Abstract:The oil and gas exploration in the Shibei subsag of Jiuquan Basin has lasted for more than 70 years, and only two small oil fields, Baiyanghe and Danbei, were discovered in the Paleogene. Although oil and gas shows have been observered in the Cretaceous of many exploratory wells, only low oil product has been obtained in a few exploratory wells, and a breakthrough in oil and gas exploration has never been achieved. Whether the sag has exploration potential and the target of oil and gas exploration remains unclear. Geochemical data of a large number of source rock samples from the sag reveal that the sag developed three sets of potential source rocks in the Lower Cretaceous, namely Zhonggou, Xiagou and Chijinpu formations. The source rocks in the Zhonggou Formation are locally distributed, are poor in organic matter, dominated by type Ⅲ kerogen, immature and have no possibility to generate oil. The source rocks in the Xiagou Formation and the Chijinpu Formation contain abundant organic matter, mainly composed of type II kerogen, and are excellent source rocks. The source rocks in the Xiagou Formation are at the stage of thermally immature to low- mature and have only the possibility to generate a small amount of low mature crude oil, while the source rocks in the Chijinpu Formation are mainly in the stage of thermally low- mature to mature and have the capability to generate low mature and mature oil. The oil generation in the sag took place mainly in the late Neogene to early Quaternary and basically stops at present. Three types of crude oil have been discovered in the Shibei subsag. The first type of crude oil is low- mature, distributed in the shallow Paleogene, and is derived from the low- mature source rocks of the upper part of the Chijinpu Formation in the sag. The second type of crude oil is medium mature, distributed in the shallow Paleogene of the western slopes of the sag, and is derived from the source rocks of the Middle Member of Xiagou Formation in the Qingnan subsag. The third type of crude oil is mature, distributed in the deep sandstone reservoirs of the Chijinbao Formation in the sag, and is derived from the mature source rocks of the Middle and Lower members of the Chijinpu Formation in the Shibei subsag. The crude oils in the Baiyanghe, Danbei and Dandong oilfields are attributed to the second type of crude oil, which are migrated into the reservoirs over long distances from the Qingnan sag. The region from the Yaerxia to the western slope of the Shibei subsag is a favorable exploration area for this type of crude oil, and the Paleogene is the main exploration target strata. The crude oils in the Chijinpu Formation of the wells Bainan- 3 and Bainan- 6 are the third type of crude oil, which are the self- generated and self- stored type of reservoirs with close- range migration. The deep Chijinpu Formation in the north- central part of the Shibei subsag is the most favorable exploration target strata for this type of crude oil, including conventional reservoirs and unconventional tight reservoirs.