The harmonic mean thermal conductivity of the whole rock column traversed by a well is calculated from the thicknesses of the different kinds of rock in the section and their respective thermal conductivities. Lithologic logs provide the thicknesses; laboratory measurements on cores, the conductivities. Heat flow can be then calculated by multiplying the mean conductivity by the temperature gradient obtained from corrected bottom-hole temperatures. In the sedimentary basins of Sumatra and Brazil the conductivities range from 4.1 to 6.5 mCal/cm°C sec but the average for all the basins is close to 5. The mean conductivity of a well depends on the sand-shale ratio and can probably be calculated without loss of precision without recourse to more measurements on samples. Conductivity determinations and heat flow in three Tertiary basins in Sumatra and in two Cretaceous basins in Brazil involving over 2,000 wells are reviewed. It is suggested that calculations of mean conductivity and heat flow be added to the existing AAPG compilation of temperature gradients in North America.