X-ray diffraction，scanning electron microscopy and casting thin observation of 78 samples from Paleozoic reservoirs in the Huanghua Depression indicate that the main clay mineral are mainlyas follow illite，illite-smectite and there are a few authigenic kaolinite and chlorite and the clay min-eral appear as film-like style and bridge type. Illite content and I/S increase with depth inereasing, which lead to the decrease of permeability of reservoir pore. Three typical of the relationship between the mercury curves and clay minerals were obtained by taking pressured-mercury testing. The claymineral assemblages imply that this area's ancient environment was dry-salt lake; the regional dia-genetic environment were alkaline; the environment being close to oil reservoir showed individually al-kali-acid-alkali process.