The Upper Cretaceous Tuscaloosa marine shale (TMS) is an oil play across central Louisiana and southwest Mississippi. The lower TMS is characterized by relatively high log resistivity (>5 ohm-m) compared to the upper part, and this elevated resistivity zone (ERZ) has become the primary target zone. This study is to investigate the cause of variation in log resistivity based on the data of petrography, mineralogy, and organic matter property and porosity. The results suggest that log resistivity is not controlled by mineralogy or porosity; rather, it is associated with oil generation during organic matter maturation. Total organic carbon (TOC) content, Rock-Eval free hydrocarbon yield (S1), and hydrogen index (HI) in the studied core increase with depth. Porosity within organic matter (OM), measured by field-emission scanning electron microscopy (FE-SEM), is also higher within the ERZ. The correlated variations among TOC content, S1 values, OM porosity, and log resistivity suggest that the higher log resistivity resulted from in situ oil generation and that the OM pores were generated during oil generation. Thermal maturity varies little in the core; whereas the downward-increasing HI indicates an increasing abundance of oil-prone type II kerogen. Higher OM porosity appears to be related to the greater proportion of type II kerogen in the ERZ. The data set demonstrates that higher contents of TOC and oil-prone kerogen are the combined factors for higher oil generation, therefore, higher log resistivity in the ERZ. The study provides a quantitative relationship between OM porosity and oil generation.