The Romanian territory had a very complex geological evolution. This evolution generated a diversified geology with extended platforms limiting, to the east and south, a folded chain where spectacular overthrusts are present. More than this, posttectonic covers (basins) common to different tectonic units are also present. Some sequences of these posttectonic covers are extended from the frontal part of the folded chain over the platforms. Most of the tectonic units, as well as the post-tectonic covers, host hydrocarbon fields. On some of the tectonic units, the oil generation is proved only by the seep presence.

The aim of this chapter is to briefly present the geology of the Romanian onshore, except its western part, namely, the Apuseni Mountains and the Pannonian basin.

Each platform (East European, Scythian, and Moesian), each the folded chains (North Dobrudjea Orogen and groups of nappes from the East and South Carpathians [Transylvanides, Median Dacides, Outer Dacides, Marginal Dacides, Moldavides]), as well as the posttectonic basins (Transylvanian Basin, Getic Depression, Focsani Depression), have been described. For all the above-mentioned units, the standard lithostratigraphy and their internal structure are discussed. More details are given for those units that proved to be important hydrocarbon producers, namely, the Moesian Platform, Getic Depression, Subcarpathian nappe, Tarcau nappe, Diapiric fold zone, and Transylvanian Basin. The petroleum systems (including source rocks, reservoirs, and traps) of most of the units are also analyzed. From almost 1000 oil and gas fields already discovered, a reduced number of fields are quoted, just in order to give an idea on the diversity of traps.

The history of the geologic evolution of the Romanian onshore related to hydrocarbon genesis and accumulation conclude the chapter.

Finally, it is to remark that, in spite of its character of synthesis, the chapter could be a useful tool for those scientists who are interested in an overview of Romanian geology and the related hydrocarbon fields.