Detailed knowledge of in situ formation thermal properties is a prerequisite for accurate temperature predictions from geothermal models of sedimentary basins. The value and regional variability of such formation thermal properties generally receive little attention: very few attention in petrophysical and even less in modelling studies. Consequently, the spatial variability of formation thermal properties is typically not considered, for neither the a priori model parameterisation nor the posteriori model calibration. This basin study determines how the thermal properties of geological formations vary spatially and how this affects the quality of modelling results compared to the results of measured temperatures in the Danish Basin. Formation petrophysical properties (thermal conductivity, radiogenic heat production, thermal diffusivity, specific heat capacity, density, and porosity) and their spatial variability in the Danish Basin are exemplarily and systematically studied by well-log interpretation techniques. Therefore, the initial computations of the mean formation well-log values (and their variability) are presented for thermal diffusivity and specific heat capacity. The analysis reveals that all the formation thermal properties display a larger variability than previously applied in geothermal or basin modelling studies. The observed maximum variability of the mean thermal formation values is up to approximately 50% (mean: 23 +/- 11%) for thermal conductivity, up to approximately 65% (mean: 34 +/- 16%) for thermal diffusivity, up to approximately 30% (mean: 16 +/- 8%) for specific heat capacity, and up to more than 100% (mean: 64 +/- 24%) for the radiogenic heat production. A strong regional thermal-conductivity variability impact was quantified by the comparison of subsequently modelled geotherms with measured borehole temperatures. When basin-wide mean formation conductivities (representing the usual assumption of constant formation values in geothermal models) are applied to such models, the misfit between the predicted and measured temperatures at the maximum borehole depth of approximately 4 km is large and averages approximately 20% (range: -21 to 22 degrees C). Application of the observed but less representative formation conductivities in terms of the 'true' overall basin average yields maximum derivations between 27 and 66% (range: -38 to 90 degrees C). The application of local formation conductivities, in contrast, yields minimum deviations generally less than < 5 degrees C, depending on consideration of regional or location-specific heat-flow values. Statistical data on the mean formation variability presented here can serve as guidelines to define reasonable variation ranges for the input or the post-processing calibration procedures for geothermal models of sedimentary basins with similar lithologies and genesis to the Danish Basin. In general, knowledge of the variability of formation thermal properties will lead to a significantly lower uncertainty in the temperature calculations, in particular but not exclusively for areas and depths where temperature observations are unavailable.