We report the occurrence of dawsonite, NaAlCO3(OH)2, in volcaniclastic rocks of the Albian hydrocarbon reservoir Castillo Formation, Golfo San Jorge Basin, neighboring the Cañadón Asfalto Basin in Patagonia; this is the first known occurrence in Argentina. Dawsonite replaces crystals pseudomorphically, and can also be found as a cement together with other carbonates, which induced a loss in the porosity of the host rock. Previous studies reported incomplete chemical reactions which suggested that oligoclase and analcime were dawsonite precursors. In the present work we document either totally achieved or incomplete chemical reactions which led to the transformation of analcime into kaolinite and dawsonite into kaolinite, respectively. These facts allowed us to deduce that the transformation of analcime into kaolinite was via dawsonite, minerals formed during different stages under a telogenetic regime. Dawsonite values of δ13C PDB (−0.1 to 1.5‰) are consistent with a magmatic source of CO2 provided by hypabyssal, basic alkaline igneous rocks of regional importance starting in the Eocene. We identify both internal and external sources of Na and Al used in the formation of dawsonite.