Recent years have seen an active research of thermal methods of monitoring the development of oil and gas fields. Among other reasons temperature changes in the oil reservoir are brought about by the manifestations of the adiabatic and Joule-Thomson effects. It may also alter due to degassing heat in case when pressure in the wellbore drops below the oil saturation pressure of the gas. The paper presents some results of experimental and theoretical study of the impact of the adiabatic and Joule-Thomson effects as well as the fluid degassing heat on the temperature field in the oil-gas reservoir. To calculate the formation temperature field in details one has to determine the thermodynamic coefficients for real formation fluids. For this end, the experimental set-up is built comprising a PVT cell to determine the Joule-Thomson and adiabatic coefficients as well as the heat of oil degassing. The experimentally obtained thermodynamic coefficients are used to calculate the evolution of temperature field in the porous medium and wellbore for various pressure gradients and gas contents. The results serve as a scientific background for development of thermal monitoring methods in oil and gas reservoirs. An example of practical application of thermal monitoring methods in oil industry is also presented.