Zheng Lunju

H2S formation and enrichment mechanisms in medium to large scale natural gas fields (reservoirs) in the Sichuan Basin

Abstract A comprehensive study on the temporal and spatial distribution of H 2 S 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 H 2 S formation mechanism which is able to predict the distribution of H 2 S. In some reservoirs, for example P 2 ch(T 1 f, have sufficient magnesic SO 4 2( and abundant hydrocarbons (palaeo pools) present at sufficiently high temperatures to cause thermochemical sulfate reduction (TSR). This is the major origin of abundant H 2 S 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 H 2 S. These formations lack a source of sulphur and energy compared to the P 2 ch(T 1 f reef flat facies reservoirs. Even though highly sour natural gas migrating from the P 2 ch(T 1 f reservoirs is present, H 2 S 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.