Data on the geothermal temperature and the temperature gradient of the earth's crust section are necessary for solving a number of fundamental and applied problems. However, the currently used methods for determining the geothermal temperature based on the temperature measured in the wellbore have known disadvantages and can only be used in a small number of wells in the field under study. The article proposes a new method for determining the geothermal temperature and the temperature gradient from the measured temperature distribution in production wells with a long enough sump. It is shown that every other well out of more than four hundred considered wells of oil fields in the Republic of Bashkortostan (Russia) has an appropriate sump. The influence of heat transfer from the working formation, metal casing and natural thermal convection in the borehole fluid on the temperature field in the sump of the production well is studied analytically and numerically. An algorithm for finding an area with undisturbed temperature distribution in the sump of the wellbore is described. The requirements for the choice of a well and corresponding sump section are given, as well as the procedure for the subsequent processing of thermograms, which makes it possible to determine the geothermal temperature and the temperature gradients in the sump. The efficiency of the proposed technique is demonstrated by two cases of field data examples.