The Targu-Jiu oil field belongs to the Internal Zone of the Getic Depression and is one of the largest oil fields in the region. In the paper are presented the mineralogical-petrographic researches made on the cores from the Oligocene-Miocene deposits of the Targu-Jiu oil field. The cores were analyzed by optical microscopy and X-ray diffraction. The mineralogical-petrographic study revealed the presence in appreciable amounts of the dioctahedral and trioctahedral phyllosilicates represented by the illite and clinochlore. This phyllosilicates was evidenced especially by X-rays diffraction. The samples analyzed are polymictic detrital rocks being composed by ruditic, arenitic and siltic clasts, and a clayey fraction. The clayey fraction plays the role of matrix for the siltic, arenitic and/or ruditic epiclasts. In the source area of the studied rocks occur both endogenous and exogenous rocks.