The continental lower Cretaceous 'A' sandstone, a major commercial petroleum reservoir in the Reconcavo basin in eastern Brazil, is a heterogeneously coarse quartzose sandstone occurring as a complex assemblage of bar-shaped lenses instead of a blanket as previously thought. Deposition occurred on intrabasin shelves and shoal ridges. A striking resemblance to the offshore bar shoestring sands of the western Mid-Continent region of the United States occurs, but the two differ materially in type of origin. Petroleum migration was controlled by the configuration of the floor of the basin upon which the sandstone was deposited. Paleostructural features thus are of major importance in exploration.