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Santos Basin Pre-Salt Cluster

File:Pre-Salt Location & Cross-Section (Tupi).jpg

An overview of the petroleum geological aspects comprising the Pre-Salt Layer within the Santos Basin.

Location of Santos Basin

Geologic History:

Tectonic Foundations

The Pre-Salt Layer was formed due to the tectonic movements which spurred the breakup of the Gondwanan supercontinent during the Early Cretaceous. The breakup was driven by heat radiating from the St. Helena plume to the north and the Tristan da Cunha plume to the south. Specific to the formation of the Pre-Salt, the Tristan da Cunha plume rose beneath the continental plate, causing thermal uplift. During this thermal uplift, tensional fractures were formed as the continental crust was thinned, giving pathway of magma to form dikes over time.

As thermal uplift continued, three rifts developed at the crest, intersecting at 120 degrees, forming a triple junction. Of the three rifts composing this triple junction, only two remained active, while the other was aborted. The two remaining active rifts composed a single accreting boundary from which seafloor spreading began, while the failed rift – today known as the Ponta Grossa Arch – became the site of the Paraná flood basalts. The rift to drift transition began in the Middle to Late Cretaceous, when rifting had completed and the mid-ocean ridge began injecting new oceanic crust.

The previously mentioned igneous dikes are today found along the coastlines of Brazil, Angola, and Namibia, and were an important tool in determining the location of the triple junction. The Pre-Salt layer itself would reside in the basins formed when the lithospheric plates drifted over the Tristan da Cunha plume, which cooled as they drifted away from the hot spot and began to sag as due to this increase in density.

Source Rock

See sections A-B of Geo-Dynamic Model

As the plates drifted over the hot spot, crustal domes subsided and collapsed as a product of thermal contraction. This accumulation of carbonates and clastics caused the basins to sag. Since these basins were formed above sea level, they formed lakes as fresh water accumulated, as well as volcanic and continental sediments. As movement continued, these lacustrine basins were stretched and subsided further, allowing the eventual encroachment of ocean water. With organic productivity booming, and salinity increasing, these basins became an opportune environment for the preservation of organic materials. These anoxic lakes, over time, deposited organic-rich shales in the middle syn-rift composing the source rock interval known as the Barremian Picarras formation. 

Reservoir Rock

See sections B-C of Geo-Dynamic Model

As continental separation continued, seawater intermittently filled low-lying regions of the expanding rift valley. This low-energy, high-salinity environment allowed for the growth of cyanobacterial colonies of unicellular algae and other microbes. These colonies produced exopolymers that are secreted as biofilm, which traps sedimentary grains into accretionary layers forming microbialite mats. Microbes composing the upper part of this mat grew upward in search of sunlight, forming the accretionary structure known as stromatolites. Over time, these microbial growths lithified to form the reservoir rocks of the Pre-Salt formation. These Aptian lacustrine carbonates deposited in the “sag section” compose the Barra Velha formation reservoir section, which in some areas is underlain by the Barremian limestone reservoir section known as the Itapema formation.

Trapping Mechanisms

See sections C-E of Geo-Dynamic Model

As the period of transition from rift to drift phase continued and basins subsided, volcanic activity from the plume created barriers along crustal fractures that confined free marine circulation of the growing South Atlantic Ocean. These barriers provided an effective lateral seal which intermittently allowed marine waters to spill over and become trapped in these subsiding basins. As the arid climate of the Aptian age evaporated these trapped waters, thick layers of salt were deposited along the continental margins of Africa and South America. These evaporate deposits overlaying the previous rift sequence created an effective top seal for the reservoir once hydrocarbons migrated. This thick section of Aptian salt is known as the Ariri formation. The end of the thick salt deposition marks the start of seafloor spreading and the resulting opening up of the restricted rift basin.

Petroleum Relevance:

Post-Salt Layer

The Post-Salt Layer is the carbonate sequence deposited over the salt layer (See sections C-D of Geo-Dynamic Model). It is interesting to note that before the discovery of the Pre-Salt, the overlying Post-Salt was the primary zone of production in the area. Oil produced from this zone is of Pre-Salt origin, and is a product of Pre-Salt oil migrating through fractures in the overlying salt layer. Though the oil is of the same origin, bacteria of the Post-Salt zone has been known to consume the lighter components of the oil that accumulates here.

Tupi Field (Lula Field)

The Tupi Field was the initial discovery well for the Pre-Salt Layer, and was discovered in October 2006 by BG Group. The well was designed to test the sag portions of the microbial carbonate pre-salt layer, and also includes an intermediate reservoir known as the Coquina Interval. It is estimated to contain five-eight billion barrels of recoverable light oil and natural gas, which is the largest estimated reserves reported in the Pre-Salt Layer to date. The well drilled through nearly 2 kilometers of salt before reaching its target depth of around 6 kilometers. From the Seismic Cross-Section, it can be seen that faults terminate at the base of the sag, allowing hydrocarbons to migrate from the Lago Feia source rock and accumulate in this zone with the thick salt layer providing a seal. The name "Tupi", the Portuguese word for "squid", was later changed to "Lula" in honor of popular former President Luiz Inacio Lula da Silva. The Field is owned and operated by Petrobras, with assistance from BG and Galp.

Other Fields of Pre-Salt Cluster

Other Fields include:

• Iara: Est. reserves: 3-4 bln bbl
• Guara: Est. reserves: 1-2 bln bbl
• Jupiter
• Azulao
• Caramba
• Bem-te-vi
• Carioca
• Parati

These fields have reservoir depths between 5,000 to 6,000 below sea level, and are overlain by a layer of salt with varying thickness (up to 2,000 meters).

Rock & Fluid Properties

• μ ≈ 1cp
• API Gravity=28-30°API
• GOR=140 -240 m^3/m^3    
• ϕ ≈ 8-20%
• k ≈ 20-500 mD
• Res. T = 40 – 100℉