Significance of sills as reservoirs in the Neuquén Basin in southern Mendoza By the late 1970s, in the Neuquén basin in southern Mendoza exploration geologists started to pay attention to the manifestations of hydrocarbons associated with intrusive bodies emplaced in rocks of Mendoza Group. At the beginning it was thought that these events were associated with natural fractures in the host rock generated by the stress caused by the intrusion of igneous material. However, was quickly found that hydrocarbons production came from the sills (Perea et al., 1984). The lack of records of such kind of reservoir worldwide, forced to develop techniques to evaluate these rocks and try to distinguish between productive and non-productive, and somehow predict their distribution and geometry. This paper attempts to summarize the working hypotheses and the experiences made in this sense