Speculative Petroleum Systems of the Southern Pelotas Basin, Offshore Uruguay Bruno Conti1*; José Alexandre de Jesus Perinotto2; Gerardo Veroslavsky3; María Gabriela Castillo2; Héctor de Santa Ana1; Matías Soto3

1Exploration & Production, ANCAP, Montevideo, Uruguay. [email protected]; [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]; [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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