Eighteen petroleum systems (PS) - particular combinations of source rock maturation and migration to reservoirs in a confined area of a sedimentary province - have been identified in Romania. Most hydrocarbons were expelled or re-migrated into the reservoirs after the Late Styrian orogenic phase , 12-14 Ma years ago. Hydrocarbons mostly accumulated in fields during the Late Sarmatian- Pliocene interval, after the Moldavian orogenic phase. Most gas in the Sarmatian-Pliocene reservoirs of the post-tectonic basins is biogenic. The thick Late Miocene-Pliocene sediments of the Carpathian foredeep and Transylvania Basin have not reached the stage of an effective thermogenic petroleum system. Evaluation of exploration history and fractal distribution laws of discovered hydrocarbons suggest future discoveries in the five major onshore PS analysed of 15 Mt of crude oil. These new discoveries will be in: foreland plays in little explored deep, central basin areas, structural, stratigraphic and combination traps of the subthrusted areas; structural traps of deeper duplexes of the Carpathian Palaeogene PS and in stratigraphic plays of biogenic PS.