Print this pageThe Role of Compressional Horizontal Stresses (σ1=SHmax) in the Evolution of the Potiguar Basin (NE Brazil) and the Age of Trap Formation for Its Main Reservoir (Açu Fm)
The Potiguar basin is found in NE Brazil, along its Equatorial margin. Onshore, the basin is made by an ENE-trending graben, filled by rift and post-rift sequences, bordered by two platforms, where only post-rift sediments are found. Basin evolution was firstly postulated to be dominated by horizontal extensional stresses: Lower Cretaceous rifting (Gondwana breakup) followed by post-rifting sedimentation, in a classical Atlantic-type passive margin. Meanwhile, several lines of evidence clearly show that compressional horizontal stresses (σ1=SHmax) play a major role in the tectono-sedimentary evolution of the basin and in the petroleum trap formation. (1) In the early nineties, structural analysis of outcrops and the development of Upper Cretaceous Açu reservoirs (main basin reservoir) showed that post-rifting strike-slip deformation and structural inversion could play a major role in the trap formation. (2) Seismicity provided focal mechanisms that indicate ongoing strike-slip stress régimes, with P axes parallel to the coast line. Breakouts are consistent with the focal mechanisms indicating that SHmax is NW-SE-trending. (3) Recent studies integrating 3-D seismics, field outcrop analyses and geochronological dating confirmed the major role played by horizontal compressional stresses in the petroleum geology of the basin. Three pulses of tectonic inversion were identified along the Carnaubais fault that marks the southeastern border of the Neocomian rift graben: (i) event 1 is intra-rifting (Valengian;σ1=WNW-ESE-trending); (ii) event 2 deformed the Neo-Aptian sag sediments (σ1=WNW-ESE-trending); (iii) event 3 is post-rifting (post-Campanian; σ1=N-S-trending). In the western plataform, the Fazenda Belém petroleum field produces from fluvial Upper Cretaceous Açu reservoirs, where structural inversion is also observed. A major NW-trending shear zone delimitates the petroleum field to the north, cuts the continental Barreiras Fm and locally exposes the underlying Upper Cretaceous Jandaira Fm (limestones) in transpressional zones. On the basis of (U-Th)/He geochronology, the Barreiras Fm deposited ca 22 Ma (detrital iron oxides) and underwent weathering ca 17 Ma (authigenic iron oxides). 40Ar/39Ar geochonology of authigenic Mn-oxides deposited in faults’ contacts between Barreiras and Jandaíra Fms showed that the weathering is ca 17 Ma and took place after fault movement. The results constrained trap formation in the Açu reservoirs between ca 17 - 22 Ma.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil