Abstract: The generation characteristics and the primary migration phase of petroliferous gas in soutern Dongying sag are studied. The exsolution conditions and affecting factors of such a gas and the changes of its drying coefficient in secondary migration stage are analyzed. The existing gas phase lies on the components of oil and gas and the pressure and temperature of petroleum system, and can be judged with P-T phase diagram. The dry coefficient of exsoluted gas from oil becomes higher than that before exsolution. Bacteria degradation may accelerate the course of such a exsolution and allows the dry coefficient to be higher. The study shows that the drying coefficient of petroliferous gas generated from the source rocks in southern Dongying sag is relative low (0.45 ～0.70), being primary migration of the oil-soluble phase; during its secondary migration, the exsoluted gas occurs in depth of 1 400 ～1 500 m. So the petroliferous gas reservoirs can only be developed over the depth of 1 400 m, with higher drying coefficient (0.95～1); while below the depth of 1 500 m, the petroliferous gas mainly dissolved in oils, and its drying coefficient lies on the drying coefficient of natural gas primarily generated from source rocks and bacteria degradation degree in this area.

Key words: Dongying sag, natural gas, phase state, gas-oil ratio