Deepwater Santos Basin Research Articles

Deep lithosphere-coupled surface deformation beneath Southeastern Brazil from amphibious magnetotellurics

The southeastern Brazilian onshore-offshore margin and underlying lithosphere are investigated by three-dimensional magnetotelluric imaging using 202 high-quality broadband and long-period amphibious MT-dataset, orthogonally traversing the regional surface deformation hills of Serra da Mantiqueira and Serra do Mar, as well as the coast-parallel onshore-offshore lineaments. Three-dimensional conductivity model reveals an electrically heterogeneous crust (up to 40 km) in the onshore Brasília and Ribeira Orogenic Belts, and ∼ 2–10 km thick-conductive-sedimentary-wedge beneath the offshore Santos Basin laying on top of a stretched and thinner resistive crust. Regionally, the lithosphere is found ∼130 km thick beneath the most northwestern part of the Brasília Orogenic Belt which is thinning out towards the deep-water Santos Basin (∼70 km). This thinning coincides with the lithosphere-asthenosphere boundary in recent seismic tomography results, reflecting the current state of the uplifted asthenosphere post-West Gondwana break-up. A major steeply dipping sub-lithospheric conductor, in conjunction with a confined asthenosphere upwelling (or lithospheric thinning), has been observed underneath Embu-Paraíba do Sul domain (Ribeira belt), which may be somehow associated with the continental scale rifting, surface uplift and the Cretaceous São Paulo-Rio de Janeiro dykes swarm. Our findings provide valuable insights into the formation of southeast Brazilian highlands and its association with mantle plume-induced lithospheric delamination (or removal) and the occurrence of alkaline magmatism, after reactivation of some pre-existing weak zones.

Active low-angle normal faults in the deep water Santos Basin, offshore Brazil: a geomechanical analogy between salt tectonics and crustal deformation

Abstract We provide a structural analysis and propose a geomechanical model for an extensional fault system observed in seismic data from the deep water Santos Basin, offshore Brazil. The system includes low-angle normal faults (LANFs) associated with underlying listric faults and salt rollers. The LANFs cut through shallow sedimentary successions up to the seafloor, reaching displacements of 1 km during the last 5 myr, at least one order of magnitude larger than the displacement of the coeval Andersonian high-angle normal faults. We suggest that the LANFs were produced by clockwise rotation of the stress field caused by the presence of salt intruded along the footwall of the main listric normal faults of the extensional system. Numerical models indicate that the proposed geomechanical model can be attained with strain weakening and widespread plastic yielding along the main normal faults. We suggest that the studied LANFs are related to salt flow in the same way that the LANFs described in regional extensional provinces are related to flow of either the lower crust or the mantle. The rheological analogy is remarkable and therefore we suggest that this work will contribute to the solution of the LANFs paradox.

An incomplete correlation between pre-salt topography, top reservoir erosion, and salt deformation in deep-water Santos Basin (SE Brazil)

In deep-water Santos Basin, SE Brazil, hypersaline conditions during the Aptian resulted in the accumulation of halite and carnallite over which stratified evaporites, carbonates and shales were folded, translated downslope and thrusted above syn-rift structures. As a result, high-quality 3D seismic data reveal an incomplete relationship between pre-salt topography and the development of folds and thrusts in Aptian salt and younger units. In the study area, three characteristics contrast with known postulates on passive margins’ fold-and-thrust belts: a) the largest thrusts do not necessarily occur where the salt is thicker, b) synthetic-to-antithetic fault ratios are atypically high on the distal margin, and c) regions of intense folding do not necessarily coincide with the position of the larger syn-rift horsts and ramps below the salt. Regions marked by important erosion and truncation of pre-salt strata, uplifted and exposed sub-aerially before the deposition of Aptian salt, can form structural lows at present or be part of horsts uplifted after the Aptian. This is an observation that suggests significant intra-salt shear drag above pre-salt structural highs during Aptian-Late Cretaceous gravitational gliding, but not on younger horsts and ramps reactivated after the main phase of salt movement. Either formed by drag or sub-aerial erosion, strata truncation below the Aptian salt does not correlate with the present-day pre-salt structure in terms of its magnitude and distribution. In addition, there is a marked increase in deformation towards the distal margin, where low-angle thrusts are ubiquitous on seismic data. The geometry and large synthetic-to-antithetic fault ratios of post-salt strata on the distal margin lead us to consider a combination of gravitational gliding of salt from the northwest and ridge push from the east as responsible for the observed styles of salt deformation.

Multidisciplinary Collaborative Concept Aids Challenging Deepwater Operations in Brazil

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 151186, ’Upgrading the Real-Time Drilling-Optimization Culture in Brazil's Challenging Deepwater Operations: The Utilization of a Remote and Rigsite Multidisciplinary Collaborative Concept,’ by Augusto Borella Hougaz, Danilo S. Gozzi, Isao Fujishima, and Klaus L. Vello, Petrobras, and Sandro Alves, Ian Thomson, Raul Krasuk, SPE, and Frank Buzzerio, Baker Hughes, prepared for the 2012 SPE/IADC Drilling Conference and Exhibition, San Diego, California, 6-8 March. The paper has not been peer reviewed. In Brazil, deepwater well-construction activity has increased significantly since the first major presalt discovery in the Santos basin in 2006. The creation of a multidisciplinary drilling-optimization group, integrated with the operator’s drilling team in 2008, and the introduction of a highly specialized downhole-drilling-dynamics measurement-while-drilling (MWD) tool changed the drilling-optimization concept in Brazil. This new optimization concept pioneered the remote (off-the-rig) downhole drilling-parameters surveillance in deepwater operations. Introduction A novel real-time optimization drilling service was implemented in 2008 with the major deepwater operator in Brazil in the Santos basin. Throughout the implementation and execution of the real-time optimization service, the importance of not only knowing where (wellbore placement) and what (formation evaluation) you are drilling but also how you are drilling, which is mainly driven by the integration between operator and service company, became apparent. After 3 years, it was clearly observed that the higher the integration between service company and operator was in a proactive and collaborative environment, the higher the drilling performance was. Today, in the deepwater Santos basin presalt environment, the real-time optimization drilling service is already part of the drilling program in almost every well, from the top hole section to the reservoir section. Real-Time Drilling-Optimization Team In 2008, to answer the “how” questions in the Santos basin, a new culture of real-time optimization arose with the concept of a multidisciplinary optimization team from the service company acting in collaboration with the major operator in Brazil in an integrated manner to achieve superior performance. To accomplish effective remote and rigsite real-time drilling optimization, it is important to use a highly specialized downhole drilling-dynamics MWD tool that records and transmits real-time dynamics data (e.g., stick/slip, lateral vibration, whirl, and axial vibration) and downhole drilling parameters (e.g., weight on bit, torque, revolutions per minute, and bending moment), which allows for a better judgment of the down-hole conditions while drilling and better support for adjusting the surface drilling parameters to optimize operations, in a continuous feedback cycle (Fig. 1).

Analysis of folding and deformation within layered evaporites in Blocks BM-S-8 & -9, Santos Basin, Brazil

Abstract The São Paulo Plateau in the deepwater Santos Basin is the site of numerous recent pre-salt petroleum discoveries. The area is characterized by a thick sequence of layered evaporites comprising primarily halite, with subordinate anhydrite and carnalite and trace amounts of other minerals. The sequence is divided into six stratigraphic packages: three relatively competent beams containing the bulk of the stronger anhydrite and three relatively weak detachment layers. Observed structural styles range from the simple to the complex, including: upright open folds, inclined thrusted folds, recumbent isoclinal folds, sheath folds and superposed folds. Multiple detachments lead to polyharmonic folding, disharmonic folding and overtightened folds. Major anticlinal structures contain acoustically transparent material surrounding disrupted, highly deformed pieces of the lower two beams. The deformation is non-coaxial, with anticlines forming a polygonal pattern and fold hinges that are highly curvilinear. The São Paulo Plateau is a contractional province that formed in response to proximal extension at the Albian Gap during convergent gravity gliding/spreading of the margin. Shortening possibly began during the waning stages of evaporite deposition, but the bulk of the movement occurred during the Santonian–Mid-Eocene. The evaporite sequence shortened much more than the cover because of extreme updip attenuation and consequent basinwards flow beneath the cover; deeper levels of evaporite exhibit more shortening due to strain partitioning across internal detachments.

Challenges in presalt depth imaging of the deepwater Santos Basin, Brazil

Several discoveries, such as Tupi, Bem-Te-Vi, Parati, and Guara, have been announced in Santos Basin off the coast of Brazil, mostly in presalt layers. These layers were well imaged by a salt-flood volume in 2003, but distortions in the base of salt (BOS) and presalt layers were still obvious. Therefore, a constant velocity model is not adequate to capture the velocity variation inside the salt bodies, which include mobile salt and evaporites. A depth migration with a complete salt model is necessary to correctly position the reservoir structures. In this paper, we discuss the challenges in building such a velocity model and share the lessons we learned while working on a data set from the deepwater Santos Basin.

The breakup of the South Atlantic Ocean: formation of failed spreading axes and blocks of thinned continental crust in the Santos Basin, Brazil and its consequences for petroleum system development

Abstract The occurrence of failed breakup basins and deepwater blocks of thinned continental crust is commonplace in the rifting and breakup of continents, as part of passive margin development. This paper examines the rifting of Pangaea–Gondwanaland and subsequent breakup to form the South Atlantic Ocean, with development of a failed breakup basin and seafloor spreading axis (the deepwater Santos Basin) and an adjacent deepwater block of thinned continental crust (the Sao Paulo Plateau) using a combination of 2D flexural backstripping and gravity inversion modelling. The effects of the varying amounts of continental crustal thinning on the contrasting depositional and petroleum systems in the Santos Basin and on the São Paulo Plateau are discussed, the former having a predominant post-breakup petroleum system compared with a pre-breakup system in the latter. An analogy is also made to a potentially similar failed breakup basin/thinned continental crustal block pairing in the Faroes region in the NE Atlantic Ocean.

Postrift sequence stratigraphy, paleogeography, and fill history of the deep-water Santos Basin, offshore southeast Brazil

Events that influenced the Santos Basin were different from the Campos Basin to the north (where the bulk of the petroleum reserves in Brazil have been discovered) and resulted in a very different stratigraphy. Because of these differences, the Campos Basin stratigraphic model is poorly suited to the Santos Basin (cf. Cobbold et al., 2001).We divide the postrift stratigraphy of the deep-water Santos Basin into 11 major sequences, each representing an average duration of approximately 11 m.y. and encompassing the middle–Late Cretaceous and Tertiary. The complex fill history of the Santos Basin was strongly influenced by the uplift of the Serra do Mar coastal mountain ranges in the Late Cretaceous and the subsequent organization of the coast-parallel Paraiba do Sul drainage system. The ancestral Paraiba do Sul tended to focus clastic influx into the northern and central Santos Basin during the Late Cretaceous and Paleogene. Focused clastic influx forced massive shelf progradation and deep-water turbidite sedimentation despite globally high-standing sea level. The southern Santos Basin, at the same time, was relatively starved of clastic influx, and drowned shelf conditions prevailed.These depositional patterns persisted until the Oligocene, when the Paraiba do Sul was captured and diverted into the Campos Basin to the north into which it currently empties. After the capture and diversion of the Paraiba do Sul, the north-central Santos Shelf was starved and drowned, and the shelf edge backstepped more than 50 km (30 mi). The southern Santos Basin, in contrast, was buried by a thick sequence of mud-prone upper Oligocene and Neogene sediments that had a great impact on source rock maturity.