Abstract:

In places with simple hydrogeological conditions,the sealing of coal bed gas is mainly determined by geological structures and cap rock lithologic propertices.In general,compressional fault could increase the content of coal-bed methane while tensional fault would decrease the content of coal bed methane.Symmetric high stress zones in parallel stripped shape would usually form in two sides far off the fault,thus causing the increase of coal bed methane content,and forming a natural curtain to block methane migration to the fault.Coal bed methane accumulation would occure above the balancing surface of anticlines and below the balancing surface of syncline.According to their expulsive pressure and permeability,the cap rocks of coal-bed methane could be divided into shielding bed,semi-shielding bed and permeable bed,which in turn could be subdivided into 9 types of lithologic assemblages.Examples of Pingdingshan coal dristrict show that,tpye-Ⅰ assemblage is more favourable for coal-bed gas;type-Ⅱ,type-Ⅳ and tpye-Ⅴ assemblages are secondany;the rest are poor.

Key words: coal-bed gas, sealing condition, structural-lithologic assemblage, Pingdingshan coal district