Large reserves of petroleum in the Oriente of Ecuador are present in sedimentary rocks deposited on a continental shelf during the Cretaceous. The petroleum was not generated in these rocks, but in the fine-grained terrigenous clastic sediments of a contemporaneous continental-rise prism deposited in deeper water farther west. The rise sediments subsequently were metamorphosed and are now part of the metamorphic rocks of the Eastern Cordillera of the Andes. At the beginning of deformation of the continental-rise sediments, caused by the onset of subduction during the Maestrichtian, most of the petroleum in the northern part of the prism was driven upward and eastward parallel with bedding. Thus much of the petroleum entered the shelf rocks laterally. More complex deformat on of the continental-rise sediments in the south prevented the escape of petroleum there. The trapped petroleum subsequently was converted to graphite by metamorphism. Quantitative calculations show that the proposed mechanism is reasonable and, thus, may have applications elsewhere and should be considered in planning exploration for petroleum in rocks deposited on continental shelves. A single carbon analysis of metamorphic rocks from the south end of the Eastern Cordillera suggests that the graphite content also diminishes southward. If true, this augurs well for the finding of oil in fields currently under exploration toward the east in Peru.