The oil fields of northwestern Peru occur in an area of much faulted Tertiary rocks, extending from the southern border of Ecuador (about 3° 30^prime south latitude) south about 225 miles (to about 6° south latitude). It is the northern end of a desert belt extending along the entire Peruvian coast, but near the northern border, merging rapidly with the semi-arid and humid forest zone of southern Ecuador.

The presence of oil in this region has been known from a very early date, and in Colonial and pre-Colonial times, several of the larger oil seeps were worked for their pitch or brea. To-day there are three general districts where commercial production is obtained: the Zorritos district in the northern part, where production is derived from the lower Miocene or upper Oligocene; farther south, the Cabo Blanco-Lobitos district; and the Negritos-La Brea district. In these latter and more important fields, the oil is derived principally from Eocene sediments.

The Tertiary of this region, which at present is the source of the entire Peruvian production, has a total thickness of about 25,000 feet. This section is exceptionally complete, generally marine and fossiliferous, and on the basis of detailed stratigraphic and paleontologic studies, it has been subdivided into about 14 mappable units or formations. The beds range in age from early Eocene to Pliocene. They rest either upon an incomplete development of Cretaceous, or, more generally, on older mountain rocks such as Pennsylvanian slates. These Pennsylvanian metamorphics, closely associated with a granite intrusion, form most of the mountain region toward the east and a few other outlying areas.

The major structural features of the region are the main Andean or mountain ranges on the east, and a Pacific fault which closely follows the edge of the continental shelf on the west. In the intervening area, the Tertiary rocks have been locally folded into smaller anticlines and synclines and extensively block faulted. The rocks are everywhere much faulted, and the local structure consists principally of many small fault blocks. Most of the faults are normal.

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Fig. 1A. Geological map of Northwest Peru.

Fig. 1B. Profile showing continental shelf and adjoining land mass at Parinas Point.

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