Abstract The Golfo San Jorge basin, located in the central Patagonia, is the oldest producing basin of Argentina. It is an intracratonic extensionla basin, where the first economic discovery took place in 1907. In the sothwestern sector, the Cerro Piedra – Cerro Guadal Norte area has a surface of 1835 sq Km, and only one field in production, the Cerro piedra field that has a daily production of 330 cubic meters of oil and 44500 cubic meters of gas from 40 wells. The reservoir unit is the bajo Barreal formation, that belongs to the Chubut Group (K) and is part of the best known play of the basin. Though 2D seismic analysis and the use of special techniques, an AVO study was made that showed an interesting response in the P normal wave incidence and the fluid factor. This way it was possible to discriminate the top and base of an important gas bearing zone in the seismic, with a good correlation in the field with the well logs. From this a new Play is recognized, the Patagonia Formation of Tertiary Age. It consist of sandstone deposit, oroginated in a marine environment, up to 35 m thick, that by wells, with a production of 165000 m3/day. This discovery is a new case in the basin and makes up an interesting exploration play in this part of the basin.