The key of assessment of shale gas is to evaluate the spatial distribution of original gas in-place (OGIP) to delineate the core area. The occurrence of shale gas is interim between tight sandstone gas and coalbed gas. Shale gas is composed of free gas in pores, adsorbed gas in the solid organic matter, solution gas in the residual oil and water. Pressure-temperature conditions control the partitioning among three gases and reciprocal transformation. The key controlling free gas, adsorbed gas and solution gas are porosity of shale, amount and maturation of organic matter, amount of residual oil in shale, respectively.