Geologic Outline and Oil Fields of Sergipe Basin, Brazil

Abstract

In the Sergipe basin of northeastern Brazil a Lower Cretaceous unconformity marks a change in general environment and tectonic style. Below this unconformity, Carboniferous to Lower Cretaceous beds are nonmarine, whereas the overlying Lower Cretaceous to Tertiary beds are dominantly marine. Intense normal faulting tectonic activity preceding the unconformity resulted in uplift and erosion which exposed Precambrian rocks in an area north of Aracaju while, in adjacent grabens, thick wedges of syntectonic conglomerates were deposited locally over older sediments. Irregularities on this unconformable surface were filled by the Carmopolis conglomerate and coarse sandstone member of the Muribeca Formation; the more extensive overlying Ibura Member evaporites of the same formati n also covered the areas where basement was still exposed. Neo-Cretaceous tectonism is characterized by small-scale faulting; the Riachuelo-Siririzinho and Vassouras-Carmopolis oil trends resulted from a combination of northwestward subsidence of basin-margin grabens and a regional southeastward tilting that started somewhat later. Oil production in this basin comes from the Carmopolis, Siririzinho, and Riachuelo fields, mostly from the Carmopolis Member. Some oil is produced also from Lower Cretaceous reservoirs in contact with the unconformity. Depth range of all reservoirs is 400-800 m. Favorable conditions for oil accumulation are the result of adequate structural evolution during Late Cretaceous time, presence of evaporites and probable oil source organic shales at the top of the reservoirs, and younger unconformities not reaching down to the trap. Locally, lateral permeability barriers or reservoir pinchouts complement the structural trap closure. The oil is of mixed base ranging from 24 to 30.5° API. Cumulative production to December 31, 1969, with Siririzinho and Riachuelo fields still being developed, was 23.65 × 106 bbl of oil. This production comes from 205 completed wells drilled in a 400-m grid.