A three-dimensional (3D) structural model for an oil-producing basin of the Brazilian equatorial margin

Abstract

Hydrocarbon discoveries in Equatorial Brazil, Equatorial Africa and French Guiana-Suriname-Guyana have recently confirmed their importance as new exploration frontiers. The Mundaú sub-basin, located on the Brazilian Equatorial Margin, comprises four producing fields in shallow waters: Xar´eu, Atum, Espada e Curim˜ a. In order to understand the structural and seismic-stratigraphic frameworks of an oil-producing region in Equatorial Brazil, this work addresses the 3D geometry and spatial distribution of main faults in the Curima˜ and Espada fields. The occurrence of hydrocarbons in the Mundaú sub-basin is compared with fields in other parts of the Brazilian Equatorial Margin and Equatorial Africa. Data from 12 wells and a 3D post-stack time-migrated multichannel seismic volume are used to define nine (9) seismic-stratigraphic units: the syn-rift Mundaú Formation (Units 1, 2, 3 and 4); the transitional Paracuru Formation (Unit 5); and the drift Ubarana (Uruburetama and Itapag´ e Members, Units 6 and 7), Tibau and Guamar´e Formations (Units 8 and 9). The study area is dominated by NW-SE planar normal faults, basinward-dipping, that form multiple half-grabens, and tilted blocks with small anticlines and synclines genetically related to a transtensional system. Three types of plays are recognised in the Mundaú sub-basins: structural, combined (structural-stratigraphic) and stratigraphic (turbiditic). In the eastern part of the study area, where the basement is shallow, no oil was found. Conversely, oil was discovered in an anticlinal trap formed over a hanging-wall block analogous to fields on the Cote ˆ D’Ivoire-Ghana transform margin. This work shows that combined traps on footwall blocks are successful plays near the shelf break of the Mundaú sub-basin, in similarity with the Espoir and Baobab fields in Ivory Coast. Furthermore, turbiditic reservoirs in drift units are analogous to the Stabroek block in Guyana and prospects in the Gulf of Guinea. The structural and petroleum-play analyses in this work are therefore crucial to understand the multiple geological processes leading to the trapping of hydrocarbons in the larger Equatorial Atlantic Ocean.