Oil fields of coastal Peru and Ecuador lie within the only significant coastal plain along the western margin of South America. This plain is underlain by chiefly marine lower Tertiary and variably marine to continental upper Tertiary sediments, and is capped by a spectacular, but locally heavily dissected, elevated Quaternary marine terrace, or Tablazo. Although evidence is meager, the marine lower Tertiary sediments are considered not to have been deposited as in a basin, but more likely as in a half-basin or marine bank on a narrow continental shelf, facing the open ocean on the west.

The principal producing areas in this belt, from north to south are (Ecuador) Ancon, and (Peru) El Alto, Lobitos, Talara-Negritos, and Portachuelo. Estimated ultimate recovery from established reserves are: Ecuador, possibly in excess of 125 million barrels of 37° gravity oil; and Peru, possibly up to 1 billion barrels of 37° gravity oil. Production comes chiefly from Eocene sandstones and conglomerates, with fracturing commonly playing a major part in reservoir permeability.

The Tertiary coastal belt is one of both common stratigraphic complications and a rather unique structural arrangement. Much of the marine Tertiary sedimentary sequence demonstrates rapid facies change and evidence of large-scale submarine landslides, resulting in common turbidity deposits and recurrent faunal zones. The structural situation is dominated by a myriad of normal faults having no regular pattern, but predominantly following a rule of faulting which favors downward movement of the updip block (in a regional sense), which is interpreted as dislocation resulting from a westerly spreading or creep of the sediments of this ancient marine bank during periodic recurrent elevations of the ancestral Amotape mountains on the east, with no massive basement buttress on the west to co tain this semi-plastic mass.

Because of both the restricted remaining undeveloped area and vertical sedimentary column in the onshore, particularly in Peru, development of additional major reserves may be expected to come chiefly from the submerged continental margin adjacent to existing production.

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