Petroleum Systems of Brazil - PGT

Petroleum Systems of Brazil - PGT

Petroleum Systems of Brazil w.pgt.com.br

Petroleum Systems of Brazil PGT - Petroleum Geoscience Technology w.pgt.com.br

Petroleum Systems of Brazil w.pgt.com.br

Petroleum Systems of Brazil w.pgt.com.br

Introduction

Brazil has the largest sedimentary area of South America, with about 24 Phanerozoic sedimentary basins with potential interest for petroleum exploration. Covering over 5,0,0km2, this sedimentary area includes basins with significant volumes of petroleum and commercial hydrocarbon production (Campos, Potiguar, Recôncavo, Sergipe-Alagoas, Solimões, Ceará, Espírito Santo, and Santos), basins with minor accumulations and/or subcommercial fields (Foz do Amazonas/Pará-Maranhão, Tucano, Camamu-Almada, Cumuruxatiba, Barreirinhas, Amazonas, Paraná, Jequitinhonha, and Parnaíba), and basins with insignificant hydrocarbon shows or even without reliable evidences of petroleum (Acre, Pelotas, Tacutu, Jatobá, Jacuípe, Marajó, and Paraíba- Pernambuco). Roughly, 27 major petroleum systems can be recognized in the 17 sedimentary basins of the first two groups (see map bellow).

Petroleum Systems of Brazil w.pgt.com.br

Source Rocks

A set of 8 major distinct hydrocarbon source systems can be recognized in the 27 petroleum systems of Brazil:

• Upper Devonian marine source system. This system is composed by marine epicontinental black shales, mostly of Frasnian age. Such rocks account for the entire oil and gas reserves of the Solimões Basin, the minor oil and gas accumulations of the Amazonas and Parnaíba basins, and the gas shows and subcommercial (?) fields of the Paraná Basin.

• Upper Permian marine source system. This system is composed by marine hypersaline organic-rich shales and marls that sourced the oil shows and tar sands occurrences of the Paraná Basin.

• Neocomian lacustrine source system. This system is composed by Neocomian rift lacustrine fresh/brackish shales that occur along most of the eastern/northeastern Brazilian Margin. Such rocks generated the oil and gas reserves of the Recôncavo Basin, the small oil and gas fields of the Tucano, Cumuruxatiba and Camamu-Almada basins, and part of the fields of the Sergipe-Alagoas, Potiguar, and Ceará basins.

• Barremian lacustrine source system. This system is composed by Barremian rift lacustrine brackish/saline shales that occur mainly along the eastern Brazilian Margin. Such rocks generated the huge oil and gas reserves of the Campos Basin, most of the oil and gas fields of the Espírito Santo Basin, and a few, but significant, oil fields in the Santos Basin.

• Aptian marine source system. This system is composed by marine hypersaline shales associated to the evaporitic sequence. Such rocks occur along most of the eastern/ northeastern Brazilian Margin, being the most important source of the oil and gas reserves of the Ceará, Potiguar, and Sergipe-Alagoas basins, and the minor accumulations of the Barreirinhas Basin.

• Albian marine source system. This system is composed by Albian marine carbonate shales and marls. Although found along most of the Brazilian Atlantic Margin, such rocks account only for small accumulations in the Foz do Amazonas/Pará-Maranhão, Jequitinhonha, and Cumuruxatiba basins.

Petroleum Systems of Brazil w.pgt.com.br

• Cenomanian/Turonian marine source system. This system is composed by marine anoxic shales and marls mostly of Cenomanian/Turonian age. As the previous system, even though found along most of the Brazilian Margin, such rocks account only for some oil and gas fields in the Santos, Espírito Santo Basin, and Sergipe-Alagoas basins.

• Tertiary marine source system. This system is composed by marine deltaic shales

(Eocene/Oligocene?) which account for the gas and light oil accumulations found in the Foz do Amazonas/Pará-Maranhão and Espírito Santo basins.

Reservoir and Seal Rocks

Eleven major reservoir/seal sequences ranging from the Devonian to the Miocene can be recognized in the Brazilian petroleum systems:

• Devonian. This sequence comprises marine sandstone reservoirs sealed by marine shales and diamictites of the same age. So far, only several oil and gas shows, and a few hydrocarbon occurrences were found in this sequence in the Amazonas, Parnaíba, Paraná and Solimões basins.

• Carboniferous. Such reservoirs occur throughout the Paleozoic basins, showing good to excelent porosity and permeability characteristics, being sealed by pelitic rocks in the Paraná and Parnaíba basins, and by evaporites (carbonate, anhydrite and halite) in the Amazonas and Solimões basins. Practically all the oil and gas reserves of the Solimões Basin were found within Carboniferous reservoirs.

The ‘’pie’’ diagram to the left shows the distribution of petroleum reserves (oil equivalent in place) according to its source. Regarding the marginal basins, practically all the petroleum was generated by pre-salt source rocks (Neocomian and Barremian rifts, and Aptian marine source systems). The remarkable contribution (73%) of the Barremian rift source system is due to the huge volumes of petroleum of the Campos Basin.

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• Carboniferous/Permian. Glacial marine sandstones sealed by pelitic rocks and diabase sills represent this sequence, which is restricted to the Paraná Basin.

• Permian. Marine deltaic sandstones, also restricted to the Paraná Basin, represent this sequence.

• Jurassic. Such reservoirs are composed by fluvial sandstones of the pre-rift sequence in the northeastern Brazilian Margin basins. The superimposed lacustrine shales of the rift sequence provide the seal. Such reservoirs contain most of the petroleum of the Recôncavo Basin, some fields in the Sergipe-Alagoas Basin, and a small accumulation in the Camamu-Almada Basin.

• Neocomian. Fluvial-deltaic and lacustrine turbiditic sandstones sealed by lacustrine shales represent this sequence. The most significant petroleum accumulations related to this reservoir sequence are located in the Recôncavo, Sergipe-Alagoas and Potiguar basins.

• Barremian. Coquina limestones sealed by lacustrine shales represent this sequence.

The most significant petroleum occurrences are located in the Campos Basin, where there are several fields with the coquinas as the main reservoir.

• Aptian. This sequence consists of fluvial to alluvial sandstones and conglomerates, which are apparently related to the Aptian unconformity, sealed by shales and evaporites. The most representative occurrences of petroleum are found in the Sergipe-Alagoas and Potiguar basins, and secondarily in the Espírito Santo Basin.

• Albian. This sequence is represented by shallow marine carbonates with remarkable porosity/permeability changes controlled by halokinesis. The most important petroleum accumulations within Albian reservoirs are located in the Santos, Campos and Espírito Santo basins. This sequence also includes fluvial sandstones sealed by shales, being the main reservoir sequence in the Potiguar Basin.

• Late Cretaceous. Turbidite sandstone reservoirs sealed by marine shales occur throughout most of the Brazilian Margin. The distribution and reservoir quality of these reservoirs are mainly controlled by the halokinesis, presence of continental sediment source areas, sea-level changes, etc. This sequence contains huge volumes of petroleum in the Campos and in the Santos basins.

• Tertiary. This sequence comprises the most prolific reservoirs of the Brazilian Margin, being generally represented by submarine turbidite fans sealed by marine shales. The

Petroleum Systems of Brazil w.pgt.com.br distribution and reservoir quality of such fans are controlled by sea-level changes, sediment availability (source areas), presence of retrograding canyons, and contour currents. These reservoirs contain most of the oil and gas reserves of the Campos Basin, as well as other minor accumulations in other marginal basins, such as the Espírito Santo, Sergipe-Alagoas, Cumuruxatiba, etc.

The ‘’pie’’ diagram to the left shows the distribution of petroleum reserves (oil equivalent in place) by the age of reservoir/ seal sequence. The noticeable dominance of the Late Cretaceous and Tertiary sequences reflects the huge volumes of petroleum found in the turbidites of the Campos Basin.

Entrapment Mechanisms

A large variety of entrapment styles occur in the described petroleum systems. In the

Paleozoic interior sags most of the petroleum is pooled in structural traps, mainly along anticlines related to the upthrow blocks of large reverse faults. Petroleum accumulations are also found within small stratigraphic traps, generally represented by sandstone lenses enclosed in the pelitic deposits of the Devonian sequence.

Along the Eastern Atlantic Margin, most of the petroleum found in the pre-rift and rift sequences (Jurassic to Aptian) is pooled within structural traps, generally in the upthrown blocks of normal faults. This is the case of several oil fields in the Sergipe-Alagoas, Recôncavo, Espírito Santo and Campos basins. In the post-rift sequence, the structural traps are essentially related to halokinesis. There is a plenty of structures related to the sliding of the Aptian salt layer and to the formation of salt domes and ridges that affected all the post-rift reservoir sequences, but mainly the Albian and the Late Cretaceous reservoirs. A significant part of the petroleum found in the Campos, Santos, and Espírito Santo basins is trapped in that type of structure. In spite of the great importance of the structural traps in the post-rift sequence, the deep-water giant oil accumulations

Petroleum Systems of Brazil w.pgt.com.br discovered in the Campos Basin are pooled within stratigraphic or structural-stratigraphic traps composed of large turbidite fans. Other stratigraphic accumulations related to turbidites and/or canyons also occur in the Sergipe-Alagoas, Espírito Santo, and Potiguar basins. Remarkable paleogeomorphic traps related to regional unconformities (mainly of Aptian and Cenomanian/ Turonian age) and to canyons are also found in the Campos, Espírito Santo, and Sergipe-Alagoas basins.

Finally, along the Equatorial Atlantic Margin, despite the abundance of structures related to wrench tectonics, most of the petroleum found in the rift sequence of the Potiguar and Ceará (Mundaú) basins is pooled within structural traps, generally composed of tilted faulted blocks. Another relevant petroleum entrapment mechanism in the Potiguar Basin is the warping and/or differential compaction of the post-rift sediments over the basement highs and upthrown blocks of faulted zones. Finally, the Amazonas Cone exhibits large gravity sliding structures, including growth-faults and shale diapirs.

Petroleum Generation and Migration

The Paleozoic and the Mesozoic/Cenozoic sedimentary basins had distinct petroleum generation and migration histories. In the Paleozoic intracratonic basins, maturation data indicate that the heat effect of the Mesozoic igneous events has strongly affected the thermal evolution of both the source rocks and the hydrocarbon accumulations. Since the Paleozoic burial history of the Devonian source rocks was just enough to start petroleum generation in most of those basins, the heat effect of the igneous intrusions (in the Solimões, Parnaíba, Amazonas and Paraná basins) as well as the overburden of lava flows (in the Paraná Basin) account for the expulsion of most of the petroleum from the source rocks and for the cracking of the already trapped hydrocarbons. Regarding petroleum migration, the major faults and mainly the regional carrier beds have played an important role in secondary migration.

In the Atlantic margin, The thermal evolution of the Mesozoic/Cenozoic sediments was controlled by interplay of burial and heat flow histories. The maturation history of the Neocomian lacustrine source rocks was strongly affected by the burial history and high heat flows associated with the rifting event. Thus, the petroleum generation starts still in the rift phase, continuing during the other phases of the basin evolution until the early

Petroleum Systems of Brazil w.pgt.com.br drift phase. The petroleum generated by the Neocomian source rocks reached the prerift (Eastern basins) and rift reservoirs via (1) the lateral contact of source rocks with basement horsts and upthrown blocks of normal faults, (2) via the lateral contact with the lacustrine turbidites sandstone bodies, and (3) via listric faults on the flanks of shale diapirs.

The thermal evolution of the Barremian lacustrine and the Aptian marine source rocks was not only affected by basement high heat flows at the end of the rift phase but also by the subsidence history of the drift sequence. Roughly, the petroleum generation started during or by the end of the Cretaceous in the areas where the Aptian/Maastrichtian section was thicker than the Tertiary section, while generation started during the Tertiary in areas where the Aptian/Maastrichtian section was notably thinner. A good example is given by the Campos Basin, where in the shallow water/slope areas, the Barremian source rocks have reached the oil window during the Late Tertiary, and by the Espírito Santo Basin, where the same age source rocks reached the oil window earlier, during the Late Cretaceous.

With regard to the migration models, the petroleum generated by Barremian source rocks migrated into the rift reservoirs through lateral contacts, unconformities and faults, and to the Aptian reservoirs via direct contacts, faults and medium/long distance lateral migration along carrier beds. The oil that accumulated in the reservoirs of the drift sequence of the Eastern basins occurred via “salt-windows” associated with extensional faults to reach the young reservoirs; and, in the Equatorial basins the hydrocarbon migrated only via unconformities and extensional faults.

Finally, the maturity history of the Albian, Late Cretaceous and Tertiary marine source rocks was essentially affected by the subsidence history of the overburden rocks. Therefore, depending on the basin, the timing of petroleum generation may vary from the Late Cretaceous to Recent. Similarly to what was discussed above for older source rocks, the timing of petroleum generation of these younger source rocks generally starts during the Late Cretaceous or by the end of Cretaceous in the areas where the Upper Cretaceous section is significantly thicker than the Tertiary section, while the generation starts in the Tertiary in areas where the Upper Cretaceous is thinner. The hydrocarbon generated by these younger source rocks reached the reservoirs via extensional faults related to salt and/or shale-induced structures.

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