Petrographic observations indicate that the distribution of cement and porosity within a Quaternary-age thrust fault in the subsurface of the Wheeler Ridge oil field in California is a function of depth and temperature and varies spatially. At depths shallower than 2.5 km (1.6 mi), porosity increases because of the abundance of open microfractures and plagioclase dissolution. At depths greater than 2.5 km (1.6 mi), the porosity in the fault zone decreases because of calcite cementation in microfractures that ultimately form vein networks. Based on delta(18)O data, we distinguished veins cemented by intraformational (lateral) flow into the fault from veins cemented by ascending fluids along the fault. Ascending, cementing fluids traveled at least 75-750 m (246-2460 ft) vertically. The petrography suggests that oil migration was the last event following dissolution and calcite cementation in the fault zone. Based on oil chemistry, whole-oil delta(13)C, API gravity, and petrographic data, we propose that hydrocarbons, presently in shallow and deep reservoirs, flowed laterally into the fault zone. Whereas hydrocarbons in shallow reservoirs flowed across the fault into the hanging wall, hydrocarbons in deep reservoirs were trapped against the fault in the footwall. The increase of API gravity with depth and lack of evidence for retrograde condensation indicate a limited vertical migration and reaccumulation of hydrocarbon, suggesting that below 2.5 km (1.6 mi), the thrust behaves as a vertical seal. The sealing properties of the thrust vary spatially and may be controlled by calcite cementation.