Based on research on space-time distribution and geochemical characteristics of sour gas fields in Sichuan Basin and relevant simulation experiment, H₂S formation mechanism is revealed and its distribution is predicted. P₂ch-T₁f reservoirs have sufficient magnesic SO₄^2- solution and abundant hydrocarbons, and at high temperature, sulfate thermochemical reduction (TSR) occurs. This is the major origin of the abundant H₂S in large to medium scale gas fields. The paper predicted that the Heichiliang reef flat facies sedimentary zone in Northeast Sichuan has highly sour natural gas reservoirs.