Progressive burial diagenesis of the Oligocene Frio Formation in Brazoria County, Texas, has resulted in extensive reaction between pore fluids and sediment in a major shift toward water/rock equilibrium. Carbon and oxygen isotopic data, combined with fluid isotopic data from the literature, indicate that quartz cement formed at 75 to 80°C and kaolinite at approximately 100°C. The zone of most rapid albitization is near 150°C. Authigenic carbonates formed over a wide range of temperatures, and those within the peak zone of hydrocarbon generation are depleted in 13C. At depths shallower than approximately 2,600 m, quartz and carbonate cementation in primary intergranular pore spaces (passive diagenesis) dominated. Below 2,600 m, within the geopressured zone, reaction of detrital components (active diagenesis) is the major process. Organic maturation, albitization, and the transition of smectite to illite are the processes that contribute most of the components required for precipitation of cements. Quartz cementation occurred quite early in the burial history of the Frio (beginning at approximately 1,500 m of burial), when rates of fluid expulsion were at a maximum and when little of the Frio sandstone section had reached the zone of albitization.