Methane weathering zone division method and influencing factors of low-rank coalbed methane in Urumqi Mining Area,Southern Junggar Basin

In order to study the characteristics and influencing factors of methane weathering zone of lowrank coalbed methane in Urumqi mining area,southern Junggar,avoid exploration risk and reduce development cost,based on the characteristics of low gas content and the coexisting of secondary biogenic gas with thermogenic gas in lowrank coal,a new method of dividing methane weathering zone was proposed by using the test data of gas content and concentration of main gases (CH4,CO2,N2).The methane content ≥1 m3/t and N2 concentration ≤20% were used as indicators to assess the economic boundary and geological boundary of methane weathering zone,respectively.The method was used to divide the depth of methane weathering zone of Hedong area and Hexi area in Urumqi,and the influence of structure,sediment and hydrogeology on the depth of methane weathering zone was discussed,and the cause of the depth difference of methane weathering zone in the two areas was analyzed.The results showed that the depth of methane weathering zone was about 370 m in Hedong and 200 m in Hexi.Before the tectonic uplift,part of thermogenic methane was generated in the two areas,the methane weathering zone was deeper.The study area experienced two strata uplift movements in Late Jurassic and Cenozoic.The roof of the coal seam in Hedong was mainly sandstone.On the one hand,it was easy for surface water to infiltrate,which generated a large amount of secondary biogenic gas in the study area.With the increase of groundwater stagnant degree,a large number of coal seam gas was enriched.On the other hand,the poor roof sealing also caused a certain amount of coalbed methane loss,resulting in the methane weathering zone in Hedong being deeper than that in Hexi.However,the mudstone roof in Hexi had a good ability to obstruct water and block gas.After the tectonic uplift,the infiltration ability of surface water was limited,resulting in the generation of a small part of secondary biogenic gas.Due to the mudstone roof had a strong ability to block gas,so that most of the coalbed methane could be preserved,and the methane weathering zone was shallow