 mW/m² (Gt Ss 1/85 borehole) and 78 mW/m² (Gt Ss 2/85 borehole) and, by adding a heat-flow component of 1.8 mW/m² for the heat production in the overburden, are in good correspondence with previously reported surface heat flow of 77 mW/m². Based on these values and the temperature log information, thermal conductivity was indirectly calculated for the entire borehole profiles. The discrepancy between laboratory-measured and computed thermal conductivities in the two boreholes is in the order of 0.24 and 0.56 W/m/K. Formation in situ thermal conductivity of the Mesozoic section ranges between 1.5 and 3.1 W/m/K.