Abstract: In China, natural gas resources mainly include coal-derived gas, oil-cracked gas, and biogenic gas (biogas). In the present study, the time-limit and yield of natural gas generation from three different origins are investigated through a thermal simulation experiment combined with other methods. Results show that the formation depth, geothermal temperature, and organic matter maturity for generation of biogas, coal-derived gas, and oil-cracked gas from different origins increase to various extents generally towards deeper depth. It is recognized that a large quantity of microorganisms were detected in deep formations of Sanhu area, Qaidam Basin, and that the secondary organic matter produced by thermal action could provide nutrients for the survival of deep microorganisms. Therefore, the upper temperature limit for biogas generation may break the conventional record of 75℃. As for gas generation from coal-measure source rocks, there exists obvious difference from marine or lacustrine organic matter with no evident peak stage of hydrocarbon generation the upper limit of organic matter maturity for gas generation rises from 2.5% previously thought to 5.0% ; total gas yield increases from less than 200 m³/t TOC formerly known to more than 330 m³/t TOC. Results of experimental simulation and kinetic research show that crude oil cracking initiates at the formation temperature of 190℃, amplifies at greater than 210℃, and ends at about 230℃; in case the thermal gradient is at a constant of 30℃/km, crude oil cracking initiates at the depth should be more than 6 000 m. These results contribute to understanding of the time-limit and yield of natural gas generation from three major origins and thus provide reference data for evaluating deep gas resources and forecasting the depth range for petroleum exploration in China.