Campos and Santos Basin
History of the Basin
The Campos basin is considered to be the most prolific basin located in Brazil due to its onshore and offshore production. It is located in the Southeastern region of Brazil, and it occupies an area of 115,000 km^2. The first oil field (Garoupa field) was discovery in 1974 on the Albian carbonate reservoirs at a depth of 120 meters (under the water). Therefore, oil production began on August 1977 in the Enchova field at a water depth of 124 meters using a semi-submersible platform.
Likewise, the Santos basin is the largest sedimentary basin operated offshore in Brazil, with a total area of approximately 350,000 km^2. The first studies for the exploration and production of petroleum for this basin were in 1970s. It contains one of the most producing areas of petroleum in the world, which is the pre-salt. There are more than twenty oil producing fields in Santos basin, and some of them are named after typical Brazilian marine species. By 2013, huge discoveries were made in Santos basin, with depths up to 6,150m (20,177 ft) in ultra deep waters.
The tectonic evolution of Campos basin is related with rifting (Mesozoic) in the South Atlantic, which resulted from the breakup of Pangea and the development of several basins. Its origin is assumed to be by domal uplift, erosion and subsequent subsidence. Furthermore, this basin is associated with three stratigraphic sequences with the stages of development described as rift, proto-oceanic, and oceanic (lower and upper part). The Neocomian rift-stage lacustrine deposits is considered the lowest sequence in which refers to the fault-controlled subsidence with occurrence of stretching that preceded the emplacement of oceanic crust, and deposition of basalt dated at 130 Ma. The Aptian proto-oceanic is characterized by a sequence of evaporitic rocks associated with intense sea-water inflows (Lagoa Feia formation). The lower part of the oceanic stage addresses the thick sequence of Albian/Cenomanian limestones with clastic intercalations, which grades upwards and basin wards into deep water marls and shales. Also, this section is associated with listric detached normal faults that sole out on the Aptian evaporites (salt). The oceanic upper cretaceous to recent clastic section is characterized by tectonic quiescence and continued subsidence with occurrence of some diastrophic structures. Also, there is a residual salt movement and increase in the intensity in deep water.
For the Santos basin, part of its deep-water region is a structural province known as the Outer High, which has geologically evolved by series of rift fault-blocks that eroded and uplifted during Late Berremian. Since it is in the pre-salt, multiple geologic events took place and formed a unique play for exploration in Brazil, with the presence of mature source rocks. There are also zones of deep crustal and mantle exhumation in Santos basin that are expected by the transition from the continental to the oceanic crust, due to the large extension of the continental crust.
The Campos Basin contains a source rock divided into two parts: one is the Lower Neocomian (Late Cretaceous era - 119 Ma to 144 Ma) having black shales deposited in a lacustrine environment with hyper-saline water, and the other is the Upper Neocomian containing calcareous black shales and marls deposited in a lacustrine environment with alkaline affinities. Furthermore, these black shales were formed in an anoxic condition allowing them to have a high-quality organic matter composition of TOC up to 9% and low sulphur type I kerogen.
The reservoirs range from fractured basalts and porous coquinas (bioclastic limestone) in the Lagoa Feia formation, to sandstones in the Campos formation, and limestones and sandstones in Macae formation. Table 1 depicts the reservoir characterization of the Santos and Campos basins in a clear and easy to follow chart.
Traps and Seals
The Campos basin shows a great correlation between trapping mechanism and tectonic evolution. The southern part of the basin consists of structural highs in which coquina rocks are affected by normal faults and the stratigraphic control is provided by the pinch out of the reservoir in the direction of the structural high. Also, basalts produce oil due to their open fractures caused by dissolution of calcite. Bonito, Bicudo, and Pampo fields display listric normal faults that created roll-overs and draping on salt. Therefore, in the Lower Tertiary and Eocene sandstones the stratigraphic control is associated with pinch-outs towards local highs and compactional effects associated with draping residual salt domes and porous carenites. Overall, the oil accumulation occurred by deep water fans distributed on the stratigraphic column from the late cretaceous to the late Tertiary. Turbiditic sand bodies of the Middle Eocene acted as hydrocarbon collector system allowing oil migration from the growth fault systems or unconformities.
HC Type/ Maturation
The majority windows for this basin are defined as 0.5–1.3%Ro of oil and 1.3–3.0%Ro of gas for kerogen Types I and II, though Type III appears at early stages of the history of the basin. Lagoa Feia Fm. is mature to generate oil and/or gas over most of the study area. The marine shales/marls of the early post-rift Macae Fm. are locally mature to generate oil, although gas maturity is not attained.
Some of the most active and large hydrocarbon plays being currently explored lie bellow a salt canopy that has variable thickness, depth and geometry, and this specifically represents a problem for Campos basin. The problem is due to the high temperatures and differential stresses that the salt sheet implies, which can be managed by using mud weights close to the overburden and by reaming the well bore. Moreover, although it is rare, casing collapse represents a consequence of a salt creep.
Another concern is the issue facing large volumes of heavy oil around 13-17 degrees API with viscosities of 20-400 cp found in post salt reservoirs in the Campos basin, both in carbonate and siliciclastic turbidites.
Sand control had been a problem in the early developmental stages of the Campos basin due to poorly consolidated turbidite reservoirs.
After the giant, new oil province comprising the pre-salt, Barremian coquinas and Aptian microbialites located in the ultra-deep water Santos basin was discovered, a series of "golden rules" were mandated by PetroBras prior to development. These rules stated that there needed to be acquisition of high resolution seismic data for detailed reservoir characterization. Along with rules pertaining to potential locations for a production system needing requirements of drilling at least two wells containing a complete logging suite, extensive coring, and downhole fluid sampling at various depths. This also included drill stem tests or injectivity tests in both wells. These were all precautionary steps taken by PetroBras in order to minimize risks based on geologic uncertainties.
Future Assessment of the Basin
Various steps have already been assessed in advanced coping of geologic risks associated with the Santos and Campos basins. In order to produce economic amounts of the high viscosity oil, engineers have implemented long horizontal or multi-lateral wells operating with high power ESPs, hydraulic pumps or submarine multiphase pumps. This along with effective heat management systems and compact oil-water separation systems have aided in economic production of high viscous oil.
The produced CO2 from the oil is being reinjected back into the ground for reservoir pressure control purposes. This also aids in the oil becoming lighter due to composition change of the oil itself.
New drilling technologies have been implemented in order to safely and efficiently extract hydrocarbons from these basins. Well designs have been subjected to resist the collapse that could be induced when drilling through thick salt layers. Special rigs have been constructed in order to reach deep target depths under harsh metocean conditions. The casing and tubings have been made to withstand harmful contaminants produced from the field such as CO2 and H2S.
Open Hole Gravel Pack has aided the possibility and advancements in horizontal wells drilled in the Campos basin. These wells have supported the construction of more detailed and intensive 3D geological models as well as the internal reservoir structure.
It is difficult to assess every risk and uncertainty connected to the Santos and Campos basins. That being said, the only way to improve development is for industry officials to keep collecting data, keep improving on technologies, and keep innovating future geological engineering ideas.
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