Estación Fernández Oro block is located in the southeastern most sector of the Neuquén Basin. The field produces mainly gas and condensate from Lajas Fm. In this area, Lajas Fm. is characterized as a tight reservoir. Accumulation is explained from the proven petroleum system Los Molles - Lajas (!). Although estimated porosity is similar throughout the formation, only the lower section produces hydrocarbons. This suggests the hypothesis of a predominantly vertical migration through faults and fractures connecting source rock with reservoir levels of Lajas Fm. Characterization and delineation of source rock becomes then very important.A 55 km2 depocenter for Los Molles Fm. was located within the block based on lithological, geochemical and seismic characteristics. Los Molles Fm. is to be found in the condensate to gas window in the area. Wells with highest EURs in the field are located just above the depocenter.Play concept was recently adjusted after the drilling of two exploratory wells. Although petrophysical properties in both wells proved to be similar, oil and gas shows during drilling are completely different. Location from the depocenter differs in each well. Well #1 was drilled just above a thickness of at least 200 m of proven source rock, whilst well #2 was drilled 4 km away from the depocenter, above an area with no source potential for Los Molles Fm.Drilling of these two exploratory wells confirms the hypothesis of migration being mainly vertical from Los Molles Fm. source rock to Lajas Fm. tight reservoirs. Presence of thick levels of source rock immediately below pay levels is emphasized. Therefore, a complete characterization of Los Molles Fm. allows an optimization of the development plan. Play concept proven in EFO should be evaluated in other areas of the basin to check plausibility, and consequently improve development plans