The Tertiary foreland basins in front of the Eastern Cordillera are filled with up to 4 km of Tertiary molasse sedimentary rocks. The basins are mainly of Miocene age and overlie older Paleozoic and Mesozoic depocenters. Three major compressional episodes are recognized: a Middle Triassic event, an Early Cretaceous event associated with major unconformities in some areas, and a regionally pervasive late Miocene-Pliocene (Quechua III) event expressed in thrusting and compressional folding over most of sub-Andean Peru.

Two families of oils are differentiated in the Marañon basin, related to Permian and Cretaceous source rocks. Three groups of oils in the Ucayali basin derive from Devonian, Carboniferous, Permian, and Triassic sources. Oil samples in the Madre de Dios basin correlate to Devonian and Carboniferous shales. A variety of trap types have been identified. The foreland can be divided into areas where preexisting faults have been reversed by late Tertiary compression and flexural uplift, and areas unaffected by this deformation where older, more subtle traps are important. To the west, the sub-Andean belt comprises regions where basement-involved thrusts predominate and other areas characterized by thin-skinned thrusting. The Oriente-Marañon-Ucayali basin complex has at least one large hydrocarbon accumulation in each trap type. The level of exploration is low, and many areas are virtually unexplored.