Distribution and genesis of mercury in natural gas of large coal derived gas fields in China

Abstract

The mercury content in natural gas samples from more than 500 gas wells in eight large gas bearing basins of China was tested, mercury release experiments on two coal samples from different areas were conducted, and the mercury content of 11 coal samples from different gas wells of Ordos Basin was tested. The mercury distribution of the coal derived gas has three features: The first is that mercury content of coal derived gas is generally much higher than that of oil derived gas, the second is that the coal derived gases from different fields vary widely in mercury content, the third is that the mercury content in coal derived gas increases with the increase of production layer depth. Mercury in coal derived natural gas mainly originates from the source rock. Besides three evidences, namely, coal derived gas mercury content is much higher than that of oil derived gas, mercury content of gas with high carbon dioxide content decreases with the increase of carbon dioxide content, and the coal bearing strata have the material base to generate natural gas with high mercury content, the pyrolysis experiment of two coal samples show that coal can produce natural gas with high mercury content during the process of thermal evolution. The mercury content of coal derived natural gas is controlled mainly by the temperature of source rock and the sulfur environment of reservoir. According to lithospheric material cycling process and oil-gas formation process, the formation of mercury in coal derived gas can be divided into four stages, transportation and deposition, shallow burial, deep burial, and preservation and destruction. Key words: China, coal derived gas, mercury, gas source rock, oil derived gas, gas genesis