Zhang Dianwei

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.

An isotope study of the accumulation mechanisms of high-sulfur gas from the Sichuan Basin, southwestern China

The mechanism of hydrogen sulfide (H2S) generation plays a key role in the exploration and development of marine high-sulfur natural gas, of which the major targets are the composition and isotope characteristics of sulfur-containing compounds. Hydrocarbon source rocks, reservoir rocks, natural gases and water-soluble gases from Sichuan Basin have been analyzed with an online method for the content of H2S and isotopic composition of different sulfur-containing compounds. The results of comparative analysis show that the sulfur-containing compounds in the source rocks are mainly formed by bacterial sulfate reduction (BSR), and the sulfur compounds in natural gas, water and reservoir are mainly formed by thermal sulfate reduction (TSR). Moreover, it has been shown that the isotopically reversion for methane and ethane in high sulfur content gas is caused by TSR. The sulfur isotopic composition of H2S in natural gas is inherited from the gypsum or brine of the same or adjacent layer, indicating that the generation and accumulation of H2S have the characteristics of either a self-generated source or a near-source.