Tight oil and gas are becoming increasingly important energy sources in North America. Spontaneous imbibition is an important mechanism in oil and gas recovery from tight reservoirs. In this study, countercurrent spontaneous imbibition experiments and nuclear magnetic resonance (NMR) were combined to study the imbibition and the fluid distribution in eight core samples. NMR is able to detect fluid distribution in different sizes of pores ranging from micropores to fractures. Before the experiments, Middle Bakken and Berea cores were saturated with air. Then imbibition experiments with one end open (OEO) and two ends closed (TEC) boundary conditions were carried out. The numerical solutions of spontaneous imbibition models were matched with experimental results by choosing reasonable parameters. The capillary pressure and relative permeability were obtained from the matching. Moreover, the gravity effect on the spontaneous imbibition was analyzed. It was found that the gravity can be neglected for tight rocks. However, it has significant impacts on high-permeability rocks. The recovery factor of OEO imbibition exhibits a linear relationship with root t. However, the TEC imbibition shows a poor linear relationship with root t at the late stage. This research provides new methods to obtain capillary pressure and relative permeability from spontaneous imbibition experiments with OEO and TEC boundary conditions and advances crucial mechanisms for the development of tight reservoirs.