Early Cretaceous Tectonostratigraphic Evolution of Rift and Sag Basins of the Campos and Santos Basins, Southeastern Brazil, Using Potential Fields, Seismic Reflection and Well Data



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The most prolific hydrocarbon accumulations discovered on the offshore margin of southeastern Brazil over the past 30 years have been found in syn-rift (Valanginian to Barremian; 136-125 Ma) and sag basin reservoirs (Aptian; 125-112 Ma) of the Campos and Santos basins. This mega-regional study of both basins focuses on determining their underlying crustal type and thicknesses and the tectonic controls on the key clastic and carbonate reservoir units in the overlying basins. This was accomplished by 3D gravity inversion integrating 300,000 km2 of depth-converted industry-obtained seismic data with potential fields data, data from 18 well logs tied to the seismic lines, and core and thin section photomicrographs. In Chapter 2, gravity inversion of the margin shows an 800-km-wide transition from: 1) unextended, 32-km-thick, continental crust of the South American continent; 2) extended, 30-11 km-thick, continental crust forming a 600-km-wide zone that includes the area of the Sao Paulo plateau; this area is underplated by a 2 to 4-km-thick, high velocity crust; and 3) normal oceanic crust with thickness of 8-10 km and Barremian (129.1) in age. Restoration of the extended area of crust is used to make an improved, pre-rift, plate reconstruction. Results from my study support magmatic underplating beneath the Sao Paulo plateau originating from the Tristan da Cunha plume. The underplating process thickened the crust beneath the plateau and allowed for a wide offshore margin. In chapter 3, I describe the use the seismic grid to map syn-rift normal faults (Barremian to Early Aptian) and associated sedimentary growth sections in the framework of fault-bend folding that account for thickness variations and structural variability affecting the syn-rift interval, as well as the overlying sag due to changes in rates of rifting during the syn-rift interval (Hautervian; ~ 138 Ma to Early Aptian; ~120 Ma). I found three distinct zones of listric normal-faults caused by differential rates of fault slip along the southeastern Brazilian margin.
In chapter 4, I use well data to examine two distinctive, carbonate facies forming an upper and lower part of the hydrocarbon-rich Barremian to Aptian (~130 Ma- ~112 Ma) sag basin unit. These vertical and lateral, carbonate facies variations and fracturing are closely related to the deeper structure of the rifted margin, and the influence of the Tristan da Cunha plume described in chapters 2 and 3. I found the Tristan da Cunha plume to control the distribution of the late Aptian carbonates, while the distribution of the early Aptian carbonates was controlled by the zones of listric normal-faults identified in chapter 3.



Brazil, Structure, Tectonics, Carbonates, Tectonostratigraphy, Campos, Santos