Although many shale oil wells have been drilled in recent years, the industry as a whole is still at the beginning of the learning curve with respect to shale oil drilling operations. Drilling through shale oil formations can be problematic and impose significant costs to operators attributed to wellbore stability problems. These problems include but are not limited to tight holes, stuck pipe, fishing, sidetracking, and well abandonment. Therefore, to more efficiently and effectively drill through such formations, operators should better understand their properties. So far, few experiments have been conducted on shale oil rock samples to better understand their properties while drilling using oil-based mud (OBM). Most experiments have been performed on common shale core samples, which are significantly different from oil shale samples in terms of mineralogy, cation exchange capacity (CEC), pore fluid, and swelling properties. This study investigates the swelling properties and CEC of shale oil core samples from the Eagle Ford field in the laboratory. Specifically, scientists submerged samples in 7% potassium-chloride (KC1) brine and OBM and then used strain gauges and the V-Shay data acquisition system to record both lateral and axial swellings for submerged and non-submerged areas. They then compared the effects of drilling fluid on the swelling properties of the rock samples and, consequently, on borehole stability during drilling operations. All of the experiments were performed in an environmental chamber, which allowed maintaining a constant temperature. The samples were taken from the vertical, horizontal, and diagonal (45°) directions relative to the layering to help determine the optimum well path that would encounter the least possible wellbore stability problems associated with swelling during drilling operations. The results demonstrate that the swelling properties and CEC of the shale oil core samples are different from the common shale core samples. The study quantified the shale/fluid properties, interactions, and effects of the different fluids on rock properties in unconventional reserves. The results of the study can help drilling engineers more precisely understand unconventional shale oil rock properties. The results of the study should also help design a more effective drilling fluid for field applications, as well as more accurately predict the mechanism of formation failure.