The Mio-Pliocene zone in the district of Ploiesti is on the border of the western plunge of the Eastern Carpathians. During more than 100 years of geologic and exploitation activity, 35 structures productive of oil and gas in the Pliocene, Miocene, and Oligocene have been found. Oil production to January 1, 1968 was 1,725 million bbl, of which 1,528 million was from the Pliocene. The most important producing structure within the region--Moreni-Gura Ocnitei--yielded about 724 million bbl.

Hydrocarbon accumulation in the district of Ploiesti may be explained by (1) the complete development of the Pliocene rocks in the form of repeated pelite and psammite sequences, which permitted the generation, accumulation, and preservation of pools, and (2) the presence of favorable structural conditions, particularly produced by diapirism of Miocene salt which was concurrent with Pliocene sedimentaion.