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.