These days, finding brand new oil/gas reservoirs is only one task that PetE's do, and probably only a minor aspect at that. Many (probably most) PetE's these days are assigned to manage existing reservoirs. For example, the general characteristics of a reservoir may be known, but not its exact dimensions, and so much of petroleum engineering is devoted to drilling new wells or modifying existing wells into the reservoir so as to learn more information about the reservoir (which then opens the question of whether the expense of performing this work is worth the value of the extra information you would receive about the reservoir). A related aspect is the proper management of the reservoir. A properly engineered reservoir (i.e. with production and injection wells drilled in the right places, with proper sensors in the proper places to monitor the evolution of the reservoir) will produce more oil than a poorly engineered one. One could say that this kind of engineering also 'finds oil' in the sense that the reservoir now can produce more oil than originally thought.