Taking Daqing oil field in the Songliao sedimentary basin as an example, the geological and geochemical conditions for the formation of a large nonmarine oil field and the factors controlling its formation are presented.

On the basis of the features in the source-reservoir framework of the horseshoe (the semi-ringlike) source area with a huge deltaic complex wedged into it, the paper also analyzes the significance in the formation of a large nonmarine oil field, due to the ideal combination of the source-reservoir-cap rocks formed with the excellent shale-sand ratio and the stable regional caps, as well as the favorable reservoir spaces set by the large complex deltaic sands and the large placanticline structure belts.

On the basis of the regularities of the generation, migration, and accumulation of nonmarine oil and gas, in the author's opinion, the formation of Daqing oil field is characterized by the following.

1. Around the field, there is a highly effective source rock not very large in volume.

2. The reservoir rocks are interbedded with the source rocks. Vertically, both rocks are largely contacted, laterally they are interfingered. The expulsion efficiency and the probabilities of oil-entrapment are high.

3. On the south, the oil was generated and was preserved there.

4. In the northern part, the lateral short-distance migration dominates.

5. Within the field, no large-scale oil migration ever occurred in a south-north direction. The oil-gas accumulation is controlled by the dynamic system of the subsurface water. The differences between the formation pressure and the saturation pressure resulted in oil and gas segregation.

On the basis of the ratio of the reservoir area to the source area, it is concluded that it is not necessary for the large nonmarine oil field to be formed to have long distance large-scale source rocks as in the case of a marine oil field.

The formation and development of the Daqing placanticline are discussed in detail. By combining the relation of the time and space for oil and gas generation, migration, and accumulation into the development history of geological structures, it is considered that the favorable coordination between the time during which oil was generated, migrated, and accumulated and the time when the structure was formed, is the basis for the formation of the Daqing oil field.