Pyrite-hydrocarbon Interaction under Hydrothermal Conditions: an Alternative Origin of H₂S and Organic Sulfur Compounds in Sedimentary Environments

详细信息    查看全文

• 作者：Kangle DING ; Ping MEI and Yue LUO
• 刊名：Acta Geologica Sinica - English Edition
• 出版年：2016
• 出版时间：December 2016
• 年：2016
• 卷：90
• 期：6
• 页码：2133-2148
• 全文大小：1972K
• ISSN：1755-6724

文摘

Sulfate rocks and organic sulfur from sedimentary organic matter are conventionally assumed as the original sulfur sources for hydrogen sulfide (H₂S) in oil and gas reservoirs. However, a few recent experiments preliminarily indicate that the association of pyrite and hydrocarbons may also have implications for H₂S generation, in which water effects and natural controls on the evolution of pyrite sulfur into OSCs and H₂S have not been evaluated. In this study, laboratory experiments were conducted from 200 to 450°C to investigate chemical interactions between pyrite and hydrocarbons under hydrothermal conditions. Based on the experimental results, preliminary mechanism and geochemical implications were tentatively discussed. Results of the experiments showed that decomposition of pyrite produced H₂S and thiophenes at as low as 330°C in the presence of water and n-pentane. High concentrations of H₂S were generated above 450°C under closed pyrolysis conditions no matter whether there is water in the designed experiments. However, much more organic sulfur compounds (OSCs) were formed in the hydrous pyrolysis than in anhydrous pyrolysis. Generally, most of sulfur liberated from pyrite at elevated temperatures was converted to H₂S. Water was beneficial to breakdown of pyrite and to decomposition of alkanes into olefins but not essential to formation of large amounts of H₂S, given the main hydrogen source derived from hydrocarbons. In addition, cracking of pyrite in the presence of 1-octene under hydrous conditions was found to proceed at 200°C, producing thiols and alkyl sulfides. Unsaturated hydrocarbons would be more reactive intermediates involved in the breakdown of pyrite than alkanes. The geochemistry of OSCs is actually controlled by various geochemical factors such as thermal maturity and the carbon chain length of the alkanes. This study indicates that the scale of H₂S gas generated in deep buried carbonate reservoirs via interactions between pyrite and natural gas should be much smaller than that of thermochemical sulfate reduction (TSR) due to the scarcity of pyrite in carbonate reservoirs and the limited amount of long-chained hydrocarbons in natural gas. Nevertheless, in some cases, OSCs and/or low contents of H₂S found in deep buried reservoirs may be associated with the deposited pyrite-bearing rock and organic matters (hydrocarbons), which still needs further investigation.