Abstract: Bayindulan sag and Wuliyasitai sag are located in the northeastern Erlian basin with only 30 km apart. However, there are great differences in organic facies between the source rocks of the two sags. The source rock of Bayindulan sag is dominated by organic facies C with the initial hydrogen index ranging from 400 to 800 mg/g, averaging 600 mg/g, and the source rock was formed in a brackish to fresh water, reduction environment with the organic maceral dominated by sapropelinite. The source rock of Wuliyasitai sag is dominated by organic facies D/E with the initial hydrogen index ranging from 200 to 400 mg/g, averaging 333 mg/g and the source rock was formed in a fresh water, oxidation environment with the organic maceral dominated by sapropelinite, exinite and vitrinite. The organic facies of source rocks can determine hydrocarbon generation characteristics. The hydrocarbon generation features of the source rocks in the 2 sags are significantly different. The crude oil of Bayindulan sag has relatively high sulfur content and asphaltenes plus resin content with the API of 15°~30°; while the API for the crude oil of Wuliyasitai sag ranges from 35° to 45° and the oil is classified as light oil. The differences of organic facies of source rocks will result in the differences of hydrocarbon generation threshold. The source rock maturity of the southern subsag of Wuliyasitai sag is higher than that of Bayindulan sag; and the threshold depth of hydrocarbon generation in Bayindulan sag is 1 500~1 600 m, whereas that of the southern subsag of Wuliyasitai sag is 1 900~2 000 m, which is deeper than those of other hydrocarbon-rich sags in Erlian basin. There are significantly different hydrocarbon generation characteristics between the source rocks of organic facies D/E and typical organic facies C (dominated by lamalginite and telalginite), the source rock of the organic facies D/E has higher hydrocarbon generation and expulsion temperatures and mainly expels light oil