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This paper analyses the diagenetic evolution of sandstones belonging to the Bajo Barreal Formation(Cretaceous) in the Golfo de San Jorge Basin (Patagonia, Argentina). The Bajo Barreal Formation includesthe main reservoirs, which are located along the western area of the basin and is composed of sandstones,conglomerates, mudstones, tuffaceous mudstones and some layers of tuffs. The principal reservoirscomprise medium-to coarse-grained sandstones, which are dominated by feldspathic litharenitesand contain minor amounts of litharenites and lithic arkoses. The authigenic minerals include kaolinite,smectite, chlorite, quartz overgrowths, microquartz and calcite, with minor proportions of megaquartz,siderite, analcime, laumontite, feldspar overgrowths and illite/smectite and chlorite/smectite mixedlayers. Secondary porosity is much more important than primary porosity and is produced by thedissolution of feldspar, lithic clasts and clay cements. The diagenetic history of the Bajo Barreal sandstonescan be divided into seven diagenetic stages, each of which is characterized by a specific assemblageof authigenic minerals and diagenetic processes. Eogenetic conditions occur in stages 1, 2, 3 and 4.Stage 1 corresponds to shallow burial characterized by the physical reduction of primary porosity bycompaction; during stage 2, rim clay cements of chlorite, smectite and clinoptilolite, as well as thinquartz overgrowths, were formed. The precipitation of pore-filling cements of kaolinite, chlorite andsmectite occurred during stage 3, while stage 4 records the intense dissolution of feldspar, lithic fragmentsand kaolinite cements. Mesogenesis occurs in diagenetic stages 5 and 6. The former correspondsto a new phase of authigenic kaolinite, while the latter records the significant dissolution of feldspar,lithic clasts and previous cements, which produced the highest values of secondary porosity. Finally,stage 7 corresponds to the highest degree of diagenesis in the Bajo Barreal Formation (mesogenesis),which resulted in the precipitation of cements of zeolites and calcite, as well as quartz and plagioclaseovergrowths.