Abstract: According to the characteristics of organic petrology, sedimentology and organic geochemistry, the source rocks of the Yanchang Formation in Zhenyuan - Jingchuan area in the south of Erdos basin are divided into three sedimentary organic facies including delta, coastal - shallow lake and semi - deep lake. The semi - seep and deep lake organic facies has high abundance of organic matters which are mainly of Ⅱ1 - Ⅱ2 type , thus posses the best hydrocarbon generating properties. The coastal - shallow lake organic facies has high content of humic substances and largely variant abundance of organic matters, thus has poor to medium hydrocarbon generating capability. The delta organic facies has low abundance of organic matters with humic substances as the main microscopic component, thus belongs to very poor to poor source rocks.