Relinquishments in Argentina will boost exploration interest

The area lies mostly in the Tarija basin, although the southern portion extends into the Cretaceous basin. Aguarague covers 15,740 sq km.

Work carried out in the area led to the discovery of additional reserves in Devonian reservoirs. The XP-1 Aguarague deep exploration well had initial production of 58 b/d of oil and 5.3 MMcfd of gas from the Devonian Icla formation and 15.5 MMcfd of gas and 15 b/d of oil from the underlying Devonian Santa Rosa formation. In both reservoirs, production was obtained through open flow. The well's TD is 5,332 m.

In the Chango Norte X-1, a drillstem test was run over a Devonian Huamapampa formation interval and flowed 19.2 MMcfd of gas plus 722 b/d of condensate.

In addition, the La Porcelana X-1001 well flowed 13.5 MMcfd of gas and 502 b/d of condensate through a 48/64 in. choke from Huamapampa on a single DST.

At the time of bidding the acreage had reserves of 59.5 million bbl of oil and 1.68 tcf of gas. Reserves as of December 1995 were 21 million bbl of oil and 1.68 tcf of gas.

Exploration possibilities in the area are the Devonian Icla, Huamapampa, and Santa Rosa formations and older strata associated with faulted structures. In addition, paleovalleys carved in Devonian sediments and filled by sediments of the Carboniferous Tupambi formation are likely to occur to the east of the area. The paleovalleys are well defined on seismic proflies recorded in the Chaco plains (Fig. 3 [40560 bytes]) and are exposed in the Subandean ranges.¹

The Palmar Largo Association area is in the Cretaceous basin (Lomas de Olmedo sub-basin) and covers 17,112 sq km (Fig. 2 [31628 bytes]).

Main reservoirs are the clastics and carbonates of the Cretaceous-Tertiary Yacoraite formation and Cretaceous Palmar Largo volcaniclastics. The distribution of the Palmar Largo formation is still unknown. Pucci outlined a distribution sketch map but was unable to locate the western border due to insufficient subsurface information.

Mujica and Zorzin³ have defined compressive structures in the platform area or Chaco plains (Jollin field) with similar structural characteristics to those present in the Subandean ranges. Hence, as the Huamapampa and Santa Rosa reservoirs produce by secondary permeability they became new objectives in that part of the Tarija basin.

Similar anomalies do appear to continue towards the east, although a new seismic coverage is necessary. It is also possible that the same anomalies extend southwards into the Palmar Largo Area (Fig. 2 [31628 bytes]) and thus establishing Paleozoic plays. Pucci⁴ indicated the possibility of mature Paleozoic sediments in the Lomas de Olmedo sub-basin.

Reserves changed from 19.8 million bbl at the time of bidding (gas was flared) to 16.2 million bbl in 1993 and 9.36 million bbl by December 1995, when gas reserves were estimated at 8.8 bcf.

At present no discoveries have been reported in the Palmar Largo Association Area.

In spite of the oil reserves reduction, the area has sufficient acreage for exploration and hence new discoveries are likely to occur in Mesozoic and Paleozoic sediments.

Neuquen basin

Yacimientos Petroliferos Fiscales's areas are located in the Neuquen basin, Argentina's most prolific in terms of proved reserves and production (Fig. 4 [64621 bytes]). They are the most prospective areas to be relinquished and likely contain commercial quantities of hydrocarbons.

The classical reservoirs in the basin correspond to the Jurassic Lajas, Lotena, and Quebrada del Sapo/Tordillo/Sierras Blancas and Cretaceous Quintuco/Loma Montosa, Mulichinco, and Agrio formations. However, in some of the areas new plays have been found in the Cretaceous Huitrin subgroup (Fig. 4 [64621 bytes] and Table 1 [10010 bytes]), both in the platform and the fold belt. The former having a main stratigraphic component and the latter is structural. The main fields are El Porton, Filo Morado in the fold belt, and Puesto Molina, Puesto Hernandez, El Trapial, etc., in the platform area.

Other plays are reservoirs of igneous rocks intruded in the Quintuco-Vaca Muerta formations that warp the strata, although no folds occur in the underlying rocks. The structure was formed by Tertiary volcanic activity and hence the hydrocarbons migrated to the traps after that geologic event. It was established that the maturation was not affected by the intrusion.

Lately in Chihuido de la Sierra Negra field one well encountered igneous rocks intruding the Huitrin subgroup and containing commercial quantities of hydrocarbons.

Vertical and horizontal migration from the Agrio shales was facilitated by faults, fractures, weakness zones, etc.

In the VII and VIII Areas the sandy Troncoso formation is one of the most prospective plays.

In the southern areas (IX and X) the plays correspond to structural and combination traps containing source and reservoir rocks of the Cuyo Group. A detailed study of the Cuyo Group is necessary in all the areas to establish maturation zones and future prospects.

Buta Ranquil

The area is an association between the YPF and Mexpetrol companies. Buta Ranquil occupies 1,609 sq km.

End-1995 reserves were 25.7 million bbl of oil and 522 bcf of gas contained in the Huitrin, Agrio, and Mulichinco reservoirs accumulated in north-south elongated and narrow folds.

Hydrocarbon possibilities may exist in areas to the east and west of the Pampa Tril-Reyes-Cara Cura structural trend (Fig. 4 [64621 bytes]) and where the Huitrin subgroup or younger units are exposed.

Gulf of San Jorge basin

The San Jorge basin is Argentina's most prolific in cumulative production.

Two areas to be relinquished are in zones close to significant accumulations. They are YPF's II and III areas.

Areas I and III are in the western tensional zone adjacent to the San Bernardo fold belt, while Area II is in the eastern tensional zone (Fig. 5 [25246 bytes]).

YPF Area I covers 5,712 sq km. Mainly small fields in Area I are in the north and south of the block.

The reserves range between 119,320 bbl of oil (Cayelli) and 8.6 bcf of gas (Paso Rio Mayo) in the north to 3 bcf of gas (Campo Zabala) and 50,000 bbl of oil (Estancia Zabala) in the south.

Undisclosed production was found in the Cerro Piedras (Bajo Barreal formation), Pampa Sissac and Estancia Essoin (D-129) formation outpost wells.

YPF's Area III occupies 568 sq km. Fields in Area III have reserves between 201,000 bbl of oil (Puesto Cerda) and 317 MMcf of gas (Rio Mayo). Due to the small size of the area, no significant discoveries are expected.

YPF's Area II covers 1,351 sq km. Although surrounded by fields of different sizes, no discoveries have been reported in Area II. It is of medium to high risk.

Austral basin

The Santa Cruz I Association Area has a surface area of 31,247 sq km (Fig. 6 [25205 bytes]) and originally contained six fields. After privatization, four fields with an average production of 1,375 b/d of oil and 1.9 MMcfd of gas were discovered.

At the time of bidding, the Santa Cruz I Association Area had 11.8 million bbl of oil and 381 bcf of gas. Reserves as of 1995 were 26.3 million bbl of oil and 350 bcf of gas.

The Santa Cruz II Association Area covers 14,000 sq km and included four fields. After the area was awarded, one field was discovered. It averages 4,578 b/d of oil and 1.16 MMcfd of gas from the Tertiary Lower Magallanes formation.

Santa Cruz II at time of bidding had 8.6 million bbl of oil and 371 bcf of gas reserves. By December 1995 reserves were 18.3 million bbl of oil and 420 bcf of gas.

In the Santa Cruz I and II areas the new fields are located in the central part of the areas, coincident with the Tertiary prospective zone named intermediate by YPF's geologists.4 However, except for Maria Ines field, which produces from Tertiary reservoirs, the rest of the production is from Cretaceous Springhill; in Bajada Fortaleza is accumulated in a thin sandstone interval located at the top of the thick, shaly Palermo Aike formation.

It is estimated that future exploration will lead to the discovery of fields with recoverable reserves ranging between 5-6.3 million bbl of oil and 8.8 MMcf-423 bcf of gas. In addition, in the western portion of the basin, adjacent to the Andes, accumulations in Tertiary strata are likely to occur.4

References

1. Stark, D., Gallardo, E., and Schulz, A., La discordancia precarbonica en la porcion Argentina de la Cuenca de Tarija, Bol. Informaciones Petroleras (YPF), Buenos Aires, Vol. 29, 1992, pp. 2-11.

2. Pucci, J.C., Geology, potential of Argentina's international bidding areas, OGJ, June 13, 1994, pp. 138-144.

3. Mujica, H., and Zorzin, V., Extension oriental de la tectonica Andina en la Llanura Saltena, Provincia de Salta, to be published in the Argentina Geological Congress and Tercer Congreso Nacional de Exploracion de Hidrocarburos, to be held in Buenos Aires in October 1996.

4. Pucci, J.C., Potential assessed in Argentina's association contract areas, OGJ, June 20, 1994, pp. 55-58.

The Author

Juan Carlos Pucci has been active in international exploration for more than 20 years. His employers included GSI, Cities Service, and Mobil Oil. He was the geologist representing the Secretary of Energy in the privatization of Argentina's upstream sector. At present he is working as exploration advisor for the Secretary of Energy and Mining. He holds an MS in petroleum geology and a PhD in geology from the University of Buenos Aires.

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