Mercury (Hg) naturally occurs in gas and oil deposits ranging from background concentration up to 5 mg/m3 in natural gas and up to 600 mg/kg in crude oil. Mercury-containing ore, coal, oil and gas deposits are generally located within the crossing zones of deep faults and transcontinental lineaments. Mercury in hydrocarbons poses a number of technological and environmental problems including Hg exposure to humans, contamination of environment and equipment, and enhancing accident risk due to the processing equipment corrosion. We studied the Hg content in ca. 80 European and Asian oil and gas deposits using the Lumex RA-915 analyzer, a universal tool for direct mercury determination in gaseous, liquid and solid media that is based on Zeeman atomic absorption spectroscopy. The evident spatial–temporal variability of mercury content in studied gas deposits with elevated Hg concentration was found. Measurements taken at different gas deposits revealed a substantial year to year variability of the average mercury concentration in producing gas wells, which ranges from 1.5 to 15 times that should be taken into account when assessing the Hg reserves in a deposit and selecting a technology of mercury removal. Besides, the short-term regular Hg variations in producing gas wells with periods of 0.3–25 h were discovered. The oscillation magnitudes range from 10 to 80% of the average content of mercury in gas. We assume that regular variations are caused by an effect of the adsorbed Hg(0) release to the gas phase due to changing mechanical stress inside the porous rocks. The regular variation of the Hg in natural gas can be considered as a possible earthquake precursor.