The northern Territory of Acre, in the Alto Jurua River area, Brazil, and the middle valley of the Ucayali River, Peru, have not been studied geologically heretofore. It is a sparsely inhabited region. Flat plains, swamps, and tropical jungle make precise geological work in a short time rather precarious. However, as this contribution emphasizes, stratigraphical and structural conditions in the northwestern Territory of Acre, particularly the Serra do Moa region, show possibilities of important petroleum accumulations and the area deserves detailed surveying.

The sediments range in age from Lower Cretaceous to Pleistocene and are of continental and marine intermittent deposition, although poorly fossiliferous. No rocks of igneous origin were observed. Tertiary and Cretaceous formations of the Pachitea, Pongo de Manseriche, and Contamana areas, Peru, can be stratigraphically correlated with those of the Serra do Moa and upper Jurua areas in the northwestern Territory of Acre.

The structural characteristics correspond with the physiographical features. The Serra do Moa Range, of Andean folding and apparently the last eastern spur of the Cordillera Oriental, is an anticline in partly Cretaceous beds, traversed by several tributaries of the upper Jurua.

Although oil seeps were not observed in the Serra do Moa region, they have been found in the headwaters of the Ahuaya River, a tributary of the upper Cashiboya River. The seeps occurred in faults in hard, red clay sandstones of the red-bed formation; the exact horizon from which the petroleum generated is not known since many seeps occur in Tertiary or Cretaceous outcrops. In the Pachitea River district, seeps have been observed and drilling is planned for the near future by a private concern. The region of the upper Jurua geologically is a part of the potentially oil-bearing structural belt extending along the eastern foothills of the Andes, south through Bolivia into Argentina and north through eastern Peru into Colombia and Venezuela.