Spontaneous imbibition of treatment fluids into rock matrix during hydraulic fracturing is believed to be one of the major reasons for the early high productions from shale oil reservoirs. In this study, we integrated Focused Ion Beam-Scanning Electron Microscope (FIB-SEM) tomography and non-destructive X-ray nanotomography techniques to study the spontaneous imbibition of oil and brine solutions in shale nanopores. First, a piece of argon-polished reservoir mudrock sample was imaged at high resolution by SEM to find areas of interest. Then, small cylindrical rock sample (20 m diameter) was cut carefully using FIB. The cylindrical core sample was then cut free from the bulk rock sample and attached to the tip of a needle. The miniature core were transferred into an X-ray CT scanner and imaged prior to exposure to any fluids (dry state). Then spontaneous imbibition tests were performed using an in-situ flooding apparatus. In order to represent fluid invasion by hydraulic fracturing in shale oil reservoirs, first, crude oil doped with 2 wt.% iodooctane was introduced to the core followed by brine (15 wt.% NaI) spontaneous imbibition. In the next step, a solution of nonionic surfactant (0.1 wt.%) was added to the core sample and extra brine imbibition was observed. The sample was scanned (voxel resolutions of 64 nm) and the pore-fluid occupancy was explored at the end of each imbibition step. The results showed that both brine and oil imbibed into the core. Nonionic surfactant increased the recovery by about 15%. This might be as a result of lower IFT and contact angle, which also has been reported in the literature. Moreover, during the last test, oil also imbibed into the pore space, due to the fact that the surface became less water wet as a result of surfactant injection prior to this test.