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Speculative Petroleum Systems of the Southern Pelotas Basin, Offshore
Uruguay
Bruno Conti1*
; José Alexandre de Jesus Perinotto2; Gerardo Veroslavsky
3; María Gabriela Castillo
2;
Héctor de Santa Ana1; Matías Soto
3
1Exploration & Production, ANCAP, Montevideo, Uruguay. [email protected];
2Universidade Estadual Paulista, Instituto de Geociências e Ciências Exatas, Rio Claro, Brasil.
[email protected]; [email protected]
3Facultad de Ciencias, Instituto de Ciencias Geológicas, [email protected];
*Corresponding author
Abstract
The Pelotas Basin, that develops in Brazil and Uruguay, still represents a frontier basin with
underexplored hydrocarbon potential. Although no oil and gas accumulations have been identified so
far, only 18 exploratory wells have been drilled in an area of more than 300,000 km2, all of them
located in the Brazilian portion. A petroleum system study could contribute to a better characterization
of the capacity of the basin to generate and accumulate hydrocarbons. Three stages have been
recognized during the basin evolution: a prerift phase which preserved Paleozoic and Mesozoic units
of the Paraná Basin; an Early Cretaceous volcano-sedimentary synrift phase; and a Cretaceous to
Cenozoic postrift phase deposited during the passive margin stage. Sequence stratigraphy was the
methodology used to interpret 2D multichannel seismic sections of the still undrilled southern segment
of the Pelotas Basin in the Uruguayan Atlantic margin. The analysis allows to identify depositional
sequences, systems tracts and the distribution of the main elements of the potential petroleum
systems. As a result six speculative petroleum systems are proposed. The first petroleum system,
related to the prerift phase, is represented by a Lower Permian restricted marine source rock and
reservoirs related to Permian to Upper Jurassic aeolian and fluvial sandstones. The second system
corresponds to the synrift phase, being constituted by a Barremian lacustrine source rock and
presents as reservoirs alluvial/fluvial sandstones of the same age. The other four proposed systems
are associated with the postrift phase, being represented by marine source rocks related to identified
Aptian-Albian, Cenomanian-Turonian and Paleocene transgressions. These systems have
predominantly siliciclastic reservoirs represented by Early Cretaceous aeolian sandstones and
Cretaceous to Cenozoic deltaic sandstones and turbidites.
Key words: Sequence Stratigraphy, Petroleum Systems, Pelotas Basin, Uruguayan Atlantic Margin
1. Introduction
The Pelotas Basin generated during the fragmentation of the Gondwana supercontinent and
subsequent opening of the South Atlantic Ocean from the Early Cretaceous onwards. It extends
through the southern Brazilian Atlantic margin, with its southernmost segment entering into the
Uruguayan continental margin. In its northern boundary, in offshore Brazil, it limits with the Santos
Basin through the Florianópolis High while in its southern boundary, in offshore Uruguay, it limits with
the Punta del Este Basin through the Polonio High.
There have not been any hydrocarbons discoveries in the basin and the existing geological
information does not allow establishing known or hypothetical petroleum systems (Magoon & Dow,
1994). However, the basin is still underexplored, with a total of 18 exploratory wells drilled , all located
in its Brazilian portion, in a surface of 250,000 km2 (considering bathymetry up to 3,000 meters), most
of them situated either in shallow waters or in the onshore segment (Figure 1). In addition, there are
no wells drilled in the Uruguayan segment of the basin (i.e., the study area), which extends over
80,000 km2. The recently acquired 2D seismic data shows a promising geology, allowing the
identification of potential source and reservoirs rocks and mainly stratigraphic traps, such as turbidites,
which shows analogies with hydrocarbon accumulation of other South Atlantic basins such as Campos
and Santos. The objective of this work is thus to evaluate the hydrocarbon potential and define the
speculative petroleum systems present in the Uruguayan portion of the Pelotas Basin.
1.1. Location of the studied area
The study area covers the southernmost portion of the Pelotas Basin. It is located in the offshore of
Uruguay, between the following parallels: 34º 20’S and 36º 40’S, and the following meridians: 50º
50’W and 52º 50’W (Figure 1).
2. Materials and Methods
The database used consisted in 6,904 km of 2D multichannel seismic sections from the Uruguay
segment of Pelotas Basin, of which 5,904 km are property of the National Oil Company of Uruguay
(ANCAP). The remaining 1,042 km of 2D seismic data correspond to multiclient data acquired by ION-
GXT. The database was visualized and interpreted using The Kingdom Suite software, version 8.8
(IHS).
The methodology used, based on the concepts of sequence stratigraphy, was defined by Hubbard et
al. (1985). It consists of four main stages:
Identification of sequence boundaries.
Interpretation of internal attributes (system tracts, maximum flooding surfaces). In this work, the four
systems tract model have been employed. Following Catuneanu (2006), these are: Lowstand System
Tract (LST), Transgressive System Tract (TST), Highstand System Tract (HST), and Falling Stage
System Tract (FSST).
Development of a geological model: Generation of maps that shows the elements of the petroleum
systems (source rocks, reservoirs, seals, etc.).
Structural analysis: Identification of migration paths and structures.
The Gaviotin x-1 well, located in shallow waters of the Punta del Este Basin, is the nearest well drilled
to the study area and was used to assign ages to the interpreted depositional sequences.
Figure 1: Location of the study area (red square) of the Pelotas Basin offshore Uruguay
and locations of wells drilled in Pelotas Basin and in other nearby basins.
3. Geological setting
The breakup of the Gondwana supercontinent, during the Cretaceous, resulted in two new continents
associated with two new tectonic plates, the South-American and African Plates. Besides that, the
process of fragmentation gave birth to the South Atlantic Ocean and a series of sedimentary basins
located in both margins of the South-American and African continents. The rifting process started in
the southern portion approximately 130 my ago, moving towards the north, being completed 20 to 30
m.y. later, during Aptian-Albian times (Bryant et al., 2012). Several of the rifts generated during this
stage, later evolved into a passive margin like Pelotas Basin (Porto & Asmus, 1976) located in the
southern segment of the Atlantic Ocean (Moulin et al., 2005). Regarding the tectonic and structural
evolution of Pelotas Basin, two main stages typical of volcanic rifted margins can be recognized:
synrift and postrift (also known as drift or passive margin).
In the proximal portion of Pelotas Basin it has been recognized, through wells, a prerift megasequence
that is not part of the evolution of the basin but can have hydrocarbon potential.
The prerift megasequence is represented by preserved Paleozoic and Mesozoic lithologies, belonging
to the intracratonic Parana Basin (called Norte Basin in onshore Uruguay), that were deposited in a
syneclise stage before the genesis of the Pelotas Basin. According to well data, the Paleozoic
megasequence preserved in the Pelotas Basin contains three lithostratigraphic units deposited in a
marine environment (Rio Bonito, Palermo and Iratí formations) during the lower Permian (Bueno et al.,
2007). The upper Paleozoic portion is represented by two units (Teresina and Rio do Rastro
formations) deposited in a fluvial-lacustrine-tidal environment, while the prerift Mesozoic sequence is
composed by Upper Jurassic aeolian-fluvial sandstones of the Botucatú Formation and the Lower
Cretaceous basaltic floods of the Serra Geral Formation (Milani et al., 1994). In relation to this
megasequence, it is of particular significance the Iratí Formation, an organic rich black shale with an
important source rock potential.
The synrift megasequence is filling half-graben structures in the proximal part of the basin. It was
drilled by a few wells, which encountered basalts of Barremian-Aptian age (Lobo et al., 2007) of the
Imbituba Formation and conglomerates of the Cassino Formation (Bueno et al., 2007). Distally, this
megasequence is represented by the seaward dipping reflectors (SDRs) (Fontana 1987, 1996). The
postrift megassequence represents the marine sedimentation of the basin and can be divided in three
main sequences according to Bueno et al. (2007): shelfal, transgressive and regressive. According to
these authors, the shelfal sequence is represented by Albian carbonate and siliciclastic deposits of the
Porto Belo formation, which were deposited in a shallow mixed-shelf environment. The transgressive
sequence extends from Albian to Oligocene, being composed by thick layers of shales of the Atlântida
and Imbé formations. The regressive sequence is formed by sandstones and siltstones of the Cidreira
Formation of Neogene age.
4. Results and discussion
4.1. Seismostratigraphic analysis
As a result of the interpretation, the three main megasequences that constitute the Pelotas Basin
(prerift, synrift and postrift) were identified and ten depositional sequences were recognized for the
postrift megasequence (Figure 2).
Figure 2: 2D dip seismic section of Pelotas Basin with interpreted megasequences.
This paper will describe the seismostratigraphic interpretation of the prerift and synrift megasequences
and some depositional sequences of the postrift that are related with the petroleum systems proposed.
4.1.1. Prerift
The prerift megasequence extends through the proximal region of the basin overlying continental
crust. Its distribution is associated with a basement high that develops in the proximal region called
Polonio High. The criteria used to identify the prerift megassequence is the same that was used in
Morales (2013) to map this package in the offshore of Uruguay: tilted, subparallel reflectors, with high
acoustic impedance contrast and without growth of the section related to faults. These criteria allow to
differentiate the prerift deposits from those of the synrift and also from the basement lithologies.
Despite the seismic resolution does not allow recognizing system tracts for this package, it is possible
to identify truncations of the reflectors on the top of the megasequence (Figure 3) which allows to
confirm that the upper limit represents an important angular unconformity.
Figure 3: Seismic character of the Prerift megassequence showing truncation of reflectors at the top of the megasequence.
The isopach map shows that the prerift megasequence thins out towards the continent, whereas the
thickest section lies in the west sector, where it is preserved beneath the hemi-grabens of the synrift
section (Figure 4).
Figure 4: Isopach map of the Prerift megassequence in meters.
It is important to notice that among the prerift lithologies drilled in the onshore portion of the Pelotas
Basin black shales of the Irati Formation were found. These shales are considered a potential source
rock for the area of study.
4.1.2. Synrift
The synrift section develops over continental crust, being basically associated with the infill of half-
grabens and the seaward dipping reflectors (SDRs). Based on the literature a Barremian-Aptian age is
attributed to this megasequence (Bueno et al., 2007). The development of the half-grabens is
restricted to the central-west sector of the area, being related with NE-SW trending antithetic normal
faults generated in the Lower Cretaceous during the fragmentation of the Gondwana supercontinent.
The genesis of the half-grabens is associated with volcanism and the later development of lacustrine
systems. The reflectors related to the half-graben infill show, in some cases, high amplitudes and high
dip angles, allowing to assume a volcanic component in its lithological composition. However, parallel
and continuous reflectors that can be related with lacustrine deposits were also identified (Figure 5).
Figure 5: Seismic character of the synrift megassequence in a halfgraben controlled by a antithetic normal fault. The
discontinuos and high amplitude reflectors at the base are associated with volcanic deposits. The lower amplitude, parallel and
continuous reflectors are related to siliciclastic deposits, alluvial/fluvial on the edges and lacustrine on the center of the
halfgraben.
The isopach map for the half-grabens infill shows two depocenters elongated in a NE-SW trend, in
which the thickest sections develop in the center of the half-grabens (Figure 6).
Figure 6: Isopach map of the synrift section associated with halfgrabens in meters.
The SDRs represents the thickest and most widespread package of the synrift megasequence,
developing in the north-central part of the area. The wedges are represented by arched reflectors
(which dip not only seaward, but also northeastward; Soto et al., 2011) with different dip angles
(Figure 7). Like the half-grabens, the SDRs are controlled by antithetic, NE-SW trending normal faults.
According to the literature (Roberts et al., 1984; Eldhom et al., 1987) this package is composed by
successive volcanic flows deposited in a continental environment, although with an inferred not
negligible sedimentary component (see below).
Figure 7: Seismic character of the synrift associated with the SDRs in dip seismic section.
The isopach map (Figure 8) shows that the thickest section of the SDRs develops in the central-east
part of the area.
Figure 8: Isopach map of the SDRs in meters.
Regarding the importance of synrift megasequence to the potential petroleum systems is worth noting
that was a potential lacustrine source rock associated with the half-grabens was identified (i.e., the
Castellanos Formation in the Santa Lucía Basin). Besides that, if the SDR package includes aeolian
intertraps interbedded with the basalts, like the case of the Kudu Gas field in the offshore of Namibia
(Bray & Lawrence, 1999), these can represent interesting potencial traps for hydrocarbon
accumulation.
4.1.3. Postrift
The postrift megassequence is the thickest of the three megasequences, reaching in the distal area
almost 6000 meters. It is considered the megassequence with the greatest exploratory potential
because it is likely to develop the most important and widespread source rocks (Aptian and Turonian),
and also goodquality and abundant reservoirs (e.g. turbidites). In addition to that, the postrift
megasequence comprises several thick and widspread levels rich in clays that represent potential
seals. The deposition of the postrift megasequence is fundamentally controlled by sea level variations
in a passive margin stage that continues nowadays.
Using the sequence stratigraphy methodology, ten sequences boundaries were identified that
individualize ten depositional sequences (Figure 9), that represents retrogradational and
progradational stacking patterns.
Figure 9: 2D dip seismic section of the Pelotas Basin (offshore Uruguay) showing the depositional sequences interpreted.
In this way, based on the seismic, were interpreted the system tracts that compose the different
depositional sequences to identify the elements of the speculative petroleum systems. The sequence
stratigraphy analysis also allowed to identify five maximum flooding surfaces (MFS) in the postrift
section (Figure 10). The MFS reveals the distribution of the potential source rock in the study area,
considering that the distal portion of these flooding surfaces represent deep marine areas with organic
rich shales. However, due to the overburden experimented by these marine shales, it is likely that the
Cretaceous source rocks and, to a lesser extent, the Paleocene source rocks had reached enough
maturity to generate and expel hydrocarbons. In this work, the interpretation and description of the
depositional sequences (that are directly related with the speculative petroleum systems proposed for
the postrift) will be presented.
Figure 11: 2D dip seismic section showing the distribution of the main maximum flooding surfaces interpreted and its location on
the stratigraphic column.
4.1.3.1. Depositional Sequence 1 (Aptian-Albian)
This sequence develops in the distal region of the basin, directly overlying oceanic crust and thins out
against the SDRs. It is limited in its base by the top basement and on the top by the sequence
boundary SB1. The base is composed by a set of parallel and continuous reflectors with a
retrogradational stacking pattern that shows onlap towards the continent against the oceanic crust and
the SDRs (Figure 11).
Figure 11: Seismic character of the depositional sequence 1 (Aptian-Albian). The parallel and continuous reflectors at the base
of the sequence make onlap against the oceanic crust and the SDRs. In the proximal section it can be distinguished a set of
reflectors making downlap over the maximum flooding surface.
These reflectors constitute a TST, which represents the first marine flooding of the basin, comprising
fundamentally fine grain marine sedimentation. On top of this set of reflectors a maximum flooding
surface (MFS 1) is developed. Leaning on this surface can be recognized a set of reflectors with
downlap terminations that shows a sigmoidal and progradational pattern. These set of reflectors are
interpreted as a HST (Figure 12), in which is expected to find fluvial, shelfal and shoreface deposits
with potentential as reservoirs.
Figure 12: Picture showing the system tracts that compounds the depositional sequence 1: TST: Transgressive system tract,
HST: Highstand system tract.
The sequence presents a constant thickness, although the thickest section is preserved in the western
part of the area (Figure 13).
Figure 13: Isopach map of the depositional sequence 1 (Aptian-Albian) in meters.
The depositional sequence 1 was deposited during Aptian-Albian age, being contemporary to the first
Oceanic Anoxic Event of the Cretaceous (OAE1). Therefore, it can be interpreted that the TST
identified is represented by organic rich marine shales that has an important potential as source rock.
4.1.3.2. Depositional sequence 2 (Cenomanian-Turonian)
This sequence develops in the distal and central region of the study area and thins out towards the
continent against the SDRs. It is limited in the base by the sequence boundary SB1 and in the top by
the sequence boundary SB2.
The sequence presents at the base a set of parallel and continuous reflectors with a retrogradational
stacking pattern that, in the distal region, lies in a concordant way against the transgressive system
tract of the previous sequence. As one moves towards the continent, these set of reflectors make
marine onlap against the high stand system tract of the previous sequence until they surpass them
(Figure 14).
Figure 14: Seismic character of depositional sequences 2 (Cenomanian-Turonian).
This set of reflectors is interpreted as a TST in which is recognized at the top the second maximum
flooding surface (MFS 2). In the proximal section of the sequence it develops a set of reflectors with a
progradational stacking pattern that advance towards the sea with downlap terminations, overlying
TST of this sequence. These set of reflectors is interpreted as a HST (Figure 15).
Figure 15: Picture showing the system tracts that compose the depositional sequences 2: TST: Transgressive system tract,
HST: Highstand system tract.
This sequence has a Cenomanian-Turonian attributed age. Associated with the TST is expected to
find organic rich marine shales deposited during the second oceanic anoxic event of the Cretaceous
(OAE2). Therefore, it has an important potential as source rock. In addition, the high stand system
tract could include lithologies, mainly sandstones, with potential as reservoir fundamentally in its
proximal section. As the depositional sequence 1, the thickest section is located in the west side of the
area (Figure 16).
Figure 16: Isopach map of the depositional sequence 2 in meters.
4.1.3.3. Depositional sequence 5 (Lower Paleocene)
The depositional sequence 5 develops in the distal region of the basin, and thins out in the proximal
section against the previous sequence (depositional sequence 4: Campanian-Maastrichtian).
The sequence 5 is limited at its base by the sequence boundary 4 (SB4) and at the top by the
sequence boundary 5 (SB5). In the distal section of the sequence can be recognized a set of
reflectors with lobe geometry that lies against the previous sequence (Figure 17) and are interpreted
as a submarine fan complex due to its paleogeographic location at the base of the paleoslope, its
lobular morphology and its internal arrange that present downlap terminations on both sides of the
body (Mitchum, 1985). This segment of the depositional sequence is interpreted as a lowstand system
tract.
Figure 17: Seismic character of the depositional sequence 5 (Lower Paleocene) showing the type of terminations that reflectors
have.
Above this tract it develops a set of continuous and parallel reflectors that present a retrogradant
pattern and make onlap against the LST and the previous sequence (Figure 18). These set of
reflectors represent a TST related with a relative ascent of the sea level. The top of this TST is marked
by a maximum flooding surface (MFS 3) related with a marine transgression during the Lower
Paleocene. At the top of this tract it can be recognized a set of reflectors with a progradational
stacking pattern that make downlap against the maximum flooding surface. This package is
interpreted as a HST. From a petroleum system point of view, the identification of a submarine fan
complex covered by a marine transgression represent very important stratigraphic traps.
Figure 18: Picture showing the system tracts that compose depositional sequence 5: LST: Lowstand system tract, TST:
Transgressive system tract, HST: Highstand system tract.
The isopach map shows that the thickest section develops in the proximal sector (Figure 19).
Figure 19: Isopach map of depositional sequence 5 (Lower Paleocene) in meters.
4.2. Structural interpretation
In relation to the presence of potential migration pathways for hydrocarbons, the occurrence of sub-
vertical faults were identifed in seismic lines. Most of these faults affect the postrift section and have
little displacement, in the order of tens to hundreds of meters (Figure 20). Given the vertical extension
of the faults and the location of the fault systems identified, it is expected that the Cretaceous source
rock (Aptian-Albian and Cenomanian-Turonian) charged fundamentally Cretaceous and Lower
Paleocene reservoirs while the Paleocene source rock, if mature, charged (and is still charging)
Cenozoic reservoirs.
Figure 20: 2D seismic section of Pelotas Basin showing major faults identified (potential migration pathways) and its relation
with the maximum flooding surfaces.
4.3. Petroleum systems proposed
As a result of the seismostratigraphic analysis six speculative petroleum systems are proposed:
IRATI-PIRAMBOIA/BOTUCATU(?)
CASSINO-CASSINO(?)
ATLÂNTIDA-IMBITUBA(?)
ATLÂNTIDA-IMBE(?)
Lower IMBE-IMBE (?)
Middle IMBE-IMBE(?)
It is important to notice that an overburden model of the potential source rocks was not developed for
this work. Therefore, in relation with the moment of generation, migration and accumulation of
hydrocarbons related with the different petroleum systems, was used overburden data of the offshore
of Uruguay modeled in Morales (2013).
Described below are the elements that constitute the speculative petroleum systems defined and their
characteristics.
4.3.1. IRATI-BOTUCATU(?) (PRERIFT)
This speculative petroleum system is associated with the prerift megassequence, being represented
by a Lower Permian source rock represented by organic rich shales of the Irati Formation, deposited
in a restricted marine environment. On the other hand, the potential reservoirs are related to Permian
to Upper Jurassic aeolian-fluvial sandstones of the Rio Bonito, Piramboia and Botucatu formations.
The seal for this system is constituted at least in part by Lower Cretaceous basalts of the Serra Geral
Formation.
The distribution for this petroleum system it would be restricted to the proximal section of the study
area (Figure 21), strongly related with the distribution of the Polonio High. The identified traps for this
petroleum system are mainly structural, related with faults or anticlines generated during crustal
deformation in the synrift phase.
In the context of the Parana Basin the Irati Formation (Mangrullo Formation in Uruguay) it is
predominantly inmature, only reaching the oil/gas window when it is associated with magmatic
intrusions. However, in the study area the Irati shale would have reach maturity due to the additional
overburden associated with the sediments of the Pelotas Basin. The migration of the generated
hydrocarbons from the source to the reservoir represents barely a distance of hundreds of meters and
it would take place through subvertical faults.
There are several risks associated with this petroleum systems, among them the integrity of the traps,
nevertheless the fundamental one is related with the preservation of the diferente elements of the
petroleum system (source rock, reservoir, seal) since the seismic resolution does not allow to
individualize system tracts.
Figure 21: The Iratí-Botucatú(?) speculative petroleum system of the prerift megasequence showing the geological model, area
of occurrence, chart of events and a related seismic section.
4.3.2. CASSINO-CASSINO(?) (SYNRIFT)
This speculative petroleum system is related with the synrift megassequence, being restricted to the
development of halfgrabens. The potential source rock is constituted by Barremian lacustrine shales
(temporarily associated with the Cassino Formation), while the reservoirs are associated with alluvial
deposits of the Cassino Formation. The seal is represented by the same lacustrine shales that are
interbedded with the alluvial deposits and also by volcanic rocks of the Imbituba Formation. This
petroleum system is restricted to the west portion of the area (Figure 22). The identified traps for this
petroleum system consist of stratigraphic pinchouts and alluvial fans deposited in lacustrine bodies.
The migration would be in a direct way since the lacustrine source rock and the alluvial reservoirs are
in contact. The main risk associated with this petroleum system is the development of a source rock
with sufficient thickness and quality to generate hydrocarbons. Additionally, the development of the
halfgrabens in the study area is reduced, barely 3 km2, therefore the generation potential is also
reduced.
Figure 22: The Cassino-Cassino(?) speculative petroleum system of the synrift megasequence
showing the geological model, area of occurrence, chart of events and a related seismic section.
4.3.3. ATLÂNTIDA-IMBITUBA(?)
This speculative petroleum system presents a source rock associated with the postrift
megassequence and reservoirs related with the synrift. The source rock is represented by Aptian-
Albian marine shales of Atlântida Formation, which are distributed in the distal portion of the basin.
These shales were deposited during the first Oceanic anoxic event of the Cretaceous (OAE 1). The
reservoirs are related with the SRDs package, being represented by aeolian sands interbedded with
volcanic flows of the Imbituba Formation. The sealing is constituted by the same volcanic flows that
toghether with the aeolian sands generate stratigraphic pinchouts. The Kudu Gas Field in the Orange
Basin, offshore Namibia is the analog for this geological model.
The main risk associated with this petroleum system is the presence of the clastic reservoir in the
SDRs. If present, the clastic intertraps would manifest in the proximal section of the SDRs while the
distal part would be constitued fundamentally by vocalnic rocks. Taking into consideration that the
Aptian source rock is onlapping the SDRs sequence (Figure 23), the migration of hydrocarbons would
take place through the unconformities between these units.
Figure 23: The Atlântida (Aptian-Albian)-Imbituba(?) speculative petroleum system with a source rock of the postrift
megasequence and reservoir rocks of the synrift megasequence, showing the geological model, area of occurrence, chart of
events and a related seismic section.
4.3.4. ATLÂNTIDA-IMBÉ(?)
This speculative petroleum system develops in the postrift megassequence and presents as source
rock the Aptian-Albian marine shales deposited during the OAE 1 of the Atlântida Formation. In this
case the reservoirs are represented by Upper Cretaceous sandstones beds associated with deltaic
progradation fronts and, most notably, turbidites of the Imbe Formation. The seals are associated with
fine grain facies of the Imbe Formation (marine shales), related with marine transgression that cover
the potential reservoirs during the Cenomanian-Turonian and Paleocene. The Atlântida-Imbé(?)
system develops in the distal portion of the study area (Figure 24). One of the main risks associated
with this petroleum system is related with the presence of migration paths that effectivly conects the
aptian source rock with the clastic reservoirs of the Imbe Formation.
Figure 24: The Atlântida (Aptian-Albian)-Imbé(?) speculative petroleum system of the postrift-megasequence showing the
geological model, area of occurrence, chart of events and a related seismic section.
4.3.5. Lower IMBÉ-IMBÉ(?)
This speculative petroleum system develops in the postrift megassequence and presents as source
rock Cenomanian-Turonian marine shales belonging to the lower section of the Imbe Formation.
These shales were deposited during the second Oceanic anoxic event of the Cretaceous (OAE2).
Meanwhile, the reservoirs are represented by Upper Cretaceous and Lower Paleocene sandstones
beds associated with deltaic progradation fronts and turbidites of the Imbe Formation. Besides that,
the marine shales of the Imbe Formation related with the Paleocene transgression would serve as
seals for the sandstones. The migration of hydrocarbons would be directly (laterally) or vertically
through faults that conects the fine grain facies with the coarse grain facies of the Imbe Formation
(Figure 25). The main risk related with this petroleum system, like the previous case, is the presence
of effective migration paths that connect the source to the reservoirs.
Figure 25: The Lower Imbé (Paleocene)-Imbé(?) speculative petroleum system of the postrift megasequence showing the
geological model, area of occurrence, chart of events and a related seismic section.
4.3.6. Middle IMBÉ-IMBÉ(?)
This speculative petroleum system develops in the upper section of the postrift megassequence and
presents as source rock Lower Paleocene marine shales belonging to the middle section of Imbe
Formation. The reservoirs are fundamentally represented by turbidites, channels and progradation
fronts deposited in the Eocene, Oligocene and Miocene. This system develops in the distal portion of
the study area (Figure 26). The main risk related with this system is associated with the maturation of
the source rock, taking into consideration that the Paleocene sequence is immature in the proximal
sector where it was drilled. However, this source rock could attain the oil window in the distal section
where the overburden is higher (Morales, 2013).
Figure 26: The Middle Imbé (Paleocene)-Imbé(?) speculative petroleum system of the postrift megasequence showing the
geological model, area of occurrence, chart of events and a related seismic section.
5. Conclusions
The seismostratigraphic interpretation performed in 2D seismic sections of the Uruguayan portion of
the Pelotas Basin shows a favorable geology for the accumulation of hydrcarbons, in a still
underexplored basin.
In the study area were identified 3 megassequence that constitute the volcanic-sedimentary infill of the
basin: Prerift, Synrift and Postrift, the latter composed by 10 depositional sequences.
There were identified potential source, reservoirs and seal rocks in the three megasequences, being
proposed six speculative petroleum system, one related with the Prerift, one with the synrift and four
with the Postrift megassequence.
The petroleum systems defined for the Prerift and Synrift present a distribution restricted to the
proximal sector, with risks related with the development of the source rock.
In the Postrift megassequence there were identified 3 potential marine source rocks, related with
maximum flooding surfaces, two of them (Aptian-Albian and Cenomanian-Turonian) are world class
source rocks. The organic content of these pelagic sequences would increase in a more distal
situation of the basin.
There were identified multiple siliciclastic reservoirs for the Postrift megassequence, fundamentally in
the Upper Cretaceous and Paleogene. Besides, were identified regional seals associated with marine
transgressions in the Cenomanian-Turonian, Paleocene and Miocene. These elements support the
conclusion that the postrift petroleum systems present a higher hydrocarbon potential. These
speculative petroleum systems develops fundamentally in the distal part of the study area.
With a few exceptions, the majority of the traps identified are stratigraphic types: pinchouts, turbidites
and channels.
The structural analyses identified a set of subvertical faults in the postrift section which could have
acted as migration pathways for hydrocarbons. The vertical extension of the faults allows to affirm that
the Lower Cretaceous source rocks would have fed fundamentally Cretaceous reservoirs, while the
Paleocene source rock would have fed the Cenozoic reservoirs. However, the migration pathways
seem to be one of the critical factors of the petroleum systems.
Acknowledgment
This article is a product of a Master of Science thesis carried out at UNESP University, Campus of Rio
Claro/SP. We would like to thank to all the professors at the University, to ANCAP for providing data
and supporting this study and to ION GXT for allowing the use of the seismic line. Finally we would
like to thank the reviewers for their valuable comments and suggestions.
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