The Daqing oil field and the principal productive zones in the northern part of the Songliao basin are composed of the depositional system of the Xingshugang delta (Lower Cretaceous Putaohua Formation). The subtle (difficult-to-detect) oil pools in the delta, distributed within the Sanzhao and Gulong synclines, developed mainly as lithologic reservoirs of large areal extent. The major geologic condition forming the lithologic reservoirs in the Xingshugang delta is their proximity to a favorable source area. Thick, mature source rocks that overlie and underlie the Putaohua Formation provide a rich source of oil. Lithologic traps, formed by both lenticular and sheet sandstone bodies are one of the necessary conditions for these large oil-bearing areas. A noselike structure in a syncline, which was synchronously developed with hydrocarbon generation and migration, is the favorable area for oil and gas enrichment.

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