Shale oil is a kind of important unconventional hydrocarbon resource in urgent need of exploitation.A systematic analysis of the forming conditions and accumulation mechanisms of shale oil in the 7th member of the Yanchang Formation (Chang 7 member) in Ordos Basin was carried out by using petro-chemistry and source rock geochemistry methods.The results show that the oil shale deposited in the member is extremely rich in organic matter with an average residual organic carbon abundance of 7.4%

and contains well-developed micro- and nano-pores for shale oil accumulation.Three main mechanisms of shale oil accumulation in the member are brought forward in this study:(1) both high organic matter productivity and strong reducing water environment served to effectively preserve organic matters in the super eutrophic lake basin during deposition

and organic shale of high abundance developed thereby provided the material basis for shale oil generation; (2) organic-rich shale characterized by strong oil-generating capacity

high conversion efficiency

and high intensity of oil expulsion in the member provided favorable conditions for the generation of shale oil in large scale; and (3) the dual occurrence mode of shale oil

i.e.

organic matter adsorption and pore retention

provided various ways for the enrichment of shale oil in the member.