Juan Carlos Pucci
Energy Secretariat and Argentina Geological Survey
Buenos Aires
The presence of a sedimentary basin between the Tandilia and Ventana hills south of Buenos Aires was recognized by several workers.1 2 3 4 5
The Argentine Secretary of Energy put four blocks in the Claromeco basin out for bids and awarded two to the Argentinian Bridas Co. Bridas committed to the acquisition of 200 km of seismic on each block.
Geology, stratigraphy
On the basis of negative anomalies, the Claromeco basin occupies an area of about 40,000 sq km and extends offshore beneath the Cretaceous Colorado basin (Fig. 1 (47621 bytes)).The Claromeco basin is located between a fold belt to the west (Ventana hills) and a foreland to the east (Tandilia hills). The basin axis, less than minus 30 milligals, has a northwest-southeast direction with its widest part extending oceanwards. The basin has a maximum thickness of 9 km in the basin axis,5 and the estimated stratigraphic sequence consists of Proterozoic, Paleozoic, and a thin cover of Cenozoic sediments.
The postulated subsurface stratigraphy is based on the rocks exposed in the Ventana and Tandilia hills (Figs. 2 (46927 bytes), 3 (35930 bytes)). In the Ventana hills, Precambrian metamorphic, granitic, and rhyolitic rocks underlay three cycles or groups of Paleozoic siliciclastics (Fig. 3 (35930 bytes)).
The lower cycle corresponds to the Cambro-Ordovician Curamalal group and consists of a basal conglomerate followed by quartzites. The cycle is unconformably overlain by the Ventana group, which is composed of coarse grained quartzites and lithic sandstones at the top. The ages vary between Ordovician and Devonian. At present it is unknown if the Pillahuinco group rests conformably over the Ventana group.
The Pillahuinco group commences with the Sauce Grande formation, which represents the Permo-Carboniferous glaciation in South America. The diamictites thickness varies between 500-250 m. The rest of the overlying formations-Piedra Azul, Bonete, and Tunas-contain typical Gondwana faunas and floras. The Upper Paleozoic sequence is connected to the north with the Chaco-Parana basin.
In the Tandilia hills, the surface stratigraphy is made up of two major units: an igneous-metamorphic basement (2 billion years before present) that is unconformably overlain by a sedimentary cover. The basement is known as Complejo Buenos Aires or Punta Mogotes (subsurface).
The sedimentary sequence consists of the Precambrian clastic-carbonatic Sierras Bayas formation and the Cambro-Ordovician quartzitic and conglomeratic Balcarce formation. Both units correspond to the La Tinta group (Fig. 3 (35930 bytes)). At least two structural interpretations are postulated:
- The Ventana range is a fold and thrust belt derived from the collision of the Patagonia and Gondwana plates1 which generated the development of the Claromeco foredeep or basin (Fig. 4 (59405 bytes)).
- The Ventana range corresponds to a folded structure6 that only one Late Devonian-Early Pennsylvanian deformation affected as the Permian sediments are only weakly deformed and illites and chlorites are syntectonically recrystallized.7 The same author postulates that the age of the strata and metamorphism increases from east to west.
- A gravity profile shows that the basin is developed between the Los Gauchos and Cristiano Muerto creeks.
Petroleum geology
The only geophysical data available are the gravity profiles published by Kostadinoff.2 5 The profiles are located along the coast and in the vicinities of the Tandilia hills.Traps
As was mentioned before, the intensity of the deformation in the Ventana hills decreases eastwards; it is then possible that gentle structures capable of trapping fluids be present in the subsurface of the Claromeco basin.In addition, in the Tandilia hills the movements of the Brazilian cycle (600 million years before present) faulted the basement and the Precambrian sedimentary cover and an important horizontal and vertical displacement resulted in a series of highs and lows.
This basement block faulting causes anticlinal structures in the post-Precambrian sediments and hence traps of this type may be found in the subsurface of western Tandilia.
Reservoir, seals
The Pillahuinco group, although highly indurated at outcrop in the Ventana hills, may have leached during uplift. The degree of metamorphism decreases and reservoir conditions improve towards the east-northeast.The carbonatic and clastic sediments cropping out in the Tandilia hills are likely to be found in the subsurface. However, no samples have been collected and therefore no reservoir analysis carried out.
Shale intervals in the Upper Paleozoic column in the Ventana hills and shale and marl intercalations in the Precambrian sediments of the Tandilia hills are the potential seal rocks.
Source rocks
The Upper Paleozoic shaly intervals of the Piedra Azul and Tunas formations are likely to occur in the Claromeco basin. Its offshore extension has been penetrated in Yacimientos Petroliferos Fiscales's Puelches well.In the Ventana outcrops, the TOC values are 1% or less, but in the Puelches well the Permian shales at below 2,600 m are in the dry gas window with TOC values between 1-1.5% and extractable bitumen remaining.
The shales and limestones of the Tandilia hills contain intervals with source rock characteristics.
In the Pampean ranges of San Luis, Prozzi and Rosso8 indicated the occurrence of carbonaceous slates with TOC values not exceeding 1%. In addition they suggest that the slates were source rocks during Precambrian-Lower Cambrian before attaining metamorphism in the Paleozoic. Following the margin of the continental areas, these units may be found in the Tandilia hills, where rocks of the same age are cropping out. It is therefore possible that the slates, which in San Luis are intercalated with turbidites, may be present in the subsurface of the eastern portion of the Claromeco basin (Fig. 5 (43034 bytes)).
The geothermal gradient varies between 5 C./100 m in the Ventana and Tandilia hills and 3 C./100 m in the offshore extension of the Claromeco basin. In the Puelches well, the geothermal gradient is higher than 3 C./100 m but is surrounded by a cooler zone (Fig. 6 (29230 bytes)). The higher geothermal gradient is probably due to volcanic rocks intercalated in the sedimentary section.
Conclusions
- With the data available, it is possible to establish the existence of a sedimentary basin between the Ventana and Tandilia hills.
- Geochemical analysis of the Puelches well core has demonstrated that the Permian shale is a potential source rock between 2,417-3,557 m depth. In addition, in the Tandilia hills, limestone and shale intervals appear to have source rock characteristics.
- Reservoir and gentle traps are likely to occur to the east of the Ventana hills.
- Although exploration in the basin would be high risk, the following recommendations for a reconnaissance program can be suggested:
The minimum program should comprise field studies and sampling, both in the Tandilia and Ventana outcrops. The program will document the thermal maturity, organic content, source, and reservoir potential of the rocks.
If the results are encouraging, a joint gravity and magnetic survey will be necessary and, depending on budget, the acquisition of seismic lines.
The seismic lines could be located between Pigue-Olavarria or Pillahuinco-Loma Negra, Coronel Dorrego-Chillan, and northwest-southeast lines between Coronel Suarez-Tres Arroyos-Claromeco and/or General Lamadrid-Juarez (Fig. 7 (29230 bytes)).
