Leverett's capillary J-function has been widely used in the petroleum industry as an effective tool for correlating capillary pressure data with rock properties. The major goals of this study are: (1) to investigate the effect of stress on the calculated values of J-function, and (2) to develop new models capable of obtaining the J-function using in situ measurements in clean and shaly heterogeneous reservoirs. Two models are developed, in which well-logging data, a good source of in situ measurements, are used to calculate the J-function. The first model was obtained using Tixier's permeability equation and Archie's equation for water saturation. The second model was derived using Tixier's permeability equation in combination with the Schlumberger shale model for water saturation determination. These models were validated using actual field data from two reservoirs in Oklahoma and Louisiana.