This article proposes that most -if not all- of the oil fields in the southwest sector of the Lomas de Olmedo Sub-basin, have an early structural control ("paleo-structures"). That characteristic is clearly recognizable in 3D seismic, however, it is difficult to map with 2D seismic or wells data alone.Timing of oil generation-expulsion in relation to the migration pathways and available traps is a key process of the Yacoraite-Yacoraite (!) petroleum system. This is evidenced in the distribution of the oil fields in relation to the structural configuration of the Yacoraite Fm, prior to the main phase of the Andean Orogeny (Pliocene-Recent).This Andean orogeny is responsible for the uplift of the Sierra de Santa Bárbara and the reactivation (tectonic inversion) of many sub-surface structures, especially towards the west of the analyzed sector. Rather than contributing to the preservation of hydrocarbon accumulations, it has affected them, possibly making them re-migrate, which has been inefficient according to the information available so far. An example of this would be the great anticline of Lomas de Olmedo that proved unproductive.There is still no precise data on the moment of expulsion of the hydrocarbon; however, it is clear that the generation pod of the Yacoraite Fm coincides with the depocenter of the Salta Gr. and with the greater initial thicknesses of the Cenozoic deposits, independently of the current structure. Thus, the critical moment would be located towards the late Miocene or early Pliocene, before the main Andean compressive phase was activatedThe so-called early structuring comprises extensional movements occurred during the accumulation of the Yacoraite and Olmedo formations as well as a series of events with subtle and active compressive characteristics during the accumulation of the Santa Bárbara Subgroup (Paleocene - Eocene) and the pre-orogenic Cenozoic