Geology of the Coast of the State of Alagoas, Brazil

Abstract

This brief article covers the writer's general survey of the coast of the state of Alagoas, in May, June, and July, 1933, and has been partly published in Portuguese.(FOOTNOTE 3) The sedimentary belt along the coast of Alagoas is the only area in the state which can be considered as offering some interest from the point of view of petroleum geology. It is limited on the west by the extensive area of basement granites and gneiss which form the rest of the state. Since 1920, 10 wells have been drilled in the Riacho Doce area on the coast of Alagoas. A study of the logs of these wells permits a modification of the previous opinions of Branner and E. Oliveira which were based only on surface observations. The formations of the sedimentary belt are Tertiary in age. The upper strata, called the Barreiras formation, form extensive red bluffs along the coast; they are poorly stratified and non-fossiliferous. The lower strata, consisting of the gray and dark shales of the Alagoas series, have been paleontologically classified as Eocene. These shales are partly bituminous and are the only known rocks in the state here considered as possibly oil-bearing. Structurally, the shales of the Alagoas series are disturbed, tilted and broken. The writer, after studying the outcrops and the logs of wells in the Riacho Doce area, admits the possibility of existence of oil in the Alagoas series, but considers the general conditions as somewhat unfavorable for the development of important production. He concludes that a deep test well is desirable, mainly for study purposes, in the most favorable locality and recommends a geophysical survey of the area. The latter is now being made.