Isotopic evidence of TSR origin for natural gas bearing high H2S contents within the Feixianguan Formation of the northeastern Sichuan Basin, southwestern China
Isotopic evidence of TSR origin for natural gas bearing high H2S contents within the Feixianguan Formation of the northeastern Sichuan Basin, southwestern China
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DOI：
10.1360/082004-147
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发表时间：
2005-11
影响因子：
5.7
通讯作者：
Z. Guangyou;Zhang Shui-chang;Liang Yingbo;D. Jin-xing;Li Jian
中科院分区：
文献类型：
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作者：
Z. Guangyou;Zhang Shui-chang;Liang Yingbo;D. Jin-xing;Li Jian
四川盆地东北部、中国西南地区，是中国含油气盆地中发现的含硫化氢(H_2S)含量较高的天然气储量最大的地区，探明控制天然气储量2000多亿立方米。这些气藏主要分布在独口河、铁山坡、罗家寨、普光等构造带上，而三叠系飞仙关组蚝滩白云岩则分布较广，H_2S含量较高，平均为9%，约17%。(T1f)是储集层。尽管许多学者认为深层碳酸盐岩储层中的硫化氢大量聚集是热化学硫酸盐还原作用的结果，但硫酸盐还原作用的过程及其残留的地质和地球化学证据仍不十分清楚。根据碳酸盐岩地层、次生方解石等的碳同位素分析，结合硫化氢、硫、石膏、铁黄铁矿等硫同位素分析，以及天然气组成、碳氢化合物的碳同位素、储集层岩石学等方面的分析，证明上述天然气是TSR的产物。硫化氢、硫和方解石是烃类气体参与TSR反应的产物。在TSR消耗碳氢化合物的过程中，烃类气体中的碳参与反应，最终转移到次生方解石中，成为次生方解石的碳源，从而导致次生方解石的碳同位素较低(−18.2‰)。TSR的中间产物硫磺及其最终产物硫化氢和黄铁矿，其硫元素均来源于飞仙关组内的硫酸盐。在硫同位素的分馏过程中，键能导致32S首先释放，释放越早，生成的硫化物(H_2S)或硫的δ~(34)S值越低。但是，对于参与反应的硬石膏，反应程度越高，释放的32S越多，剩余的32S越少，δ34S越多。测试结果证实了硫同位素动态分馏过程。
The northeastern area of Sichuan Basin, southwestern China, is the area with the maximal reserve of natural gas containing higher hydrogen sulphide (H2S) that has been found among the petroliferous basins of China, with the proven and controlled gas reserve of more than 200 billion cubic meters. These gas pools, with higher H2S contents averaging 9%, some 17%, are mainly distributed on structural belts of Dukouhe, Tieshanpo, Luojiazhai, Puguang, etc., while the oolitic-shoal dolomite of the Triassic Feixianguan Fm. (T1f) is the reservoir. Although many scholars regard the plentiful accumulation of H2S within the deep carbonate reservoir as the result of Thermochemical Sulfate Reduction (TSR), however, the process of TSR as well as its residual geological and geochemical evidence is still not quite clear. Based on the carbon isotopic analysis of carbonate strata and secondary calcite, etc., together with the analysis of sulfur isotopes within H2S, sulphur, gypsum, iron pyrites, etc., as well as other aspects including the natural gas composition, carbon isotopes of hydrocarbons reservoir petrology, etc., it has been proved that the above natural gas is a product of TSR. The H2S, sulphur and calcite result from the participation of TSR reactions by hydrocarbon gas. During the process for hydrocarbons being consumed due to TSR, the carbons within the hydrocarbon gas participate in the reactions and finally are transferred into the secondary calcite, and become the carbon source of secondary calcite, consequently causing the carbon isotopes of the secondary calcite to be lower (−18.2‰). As for both the intermediate product of TSR, i.e. sulfur, and its final products, i.e. H2S and iron pyrites, their sulfur elements are all sourced from the sulfate within the Feixianguan Fm. During the fractional processes of sulfur isotopes, the bond energy leads to the 32S being released firstly, and the earlier it is released, the lower δ 34S values for the generated sulphide (H2S) or sulfur will be. However, for the anhydrite that participates in reactions, the higher the reaction degree, the more 32S is released, while the less 32S remains and the more δ 34S is increased. The testing results have proved the process of the dynamic fractionation of sulfur isotopes.