Organic matter transformation ratio, hydrocarbon expulsion efficiency and shale oil enrichment type in Chang 73 shale of Upper Triassic Yanchang Formation in Ordos Basin, NW China

The major enrichment type of shale oil in the Chang 7(3) shale of Upper Triassic Yanchang Formation in the Ordos Basin is unknown. This paper analyzes the organic matter transformation ratio, hydrocarbon expulsion efficiency and roof/floor sealing conditions of the Chang 7(3) shale, and evaluates the major enrichment type of shale oil in this interval. The average o-ganic matter transformation ratio of the Chang 7(3) shale is about 45%; in other words, more than 50% of the organic matters have not transformed to hydrocarbons, and the lower the maturity, the greater the proportion of untransformed organic mat-ters. The cumulative hydrocarbon expulsion efficiency of the transformed hydrocarbon is 27.5% on average, and the total pro-portion of untransformed organic matters plus retained hydrocarbons is greater than 70%. The relative hydrocarbon expulsion efficiency of the Chang 7(3) shale is 60% on average, that is, about 40% of hydrocarbons retain in the shale. The Chang 7(3) shale corresponds to Chang 71+2 and Chang 8 sandstones as the roof and floor, respectively, and is further overlaid by Chang 6 shale, where extensive low porosity and low permeability-tight oil reservoirs have formed in the parts with relatively good porosity and permeability. Moreover, the Chang 7(3) shale is tested to be in a negative pressure system (the pressure coefficient of 0.80-0.85). Therefore, the roof/floor sealing conditions of the Chang 7(3) shale are poor. The retained hydrocarbons appear mostly in absorbed status, with low mobility. It is concluded that the medium-high mature shale oil is not the major enrich-ment type of shale oil in the Chang 7(3) shale, but there may be enrichment opportunity for shale oil with good mobility in the areas where the sealing conditions are good without faults and fractures and oil reservoirs are formed off Chang 71+2, Chang 6 and Chang 8. Furthermore, low-medium mature shale oil is believed to have great potential and is the major enrichment type of shale oil in the Chang 7(3) shale. It is recommended to prepare relevant in-situ conversion technologies by pilot test and figure out the resource availability and distribution.