FIELD: monitoring systems.SUBSTANCE: invention relates to methods for monitoring underground formation in which non-conventional hydrocarbons are produced. Essence: inert gas diffusion model and target hydrocarbon diffusion model are selected. Each model describes the change in concentration as a function of time, depth and diffusion coefficient. Prior to the production of hydrocarbons, at least one fluid sample present in the zone to be monitored is taken from the soil layer above the underground formation. Composition of at least one inert gas is measured within the sample. During and/or after production of hydrocarbons, at least one second fluid sample present in the zone to be monitored is taken. Concentration of inert gas within the second sample is measured. Wherein the second sample is taken and the concentration of inert gas is measured within said sample repeatedly at different times. When the concentration of inert gas increases, the effective diffusion coefficient of the inert gas diffusion model i
