The main reservoir rocks in giant oil fields of the world are sandstones and carbo-nate rocks. In Japanese oil fields, however, the reservoir rocks are such sedimentary rocks as sandstones (59per cent), tuffs (37per cent) and carbonate rocks (1per cent), and volcanic rocks 3per cent). To form good reservoirs in a basin, these rocks must have high porosity and permeability and be developed extensively with adequate thickness. The latter refers to vertical and horizontal dimensions of reservoirs in a basin, and is controlled chiefly by geological and geographical factors during deposition. The former includes petrophysical properties of reservoirs and are a modifica-tion of primary petrophysical properties resulting from subsequent physical and chemical processes after deposition. These processes are compaction, fracturing, solution, cementation, and transform-ations such as dolomitization and calcitization. Generally, most important and essential mass properties of reservoir rocks, i.e. effective porosity and permeability, are mutually related. Also, effective porosity is positively correlative with absolute porosity. This indicates that absolute po-rosity must be one of the important factors for the evaluation of reservoir rocks. The writer and his colleagues, therefore, have studied the origin of porosity in reservoirs of various sedimentary basins in Japan and Canada from a petrophysical viewpoints. As a result, he came to a conclusion that the areal and quantitative analysis of each processes relating to origin of absolute porosity must be most important for the evaluation of reservoirs. Examples of this evaluation, the Cretaceous sandstones of central Hokkaido, Japan, have been discussed in this report. Limy lithic wackes of the Cretaceous Ezo Group of Hokkaido have moderate grain density (2.68g/cm3 in average) and low absolute porosity (4.7per ecnt in average). The most important processes that decreased the porosity were compaction and areal cementation by calcite and chlorite. Porosity increased slightly as a result of solution and fracturing. There-fore, these sandstones may be poor as a petroleum reservoir.