Abstract: The source rocks of lower Es³ and upper Es⁴ are high quality source rock in the Dongying Sag of the Bohaiwan Basin.It was considered that the crude oil on south slope of the Dongying Sag mainly came from Es⁴ source rock;while it was argued that whether Ek² can generate hydrocarbon.The correlation between oil and source rock shows that Ek² source rock differs from Es³ source rock with higher gammacerane and differs from Es⁴ source rock with higher trishomohopane and S configuration of C₂₉ sterane.There is consanguinity between crude oil in deeper layers and Ek² source rock.Although Ek² source rock was found belongs to bad source rock with lower organic carbon,poorer type of kerogen and higher maturity,it optimizes toward the sag center based on biomarkers.So it is presumed that there would be good source rock of Ek² in its sedimentary center,and the genetic potential of Ek² would be large.