Thrust Faults and Ruptured Folds in Romanian Oil Fields

Abstract

Overthrust faults and ruptured, asymmetrical salt folds over-riding one flank occur in the Romanian oil fields where deep drilling gives control on subsurface conditions. The folding developed very gradually during early Pliocene along axial lines already existing in the Miocene and older Tertiary rocks; thus accumulation of oil and gas in the lower Pliocene (Meotian) and middle Pliocene (Dacian) must have taken place before the large-scale folding and faulting (Wallachian) of the last stage (Levantine) with vertical movements of several thousand feet. Salt and salt breccia were squeezed into or through some of the folds or rupture lines, for the weight of the Pliocene was enough to bring the salt masses to a plastic state. With 10,000-15,000 feet of sediments, most of which were unconsolidated, lying on a sloping foundation of the Carpathian foreland, it is easy to visualize the long strike folds pressing forward and down the slope as the uplift of the Carpathians reached its climax. Salt and salt breccia, along with fluids and gases entrained along lines of movement, served as lubricants. We may assume a buoyant effect of submergence to lighten the load, for the Levantine was still a lake when much of this happened. There is little indication of movement of reservoir contents far from the original places of accumulation. Where faulting has afforded communication between Meotian paraffinic and Dacian non-paraffinic oil-bearing sands, crudes of mixed base are found in the latter. The Baicoi field has an interesting structural feature where large crestal and north flank production was found about 5,000 feet above the faulted south flank, with the same prolific oil sands. The origin of the salt is a long disputed question. In recent years many Romanian geologists ascribe it to the Tortonian, in which lagoonal salt beds occur. Evaporites are also reported in the Helvetian and in the Aquitanian. In either or both cases, they could take part in the diapir movements. The writer believes the original salt masses started from pre-Aptian Mesozoic.