Research On Microseepage Of Methane And Its Flux Evaluation In Dawanqi Oilfield

Methane is one of the most important greenhouse gases,which attracts the attention all over the world.In the past decade,the source and sink of geological methane has gradually become one of the hotspots of global greenhouse effect research.It is of great significance to explore and reveal the microseepage of methane in petroliferous basins.In this study,the surface methane flux,the variation of gas concentration,and the carbon isotope composition of methane and carbon dioxide in different depths of the underground were studied in Dawanqi oilfield,Tarim Basin,China.The results show that methane emission is common.74.32%of the sampling points show methane emission,and 25.68%of the sampling points show negative flux.Through the analysis of the monitoring results of the methane flux and the petroleum geological data,it can be seen that the methane flux is affected by the geological structure,the thickness of the oil layer,the buried depth of the oil and gas reservoir,the gas oil ratio of the oil reservoir and the seepage of the overlying strata.From the analysis results of the methane flux and the soil gas and adsorbed gas,the correlation between the methane flux and the two is very weak,which also proves that the influencing factors of the surface methane flux in are extremely complex.In this study,soil gas,adsorbed gas and carbon isotope in different depths of the near surface were studied.The results show that the concentrations of methane,hydrocarbon gas and carbon dioxide in soil gas of Dawanqi oilfield are much higher than those in the background area,indicating the presence of methane and hydrocarbon gas from the underground oil and gas reservoir.And the high concentration of carbon dioxide indicates that part of hydrocarbon gas is oxidized in the process of upward migration.At the same time,the concentration of methane in soil gas and adsorbed gas,and carbon dioxide in soil gas all showed a significant downward trend.The ?13C of methane in soil gas is between-46.3‰--18.9‰,which belongs to thermogenic gas;the ?13C of carbon dioxide is between-23‰?-9.59‰,which indicates that carbon dioxide comes from organic matter.All the above results show that there is microseepage methane from deep oil and gas reservoir in Dawanqi oilfield.In summer,it is the source of methane,while in winter,it is the sink of methane.According to the methane flux in summer and winter,the methane flux is estimated to be 69.56t/y in Dawanqi oilfield.The results enrich the microseepage methane flux database of global petroliferous basins.