Martin Keeley, Malcolm Light
Intera Information Technologies Inc.
Henley-on-Thames, U.K.
Stanley Hogg
Cullen, Valdez Rojas y Asociados
Buenos Aires
Carlos Urien
Urien & Associates
Buenos Aires
The announcement last fall by Pres. Carlos Menem of a new exploration licensing round, known as the Plan Argentina, triggered an unprecedented wave of interest in the opportunities.
Argentina is a vast under-explored country with an area of more than 1 million sq miles.
With tax reduced, royalty pegged at a maximum 12%, and equity participation from the state through Yacimientos Petroliferos Fiscales eliminated, the Ministry of Hydrocarbons and Fuels in Buenos Aires is expecting to award most of the 145 exploration concession blocks by yearend 1992 (Fig. 1).
ln preparation for Plan Argentina, the ministry contracted Intera and Cullen, Valdez Rojas y Asociados of Buenos Aires to fully review the exploration potential of the open blocks covering 15 onshore and offshore sedimentary basins. The results are synthesized here, with the emphasis on changes in perceived prospectivity.
ARGENTINA'S BASINS
Three classes of basin are recognized in Argentina.
PALEOZOIC INTRA-CRATONIC SAG BASINS. These basins are formed within the greater Gondwanaland prior to Atlantic rifting. A thick sequence of mixed Paleozoic clastics accumulated within a vast interior crustal depression with anoxic flooding events of Silurian and Devonian age.
Permo-Carboniferous glacial sediments are succeeded by organic-rich lacustrine muds.
The northward migration and collision of exotic terranes with Gondwanaland in the Patagonia region during the Permo-Triassic created an orogenic belt across what is now central Argentina and into southernmost Africa: the Ventana-Cape fold belt.
Late Jurassic crustal doming immediately prior to Atlantic rifting led to the erosion of earlier strata from the eastern margin of southern South America.
Paleozoic remnants are now preserved to the west in the Chaco-Parana basin (northern Argentina, southern Brazil, western Uruguay, and eastern Paraguay), the Claromeco basin (east central Argentina), and beneath the Andean foreland Tertiary basins of western Argentina.
Oil production is recorded from Ordovician, Silurian, and Devonian sandstones in the Tarija basin, northern Argentina. The Paleozoic intervals believed to be the source of these oils are also found in the Bolsones basin to the south as well as throughout the Chaco-Parana basin.
PACIFIC SUBDUCTION BACK ARC BASINS. The Pacific plate was obliquely subducted northwards beneath the Brazilian shield during the late Paleozoic and Mesozoic.
The process enabled exotic terranes and volcanic arcs to be progressively accreted onto the shield. Associated back-arc extension and oblique transtension generated belts of narrow troughs that young south.
During the Triassic and Jurassic these troughs were the site of restricted sedimentation, leading to the accumulation of the most important oil and gas source rocks in Argentina.
On further extension, these troughs matured into broader asymmetrical foreland basins, with late Cretaceous-Paleogene marine clastic infill, providing reservoir and seal associations.
During the Miocene the Pacific subduction vector swung around to the east, thereby inducing a phase of transpression and granitic emplacement: the Andean orogeny. It was also associated with a new belt of broad back-arc basins, running north-south, the Neuquen, Cuyo, Bolsones, and Nirihuau basins.
Late Cretaceous-Tertiary sedimentation was strongly controlled by eustacy, ensuring a cyclical interbedding of fine and coarse-grained clastics. Thick Neogene burial of the earlier Triassic-Jurassic basins and high heat flow brought about rapid source rock maturation synchronous with and post-trap formation.
Oil production from this class of basin accounts for the great majority of Argentine oil production, currently more than 500,000 b/d: Austral (Magellanes), San Jorge, Neuquen, and Cuyo basins.
ATLANTIC RIFT BASINS, MODIFIED BY BASEMENT FABRIC. Nine basins lie along the eastern shelf of Argentina onshore and offshore bordering the Atlantic. All were formed during latest Jurassic and early Cretaceous in response to the rift and eventual separation between the South American and African continents.
The basins have various orientations, the result of interference between extension rifting stress and the basement fabric, which was imparted from earlier accretionary processes (see above).
The Salado and Colorado basins in the north and the North Malvinas (San Julian) basin in the south are oriented orthogonal to the Atlantic margin, whereas the Valdes, Rawson, and West Malvinas basins lie nearly parallel to it.
All these basins, however, have very similar rift fill clastic sequences reaching more than 4 km in thickness
The dominant rift fill is sandstone, resulting from rift shoulder and hinterland erosion. Higher sea levels in the late Cretaceous and Paleogene enabled the accumulation of thick fine-grained sequences.
Oil shows are rare in these basins, partly because there is so little well evidence. However there is some well geochemical evidence to support the presence of source rocks at the base of the Colorado rift sequence.
The sedimentological model for deposition during the early rift phase would certainly call for restricted marine or lacustrine deposition in the outer Colorado basin.
Indeed lacustrine bituminous carbonates are recorded from outcrop at the base of the rift succession of the Santa Lucia basin in southern Uruguay, thereby providing a potential analog for all the Atlantic rift basins of Argentina.
PROSPECTIVITY OF BLOCKS
More than 145 open onshore and offshore blocks in 15 basins are available in Plan Argentina.
Here is a rundown of their prospectivity.
AUSTRAL BASIN
Four moderate risk open blocks lie basinward of the hingeline.
Most oil and gas discoveries to date have been made on the shallow eastern side of this line, but these hydrocarbons were sourced from the Lower Cretaceous kitchen to the west.
SAN JORGE BASIN
Six open moderate risk blocks lie in the west of this basin, the Rio Mayo subbasin, in which there has been very little exploratory drilling to date.
However, surface geochemistry proves the existence of the same excellent Triassic and Jurassic source rocks as are so prolific in the east of the basin.
NIRIHUAU BASIN
One large open low risk block covers this small mid-Tertiary back-arc basin, in which both surface geochemistry and the three wells drilled demonstrate prolific capacity for oil generation.
NEUQUEN BASIN
Four open low risk blocks lie in the Andean foredeep on the western margin of this most important of Argentine oil and gas producing basins.
Difficult surface conditions in the west have hindered exploration to date.
CUYO BASIN
Eight low risk blocks are distributed over this the second most important hydrocarbon producing basin in Argentina.
All blocks lie close to existing fields with Triassic, Jurassic, and Lower Cretaceous sandstone reservoirs.
Operators continue to make discoveries by applying modern geophysical techniques and stratigraphic trapping concepts.
TWO BASIN COMPLEX
Fifteen moderate to high risk blocks are open over the poorly known Bolsones-San Luis complex of basins in the Andean foreland.
Preliminary field work does support the presence of good subsurface plays involving both the Upper Paleozoic and early Mesozoic successions, but geophysical coverage is poor.
PUNA BASIN
Only two moderate to high risk blocks are open in this highly deformed Andean foreland basin.
Again, in the absence of proved reserves, reconnaissance work lends encouragement. Similar plays to the Bolsones and Oran basins are anticipated.
ORAN [NOROESTE] BASIN
Seven open moderate to high risk blocks lie around the shelf margins of this Andean foreland basin.
Proven plays produce oil and gas from a variety of Devonian, Carboniferous, and Cretaceous reservoirs.
CHACO-PARANA BASIN
This vast Paleozoic intracratonic sag basin covers more than 6 million sq km in Argentina alone (Fig. 2).
The basin has received little exploration, partly because the eastern half is covered by Cretaceous plateau basalts. Also, the few wells drilled to date have not been encouraging. However, the Brazilians have had recent success with their Irati-sourced (Permian) gas/condensate play.
There are 39 high risk blocks open over this basin; if elephants remain to be found in Argentina, it must surely be here.
SALADO BASIN
This Atlantic rift basin lies adjacent to Buenos Aires with its domestic and industrial consumers.
It also lies closest to the base-rift oil prone facies proven in the Santa Lucia basin, Uruguay.
Seven onshore and one offshore moderate risk blocks are open in this basin.
COLORADO-CLAROMECO
The asymmetrical Permo-Carboniferous Claromeco basin passes offshore beneath the Atlantic rift Colorado basin.
Except for inshore, the Colorado basin is underexplored, yet the distal central and eastern subbasins yield the most prospective facies. Onshore eight high risk blocks are open, and offshore five high to moderate risk blocks are available, with potentially high reserves.
VALDEZ-RAWSON BASINS
Six moderate to high risk open blocks cover these two poorly known Atlantic rift basins.
However, good seismic coverage proves up several structures that will be prospective if adequate source facies are present in the basins.
PATAGONIAN SHELF
Eighteen moderate to high risk blocks are open on this shelf, around the margins of the San Jorge, North Malvinas (San Julian), and West Malvinas basins (Fig. 3).
Given that the long range migration is already proven within these basins, extensional structures on the shelf-basin transition could be charged with hydrocarbons.
Some of the same oil bearing horizons known in the San Jorge and West Malvinas basins extend well onto the shelf and are capped by thick fine grained Tertiary clastics.
CONTINENTAL SLOPE
Only nine high risk open blocks lie in water deeper than 200 m.
They lie astride an important slope-parallel basement ridge formed during early rifting. Landward of this synrift restricted deposition is anticipated.
Coupled with the Cretaceous anoxic intervals proven by Deep Sea Drilling Project drill holes to lie seaward, this ridge forms an important regional focus of potential hydrocarbon migration, draped by both fine and coarse-grained Tertiary clastics.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
