Natural Emissions of Methane and Source Identification from Oil-Water Interface of Yakela Condensed Oil/Gas Field

In order to assess correctly the flux of CH 4 emitted naturally from oil/gas basin,a static flux chamber method was applied to study natural emissions of methane to the atmosphere in oil-water interface of Yakela condensed oil/gas field in Tarim basin,Xinjiang.Using an online method which couples a gas chromatography/high-temperature conversion/isotope ratio mass spectrometry(GC/C/MS)together,13 C/12 C ratios of the methane in flux chambers were measured.The results demonstrate that oxidation environments of soil and water are different in the oil-water interface because chambers lie in different places.So,fluxes of methane emission in the chambers are very different.The maximum daily flux of methane emission is 2.28 mg/m2·d and the minimum-1.32 mg/m2·d.The average daily flux of methane emission is 0.51 mg/m2·d with the standard deviation of 1.23 mg/m2·d.Although there are large differences in flux of methane emission in the oil-water interface,daily change laws are similar:the methane emission flux reaches the maximum at 5:00-6:00 am,and then it decreases gradually,and reaches the minimum at 17:00-18:00 pm,and then it gradually increases.Measurement of carbon isotope of methane shows that δ 13 C of methane becomes heavier with decreasing concentration in the afternoon and with increasing concentration in the morning.Thus it can be seen that the soil above the oil-water interface becomes a more oxidation environment because water contains O 2.Methane is absorbed and oxidated constantly during migration to the earth's surface from deep oil/gas reservoir through microseepage and pervasion.Only a little methane which is not oxidized and degraded is discharged into the atmosphere through diffusing and convection.Even all methane is absorbed by soil in some areas,which leads to the sink of atmospheric methane.