Abstract: Deep heavy oil reservoirs of the third member of the Shahejie Formation (Sha 3 Member) in the Sanhecun block of the Zhanhua sag exhibit complex resistivity responses, with some intervals showing high resistivity and others low. The controlling factors remain unclear, seriously hindering log-based fluid identification and saturation evaluation. Using core lithologic descriptions, petrophysical data, X-ray diffraction, production tests, and well logs, this study examines the lithology, petrophysical properties, oil-bearing characteristics, and their interrelationships in the heavy oil reservoirs of the Sha 3 Member and investigates resistivity differences and their controlling factors. Results show that the heavy oil reservoirs are mainly fine sandstone, dominated by quartz with minor feldspar and clay minerals, and are characterized by medium to high porosity and permeability. Pebbled coarse sandstone and pebbled medium sandstone have the best petrophysical properties, followed by pebbled fine sandstone and fine sandstone, whereas conglomerate has the poorest. Oil-rich and oil-soaked samples have relatively good petrophysical properties, whereas oil-stained and oil-trace samples have poor properties. Oil-rich samples occur mainly in pebbled fine sandstone, while oil-soaked and oil-stained samples occur mainly in fine sandstone. Resistivity differences are primarily dependent on lithology. Resistivity values of heavy oil layers are generally higher than 40 Ω·m in medium conglomerate and pebbled coarse sandstone and lower than 30 Ω·m in pebbled fine sandstone and fine sandstone. Resistivity responses are jointly controlled by lithology, petrophysical properties, oil-bearing characteristics, and the occurrence state of oil and water. In particular, for similar lithologies, the effects of heavy oil–water interactions and their occurrence modes on resistivity differences should not be overlooked. These insights into resistivity differences and their controlling factors provide a basis for determining the resistivity cutoff of effective heavy oil reservoirs and developing log-based saturation evaluation methods, and offer implications for the development of analogous heavy oil reservoirs.