Na-bentonite and sea water were mixed and compacted under the programming temperature and pressure to clear the physical conditions of transformation of clay minerals and the primary migration of hydrocarbons during diagenesis experimentally. Water content in the samples decreased rapidly after the short period of compaction. Then it decreased gradually with time and reached the first equilibrium at about 30 per cent of absolute porosity. Porosity decreased below 15 per cent by continuous compaction. Content of cations and anions in the expelled water differed from those in the sea water. Though Na+, Ca++, Cl- and SO4-- were stable in their content, HCO3- increased and Mg++ and K+ decreased in content with the rise of pressure and temperature. Chemical composition of compacted samples were different slightly from those of original bentonite. With the rise of temperature and pressure, CaO and Na2O decreased in their content, while K2O was stable. Content of MgO increased by compaction below 700kg/cm2 and 80°C, while it decreased by compaction beyond this pressure and temperature. Mineralogical study of compacted samples by X-ray diffraction and thermal analysis showed that the structure and layered water in montmorillonite were almost the same as those in original mineral.