Abstract: Wettability of rocks can be determined through experimental measurements indoors. However there is no oil-wet rock underground. By experiment, the degree of water-wettability of rocks can only be measured rather than the oil-wet property of rocks. The water-wet property of rocks makes them possible that oil and gas migrate to traps successfully, and the pressure of reservoirs distributes normally. If the rock underground is oil-wet, oil and gas will scatter along the migration way and no hydrocarbon will be accumulated finally. In oil-wet reservoirs, the distribution of formation pressure will appear as incredible phenomenon. It is concluded that there is no oil-wet rock underground, and the rocks underground can only be water-wet in nature.

Key words: rock, reservoir, wettability, oil-wet, water-wet, pressure distribution, oil-gas migration