Determination of dynamic fluid intrusion in tight oil reservoirs requires a very careful and precise estimation of changes in fluid volumes during spontaneous and forced imbibition. Because of the extremely low permeability of these mudrocks, only very small volumes of fluid can flow through the matrix. To overcome this limitation a new approach to measuring minute increments in oil and water saturation is proposed. This method utilizes the transient decay in pressure within the bulk fluid surrounding the rock during confinement to estimate the volume of fluid intrusion within the tight rock. Pressure decay data for shale samples from the Eagle Ford, Utica and Bakken formations, each of varying particle diameter was obtained experimentally from batch tests and the total pressure drop was used to measure oil saturations and a pseudo capillary pressure curve was derived from the pressure and saturation data for the pore system comprising of oil vapor as the non-wetting phase and oil as the wetting phase. These measurements were confirmed with NMR measurements made on the same samples (before and after oil saturation). The results show that shale formations with larger permeability tend to absorb more oil during spontaneous imbibition as compared to forced imbibition. Furthermore, it was observed that the duration of oil intrusion at lower initial pressures during forced imbibition was shorter than the time interval for oil intrusion at higher initial pressures for the same particle size. Also, in relation to particle size, it was discovered that significant oil intrusion occurred at lower pressures for large shale crushed samples during forced imbibition, in contrast to smaller samples which showed more oil intrusion at higher pressures during forced imbibition. The reason for this is because larger sized samples possess micro cracks which serve as extra channels through which oil can penetrate. Introduction Organic-rich shale resources are abundant in the United States and they have become a huge source of oil and gas owing to the vast amount of fossil fuel reserve potentially trapped within these formations. Despite the enormous amounts of oil and gas present within these shale plays, the ease of exploiting this resource still remains a challenge because of the extremely tight nature of these formations. In spite of latest technological advances in hydraulic fracturing and horizontal drilling, primary recovery factors from these unconventional reservoirs are still low.