DISTRIBUTION, CLASSIFICATION AND ORIGIN OF NATURAL GAS WITH HYDROGEN SULPHIDE IN CHINA

Abstract: In china, hydrogen sulphide contents of natural gas are obviously controlled by reservoir lithology. H₂S contents of natural gas in clastic rocks are very low even down to zero. The highest H₂S contents of natural gas in those rocks are 0.68%, which can be found in Upper Triassic Xiangxi sandstones in Sichuan Basin.Carbonate reservoirs commonly contain H₂S, sometimes with high contents. Natural gas containing H₂S more than 1 % is all in carbonate rocks ( Fig. 1 ) in China, which consist of Sinian system ( Dengying Formation ) , Ordovician system ( Nanjinguan Formation ) , Permian system ( Maokou and Changxing Formations ) , Triassic system ( Feixianguan, Jialingjiang and Leikoupo Formations ) and lower Tertiary system ( Kongdian and Shahejie Formations ) in succession of strata. The highest H₂S contents of natural gas in carbonate reservoirs are about 92%, which are accumulated in Kongdian Formation of lower Tertiary. H₂S reservoirs of natural gas higher than 1 % cover a quarter of the total non-associated gas reserves in China. At present, it is one of the main sources for sulphur-producing. According to the H₂S contents of natural gas, and their characteristics of strati-graphic suite, with reference to the original phase in gas pools or in natural gas, and the demands of desulphurization process after gas development, gas pools containing H₂S can be classified as follows: 1. H₂S gas pools: H₂S contents in natural gas are over 70%, which are distributed in sulfate-carbonate sequence, for example, H₂S contents in Zhaolanzhuang gas pools are about 92%. Underground H₂S is in liquid phase. 2. High H₂S gas pools: H₂S contents in natural gas are about 2-70%, which are also accumlated in stratigraphic suite of sulfate-carbonate, such a gas pools of Changxing, Jialingjiang and Leikoupo Formations in Sichuan Basin. Underground H₂S is not in liquid phase. 3. Low H₂S gas pools: H₂S contents in natural gas are about 0.5-2 %, most of them are less than 1.5%,which developed in carbonate sequence,such as the Weiyuan gas pool in Sichuan Basin. 4. poor H₂S gas pools: H₂S contents are only a little, about 0-0.5%, most of them are less than 0.05%, which developed in carbonate sequence as well as clastic sequence. 5. Gas pools without H₂S or non H₂S gas pools, which mainly developed in clastic strata as well as carbonate reservoirs. Maximum H₂S contents of natural gas for individual stratum are listed in Table 3 . Based on the characteristics of stratigraphic suites containing H₂S gas, reservoir behaviours and geochemical characteristics of gas generation, the genesis of H₂S can be classified into four types. 1. Nonbiogenic reduction type Under reducing environment with high temperature, hydrocarbon or organic matters are able to convert sulfates into H₂S gas, So H₂S gas pools and high H₂S gas pools are commonly formed in these conditions. 2. Biogenic reduction type Hydrocarbon contained in sulfate stratum was converted into H₂S by decomposition of sulfate bacterial reduction. 3. Thermal cracking type Oil and sapropel types of kerogen in carbonate sequence underwent thermal cracking during the stage of over heating. After the thermal cracking, only a small amount of H₂S gas could be formed, because the source of sulphur in carbonates is not so abundant as that in sulfatec-arbonate sequence, thus, all these gas pools belong to low H₂S gas pool. 4. Secondary type H₂S-generating was not in relation to reservoirs, it migrated from other strata into clastic reservoirs in various ways, so all gas pools are poor H₂S gas pools.