Shale gas is unconventional natural gas

mainly CH

4

occurring in organic shale. The adsorbed gas is the main source for later production of shale gas. Therefore

studying the adsorption mechanism of shale plays an important role in shale gas development. By using type Ⅱ kerogen molecules

a type Ⅱ kerogen model is established. Then

Monte Carlo method and molecular simulation method are used to study the micro adsorption behavior and mechanism of CH

4

in type Ⅱ kerogen. Experimental data are used to verify this model

and the effec

ts of pore size

temperature and pressure on the adsorption behavior are investigated. The findings are as follows: ① The higher the pore size

the greater the excess adsorption capacity of CH

4

. The higher the temperature

the lower the excess adsorption capacity of CH

4

. With the increase of pressure

the absolute adsorption amount of CH

4

increases rapidly at first and then gently

and the excess adsorption amount of CH

4

increases first and then decreases. ② The adsorption heat of CH

4

decreases with the increase of pore size. The adsorption of CH

4

in kerogen is physical adsorption. ③ When the pore size is smaller than 1 nm

CH

4

is the adsorption phase in kerogen; when the pore size is larger than one nanometer（1 nm）

CH

4

is the coexistence of adsorption phase and free phase in kerogen.