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Abstract: A comprehensive study on the temporal and spatial distribution of H2S accumulations and the geochemical characteristics of sour gas fields (reservoirs) in the Sichuan Basin has been conducted. It has been followed by a series of carefully designed simulation experiments to elucidate an H2S formation mechanism which is able to predict the distribution of H2S. In some reservoirs, for example P2ch(T1f, have sufficient magnesic SO4 2( and abundant hydrocarbons (palaeo pools) present at sufficiently high temperatures to cause thermochemical sulfate reduction (TSR). This is the major origin of abundant H2S in large to medium scale gas fields. The formation mechanism of highly sour gas fields (reservoirs) is an in-situ reaction under conditions where sufficient source, enough energy supply, suitable reservoir volume and severely reducing environments occur. However, in some areas or strata, such as slope belts or on continental shelf deposits, or formations such as the Jialingjia and Leikoupo Formations, which are dominantly compact carbonatite and have contemporaneously different facies, it is difficult to form abundant H2S. These formations lack a source of sulphur and energy compared to the P2ch(T1f reef flat facies reservoirs. Even though highly sour natural gas migrating from the P2ch(T1f reservoirs is present, H2S is often consumed due to long distance migration and results in hydrocarbon-rich sweet natural gas. This study predicts that the Heichiliang reef flat facies sedimentary zone in northeastern Sichuan to have highly sour natural gas reservoirs. [ABSTRACT FROM AUTHOR]