Argentina: The Final Atlantic Margin Frontier
Argentina has the least explored continental margin along the Atlantic Ocean. Its coastline extends for some 1,700 km in a north-east to south-west direction from the Uruguay border to the South Scotian Foldbelt (Figure 1 and Figure 2); and yet there are no wells drilled in more than 200m water depth. Government policies of oil pricing and export restrictions are partly responsible for this lack of activity. However, with the recent change of government, the state company Enarsa has handed back all its offshore blocks to the government. Argentina may now be looking to attract foreign explorers; and here we offer some new perspectives on the exploration potential of the Atlantic margin.
The Argentine margin is transected by a series of isolated rifts within the continental shelf (Figure 2) which are filled with non-marine sediments. They initiated in mid-Jurassic times, and rifting continued into the Early Cretaceous. The rifts are generally highly oblique to the Atlantic shelf edge indicating an approximately north-south extension at this time. The north to south trending Rawson-Valdes Rift is an exception, but this may have developed as a pull-apart basin along a north-west to south-east trending shear zone. These rifts have been the main focus of oil exploration to date, and all of them have at least one exploration well.
During the Early Cretaceous South Atlantic rifting event the extension direction switched to approximately east-west and a narrow zone of small north-south trending rifts developed along the present continental shelf edge shortly before ocean spreading initiated. The exact timing of the onset of ocean spreading is difficult to determine because of the lack of magnetic reversals at this time. Also, the first magnetic anomalies are located on seaward dipping reflector (SDR) volcanics, and not typical oceanic crust. Suggested spreading onset ages range from 140 to 130 Ma.
Several Underexplored Rift Basins
Salado Basin: This northernmost basin has been tested by only six onshore and four offshore wells. All the onshore ones were dry, with two reported to have minor oil shows. The offshore Samar-1X well encountered some residual oil in Cretaceous and Paleocene sandstones, while Dorado-1X was drilled on a basement high and did not test the rift sequence. Paleozoic sediments were encountered in the adjacent Punta del Este graben in Uruguay, and similar shales and sandstones and extrusive volcanics can be expected in the Salado. The non-marine syn-rift and early Aptian-Albian drift sequences consist of red sandstones and shales. There is no known source in this basin.
Colorado Basin: The Colorado Basin is the largest and deepest of the Jurassic rifts with up to 13 km of pre-rift Paleozoic strata and Jurassic-Early Cretaceous syn-rift fill (Figure 3). Twelve wells have been drilled in the basin. The Cruz del Sur-1 well recovered a small amount of good quality (39° API) moveable oil from an Early Cretaceous syn-rift sandstone reservoir. The oil is believed to be sourced from a Permian shale, and Paleozoic strata are imaged on the seismic data as a parallel bedded pre-rift sequence in the offshore Colorado and Salado Basins. Permian strata also crop out in the Claromeco Basin.
A Neocomian source rock with some hydrocarbon potential was also encountered in the Cruz del Sur-1 well, but was immature at the well location. A 25m-thick source interval was reported to have an average TOC value of 2.4%. Neocomian source rocks are more deeply buried in the center of the Colorado Rift, and could source hydrocarbons which migrated into rotated fault terrace plays, which are present along the basin margins. The Colorado Basin is separated from the Atlantic margin by an outer high. This high could also access a syn-rift source rock kitchen. Subtle closures have been imaged above the syn-rift faults at Early Cretaceous level over this high and later small reactivations caused faulting extending up to the Cretaceous level to allow migration into potential shallow water reservoirs. Bright, structurally conformable amplitude anomalies have been identified by YPF over at least one of these large Cretaceous level closures in their previously-operated Colorado blocks. The prospects lie in shallow water, and have good access to Neocomian source rocks sitting in the oil window (Figure 3).
Rawson-Valdes Basin: This is a complex north-south trending system of rifts with a similar history to Salado and Colorado. Two dry holes have been drilled in the basin. The Tayra-1 well was a good test of a rotated fault block play and was adjacent to a 5 km-thick graben sequence. The onshore part of the basin is floored by large volumes of Mid- Jurassic volcanics. There are no proven source rocks in the Rawson-Valdes rift, but the southern Rawson rift has not been drilled yet.
San Jorge Basin: San Jorge is the only basin with proven commercial volumes of hydrocarbons, with more than 4 MMbo recoverable reserves (Fitzgerald et al., 1990). The basin opened in a north-south direction during Late Jurassic to Early Cretaceous rifting (rift trend is east-west). The western half of the basin was affected by Late Cretaceous to Miocene Andean compression, which produced a north-south trending foldbelt in the Andean foreland. This combination of orthogonal extensional rifting and compression has produced numerous structural traps in this basin.
The main source rocks are syn-rift lacustrine Late Jurassic to Early Cretaceous Aguada Bandera Formation and Barremian to Aptian age Pozo D-129 Formation, which were deposited during rifting. The current oil production is all onshore. The basin extends offshore, but the rift fill is generally less than 2.5 km, and any potential source will probably be immature. Approximately 25 dry holes have been drilled in the offshore basin.
Reservoirs are mainly discontinuous fluvial and deltaic sandstones of Tertiary and Cretaceous age. Oil recovery is difficult from such reservoirs, and over 30,000 wells have been drilled in this basin. Unconventional shale oil and gas reserves are also present (Kuuskraa et al., 2013).
San Julian Basin: San Julian is the smallest and southernmost Jurassic rift, with some indication of an earlier Permian phase of rifting. Early Jurassic volcanism marked the initiation of the Jurassic rift phase. An Early Cretaceous transpression caused inversion of the basin, and a late Miocene Andean compressive reactivation also occurred.
The syn-rift Jurassic strata mainly consist of continental sandstones and interbedded lavas and pyroclastics, with some intercalated calcarenites in the late Jurassic (Figueiredo et al., 1996). The lavas have been dated in the onshore Deseado Basin at 168–170 Ma. Total sediment thickness reaches up to 6 km.
San Julian-1 was drilled on a transpressional fold adjacent to deeper graben fill. Petrobras considered this to be the best location to drill within the basin, but it was dry. Some thin source rocks (1.5–5.4% TOC) were encountered in the well in the Neocomian-aged Bajo Grande Formation, but this is generally immature throughout the basin due to lack of burial (Soares et al., 2000). The north-eastern segment of the rift has an older untested section with potential for a deeper source in the Permian sourcing Triassic reservoirs.
The adjacent Deseado onshore basin has good surface oil seeps in Cretaceous strata at Los Navarros and Santa Catalina, but the source of the biodegraded oil is not known.
Atlantic Margin Post-Rift Potential
The break-up of the South Atlantic Ocean initiated between Africa and Argentina in the Early Cretaceous (ca. 130–135 Ma) with the development of a relatively narrow zone of rifting (50 km) and small grabens 5–10 km wide and 1–2 km deep, which are aligned along the present continental shelf edge. This style of extension is in stark contrast to the Brazilian margin farther north where the rifted margins extend for several hundred km width, and the large fault blocks reach up to 50 km wide and 5 km-deep rifts developed.
The reason for this is probably the presence of a linear zone of hot upwelling mantle which extended southward from the Parana plume head in southern Brazil. This upwelling and sub-aerial volcanism occurred at 135–130 Ma. Crustal extension was achieved by dyking with rapid magma injection to fill the accommodation space. The narrow Atlantic rift zone and wide seaward dipping reflector sequences are therefore unlikely to have any significant syn-rift hydrocarbon potential as no source rocks or reservoirs are predicted.
However, the overlying Cretaceous to Recent deepwater sediment pile reaches up to 6 km in places and has significant exploration potential, but is untested. Good source rocks of Barremian to Aptian age have recently been discovered in wells on the conjugate Namibian margin in the deepwater (Mello, 2012); and similar source rocks may be present in Argentina, although there is no well data to support their presence so far. There is also good evidence of large channels with bright reflectivity cutting through the continental slope onto the oceanic abyssal plain at many localities. Carbonate build-up reservoirs may also be present on top of the SDRs when the first marine incursion occurred into the basin.
Bold Explorers Required
The Argentine margin is the least explored segment of the Atlantic Ocean. Initial exploration efforts have focused on the shallow water internal rifts, which indicate the Colorado has the best potential. This is the deepest basin and has some fair source rock potential within the rift. No viable source rocks have been proven in the other Jurassic rifts to date. However, the deeper half grabens of the Jurassic rifts have never been drilled.
The true Atlantic margin has very little potential in the syn-rift, but the post-rift sequence (Barremian to Recent) may contain a Barremian to Aptian source rock. The Early Cretaceous post-rift interval is buried to 4 km along the Atlantic hinge so the source may be mature (see Figure 2 for sediment thick locations). Reservoir channels have been mapped in the deepwater on 2D seismic (our own work). A bold well is required to test this play.