Spontaneous capillary imbibition plays a significant role in oil displacement mechanism during water flooding operation in extra-low permeability sandstone reservoirs. However, after long-term of waterflooding, most of the oil blobs still remain within the pore space. In this context, quantitative assessment of the micro-occurrence of remaining oil in pore space is indispensable to conduct for an efficient waterflooding operation. In this work, the core sample with 3 mm x 10 mm dimensions was prepared from the southeast area of Ordos Basin Chang6 extra-low permeability formation. Subsequently, high-resolution X-rays CT scanning technology was employed to visualize the changes in fluid distribution during spontaneous imbibition process. By means of image segmentation and three-dimensional (3-D) image reconstruction technique, the images of the remaining oil in the 3-D pore space were obtained. After that, the shape factor and relative volume factor were used to quantify the micro-occurrence of remaining oil in the 3-D pore space. The experimental results show that the total volume of oil was declined to 2.27 x 10(7) mu m(3) from 4.25 x 10(7) mu m(3) after spontaneous imbibition in the core sample. Consequently, the continuous oil phase was disrupted by the water in pore space and segmented oil blocks were appeared. Meanwhile, the number of oil blocks was increased to 6027 from 958 that led to severe Jamin's effect and spontaneous imbibition was resultantly came to an end. While in 3-D pore space, three different kinds of micro-occurrences of remaining oil were spotted, such as network (54.31%), cluster (35.28%), and isolated (10.51%). The network and cluster type of remaining oil micro-occurrences are considered as dominant types owing to the large volume and better communication. Because of abrupt capillary force reduction in suddenly enlarge channel and multiple Jamin's effects, the network and cluster remaining oil would mainly be confined in large channels (over than peak pore radius). Therefore, our work provides some beneficial understanding of the remaining oil micro-occurrence in pore space after imbibition.