Abstract: In many basins with the thin laminated groups of sands imbedded in organic-rich shale,shale and sandstone pay zones communicate with each other,and should be taken as inseparable parts of one pressure system. In order to calculate the original gas reserves in place,the differences of pores and fluid between shale and sandstone were com- pared,and the reservoir was divided into inorganic matrix and organic matrix,on which the volume model was estab- lished. According to the principle of material balance,the Langmuir adsorption isotherm model and Palmer & Mansoori model,the change of porosity and fluid with pressure drop in both matrices were discussed,and the material balance equation for reservoir with shale gas and laminated sandstone pay zones was derived herein. The field case study proves that new material balance equation can be applied to calculate original free gas and adsorbed gas reserves in shale or sandstone,and estimate the desorbed gas in development process.