TECTONICS AND SEDIMENTATION
OF THE CHACO BASIN, PARAGUAY.
ITS HYDROCARBON POTENTIAL.
The Chaco basin occupies 246.725 km² in western Paraguay. The evolution and distribution of
the stratigraphic sequences and the tectonic-structural style of the Chaco basin are based on
outcrop observations in the northern Chaco, and hydrocarbon and groundwater exploration
The Curupaity-Carandaity subbasins represent well developed Paleozoic subsidence areas in
the northwestern Chaco. Thick Mesozoic sediments are predominantely deposited in the
Pirity-Pilar subbasins to the south and in the shallow Bahia Negra platform and San Pedro low
to the east; and are separated by structural highs and uplifts. Uppermost Proterozoic to recent
sediments are present in the Chaco basin; the crystalline basement is not reached.
The emplacement on a heterogenous continental crust and the sedimentary distribution and
subsidence history of the Chaco basin (four superimposed phases) is controled by northwest
and northeast-oriented structural lineaments originated during the eocambrian compressional
Mesozoic South Atlantic cycle rift tectonics, predominantely oriented towards northeast
reorganized the structural pattern of the Chaco basin as asymmetric graben systems
developed and Paleozoic subsidence zones stabilized.
The Cenozoic Andean cycle uplift registers some structural readjustment features in the area.
The Chaco becomes established as a modern foreland basin.
Paleozoic subsidence in the northwestern Curupaity-Carandaity subbasins accommodated
uppermost Proterozoic to Lower Permian marine shales, sandstones and carbonates, with
increasing continental influence upwards.
Upper Jurassic to Lower Eocene continental clastic sediments accompanied by local
magmatites accumulated in Cretaceous rift basins (Pirity-Pilar subbasins) and interrupted by
local marine incursions transgress Paleozoic subbasins and highs.
Middle Eocene to recent continental sediments, with local Miocene marine deposits in
southeastern areas, represent the youngest stratigraphic units in the Chaco basin.
Targets for hydrocarbon exploration depend on the presence of source rock in marine Upper
Devonian shales and Upper Cretaceous shales and carbonates. Main potential areas include
Carboniferous channel sandstones in mature portions of the Curupaity-Carandaity subbasins
and adjacent flanks. Combined structural-stratigraphic traps in Mesozoic sandstone reservoirs
within the Pirity subbasin may form local hydrocarbon accumulations.
Normal faulted Upper Devonian marine shales are unconformably overlain by Mesozoic
sandstone reservoirs on the southeastern flank of the Boquerón high that represent a viable
La Cuenca del Chaco ocupa la región occidental de la República del Paraguay (246.725 km²).
La evolución y distribución de la secuencia estratigráfica y el estilo tectónico-estructural de la
Cuenca del Chaco se basa principalmente en datos de actividades exploratorias para
hidrocarburos y aguas subterráneas, y en menor escala en observaciones directas de
unidades aflorantes en el norte del Chaco.
Las subcuencas de Curupaity y Carandaity representan áreas paleozoicas de subsidencia
bien desarrolladas al noroeste del Chaco; potentes sedimentos mesozoicos se depositaron
predominantemente en las subcuencas de Pirity y Pilar al sur, como tambien en la plataforma
de Bahía Negra y el bajo de San Pedro al este; estando separados por altos estructurales.
Sedimentos desde el Proterozoico superior al reciente son observados en la Cuenca del
Chaco; el basamento cristalino no fue identificado.
El emplazamiento sobre una corteza continental heterogénea, la distribución sedimentaria y la
historia de subsidencia (cuatro fases sobrepuestas) de la Cuenca del Chaco son controlados
por lineamientos estructurales orientados al noroeste y noreste, originados por el
compresional Ciclo Brasiliano en el Eocámbrico.
Una tectónica distensional mesozoica del Ciclo Sudatlántico, con orientaciones predominantes
al noroeste reorganiza la disposición estructural de la Cuenca del Chaco abriendo sistemas de
subcuencas asimétricas (grabens) y estabilizando áreas paleozoicas de subsidencia.
El compresional Ciclo Andino cenozóico registra algunos reajustes estructurales en el area. El
Chaco se transforma en una planicie promontoria moderna.
Cotas importantes de subsidencia paleozoica se registran en las subcuencas noroccidentales
de Curupaity y Carandaity, depositándose lutitas, areniscas y carbonatos desde el
Proterozoico superior al Pérmico inferior, con influencia continental aumentando hacia arriba.
Sedimentos clásticos continentales, acompañados por incursiones marinas locales y
magmatitas aisladas del Jurásico superior al Eoceno medio rellenan subcuencas mesozoicas
de orígen distensional (subcuencas de Pirity y Pilar), recubriendo subcuencas y altos
Rellenos sedimentarios del Eoceno medio al reciente, con participación marina local al
sureste, representan las secuencias estratigráficas más modernas de la Cuenca del Chaco.
Areas potenciales para la exploración de hidrocarburos dependen de la presencia de rocas
generadoras de orígen marino en lutitas del Devónico superior y lutitas-carbonatos del
Cretácico superior. Areas de mayor potencial de reservorio incluyen areniscas de paleocauces
carboníferos en niveles maduros en el interior y los flancos de las subcuencas de Curupaity y
Carandaity. Reservorios combinados en trampas estratigráficas-estructurales dentro de
areniscas mesozoicas de la subcuenca de Pirity indicarían posibilidades para acumulaciones
locales de hidrocarburos.
Bloques fallados de lutitas marinas del Devónico superior, recubiertos discordantemente por
reservorios de areniscas mesozoicas en el flanco suroriental del alto de Boquerón representan
areas viables para la exploracion de hidrocarburos.
The Paraguayan Chaco basin constitutes the western part of the Republic of Paraguay and
comprises 246.725 km² (60.8% of the national territory; Figure 1 ). The Chaco is an extensive
Quaternary plain, with a gentle inclination of 3
to the southeast and an average elevation of
160 meters above sea level. Outcrops of Paleozoic and Mesozoic stratigraphic sequences are
found in the northern Chaco and along the Paraguay river (Figure 2); topographic highs may
reach altitudes up to 450 meters.
The Paraguayan Chaco borders in the north and west to Bolivia; in the south along the
Pilcomayo river to Argentina; and in the east along the Paraguay river to eastern Paraguay
The Chaco basin represents a modern foreland basin of the Andean ranges, located between
the Andean belt to the west, the Brazilian shield to the northeast, the Paraná basin to the east
and the Pampa basins to the south.
The present knowledge of the stratigraphic and tectonic evolution is primarily the result of
hydrocarbon and groundwater exploration activities (Wiens, 1989; PNUD, 1978). Several
hundred shallow to deep water exploration wells were drilled throughout the Chaco.
Hydrocarbon exploration activities from 1947 to 1993 include a total of forty exploration wells
(Table 1), 11.500 kilometers of seismic lines, and a regional coverage by aeromagnetic
surveys in the southwestern, western, and northern Chaco. Promising exploration targets are
identified, but not commercial hydrocarbon deposits have yet been discovered.
Four subbasins are located in the Paraguayan Chaco (Figure 3): the Curupaity subbasin in the
north (three hydrocarbon exploration wells) and the Carandaity subbasin in the west (twenty-
seven hydrocarbon exploration wells) constitute Paleozoic basins; the Pirity subbasin in the
southwest (eight hydrocarbon exploration wells) and the Pilar subbasin to the south (no wells)
comprise Mesozoic rift basins. The Paleozoic-Mesozoic San Pedro low in the east (extending
westward from the Paraná basin; no wells on the Chaco side) and the Paleozoic Bahía Negra
platform in the northeast (no wells) remain highly interpretative.
The subbasins and lows are separated by arching highs and uplifts (Figure 3): the central
Chaco uplift (1 hydrocarbon exploration well) merging into the Fuerte Olimpo high , the
Lagerenza high and the Boquerón high ; the Presidente Hayes uplift (1 hydrocarbon
exploration well); and the western margin of the outcropping Rio Apa subcraton .
The tectonic style of the Chaco basin is characterized primarily by northwest-southeast and
northeast-southwest-oriented structural lineaments of eocambrian orogenetic compressional
origin (Brasiliano cycle). Episodic reactivation throughout Phanerozoic time superimposed four
distinct subsidence phases (Early Paleozoic, Late Paleozoic, Late Mesozoic and Cenozoic;
Figures 4 and 5), separated by erosional unconformities, and periods of non deposition or low
sedimentation rates. While the Paleozoic phases reflect quiet subsidence with local structural
readjustments , the Mesozoic phase indicates a general reorganization of the structural style
by major rift tectonics along predominant northeast oriented lineaments (South Atlantic cycle).
The Cenozoic phase is caused by the Andean uplift (Andean cycle) with some regional
The Phanerozoic Chaco basin can be characterized stratigraphically by three major geological
events (Figures 4 and 5):
1. Clastic and carbonate Paleozoic sedimentation from Uppermost Proterozoic to Early
Permian in marine to continental environments in a continental platform setting and subsiding
2. Clastic to carbonate sedimentation from Late Jurassic / Early Cretaceous to Middle Eocene
of thick continental deposits within rift-originated subbasins with local marine incursions
transgressing Paleozoic subbasins and highs.
3. Clastic to evaporitic sedimentation with local marine deposits throughout the Chaco basin in
a modern foreland environment from Middle Eocene to Quaternary.
TECTONIC EVOLUTION OF THE PARAGUAYAN CHACO
The tectonic history of the Phanerozoic Chaco basin started during the very intense
thermotectonic Brasiliano cycle (680-450 Ma), depositing carbonate-clastic transgressive
sequences of the Eocambrian Itapucumí Group that formed in response to "pulsing"
compressional-extensional tectonics (Zalan, 1987). Compressional tectonics with thrusting
features caused through granitisation the emplacement of important acid magmatites (outcrops
in southeastern and northeastern Paraguay; 680-580 Ma; Cordani, 1984; Wiens, 1986).
Stabilization and cooling took place from 500-450 Ma and probably represent the mechanism
of initial basin subsidence that created the depression of the Chaco and Paraná basins (473
Ma; Zalan, 1987).
The tectonic result of the transcurrent Brasiliano cycle is reflected by the rupture of the
Precambrian-Eocambrian basement, and fragmentation into tectonic blocks that are oriented
primarily towards northwest-southeast and northeast-southwest , as opposed to predominant
north-south and east-west basement structural trends.
The complex block pattern can be observed today as in crystalline highs (Río Apa and Río
Tebicuary subcratons); and in platforms (the carbonate Itapucumí Group along the
northeastern Chaco and the central Chaco uplift), as well as in subsiding areas (Carandaity
and Curupaity subbasins; western and northern Chaco respectively), that form the first
evidence of differential Phanerozoic subsidence (Figures 5, 6, 18 and 19).
The Chaco basin is characterized by four distinct superimposed subsidence phases
(Ordovician-Devonian, Carboniferous-Permian, Late Jurassic / Early Cretaceous-Middle
Eocene, and Middle Eocene-Quaternary). These subsidence phases are separated by periods
of erosion, nondeposition or low sedimentation rates.
During most of the Paleozoic the Paraguayan Chaco basin was part of a tectonically relative
stable area with shallow-marine to continental environments, that extended to the south of the
Brazilian shield (Figures 1, 5 and 6). The Paleozoic sedimentary sequence in the Paraguayan
Chaco basin comprises clastic sediments and subordinated carbonates.
1st phase: Ordovician-Devonian
An almost complete Ordovician sequence in the Don Quixote-1 well (Cerro León Group,
Carandaity subbasin) and middle to Upper Ordovician sediments in the Asunción-1 well
(Caacupé Group, San Pedro low), as opposed to an incomplete or non existent Ordovician
sequence in other wells and outcrops, suggest differential subsidence and minor erosional
Outcrop observations in southeastern Paraguay and the northern Paraguayan Chaco suggest
an almost continuous sedimentation from Ordovician through Devonian times, as the result of
a mayor fluctuating transgression-regression cycle. A large gulf along a passive continental
margin extended from a protopacific ocean to the west covering the general area of the
Chaco and Paraná basins.
The regional Lower Silurian unconformity in the Chaco basin (Zapla tillites; Russo et al., 1979)
has so far not been observed in the Paraguayan Chaco, but could be present in the main
Paleozoic basins (Curupaity and Carandaity subbasins).
Sea-level changes and local Devonian transgressive pulses (San Alfredo Group) are restricted
to the western and northern Chaco, and are also observed in the Paraná basin (San Pedro
low). Continental "Devonian red beds" in the López-1 well (Harris, 1959) have not been
In conclusion, a large sedimentary platform established in the central, eastern and southern
Chaco since the Ordovician and continued until the Mesozoic.
Continuous subsidence is only observed in the Curupaity and Carandaity subbasins (northern
and western Chaco), the San Pedro low and the main Paraná basin (eastern Paraguay).
2nd phase: Carboniferous-Permian
A pronounced angular unconformity and erosional surface is present at the Devonian-
Carboniferous contact in widespread areas of the northwestern Paraguayan Chaco. A differen-
tial readjustment of structural basin blocks causes the incipient uplift of the Boquerón, Fuerte
Olimpo and Lagerenza highs, resulting in unconformities between voluminous Upper
Carboniferous continental glacial sediments and local Lower Permian shallow-marine
sequences (Palmar de las Islas Group) that rest on Devonian shales (Lobo et al., 1976).
Only in the deeper parts of the western Carandaity subbasin a complete section from
Devonian to Upper Carboniferous and Lower Permian sediments is preserved. In the Curupaity
subbasin an erosional unconformity marks the contact between Carboniferous sequences and
The western and northern areas of the Paraguayan Chaco started to subside separately in the
Late Paleozoic. In contrast, the eastern, central, and southern regions remained stable, and
were locally influenced by erosion or the subsidence effects of the Paraná basin (San Pedro
During the Paleozoic, sedimentation in the Chaco basin was controlled by northwest and
northeast Eocambrian block-type structural trends. Even if vertical and horizontal movements
during this time only occured on a small scale, they had sufficient magnitude to guide the
location of depocenters and intrabasinal highs, as well as the distribution of sedimentary facies
and the migration or inversion of morphological features.
3rd phase: Late Jurassic/Early Cretaceous-Middle Eocene
Sedimentation reinitiated during Triassic as fluvial-eolian deposits, filling morphological
depressions (Bianucci et al., 1981). The environment of deposition changed later to desert
conditions, mainly in the northern Chaco areas, covering all basins, highs and platforms (lower
Adrian Jara Formation).
A major Mesozoic extensional tectonic event (South Atlantic cycle, 230-65 Ma) corresponds to
the deposition of the main Adrian Jara Formation in the Curupaity subbasin, and the Berta,
Palo Santo and Santa Barbara formations in the Pirity subbasin, as well as to poorly identified
red beds in the central, and southeastern Paraguayan Chaco. The tectonic style reflects a
well-developed rift system that envolved from Early Cretaceous to Middle Eocene (Figure 6).
Mesozoic rift tectonics marked a structural reorganization and the change in sedimentary
history along the tectonic pattern in the Paraguayan Chaco. The general configurations of the
existing basins and highs are modified substantially and new tectonic units develop: the Car-
andaity and Curupaity subbasins become more stable, indicating minor sedimentation rates;
arching of existing highs is observed and new depositional centers are formed.
Three new northeast-southwest-oriented subbasins become active through Mesozoic
extensional tectonics: the Pirity and Pilar subbasins, and the Bahía Negra platform.
Continental sedimentation is predominant. In the Pirity subbasin there is a short marine
transgression from the southwest during the Late Cretaceous. Local basic to alkaline
magmatism is characteristic (135-108 Ma and 70
The Mesozoic rift phase is considered to be the most important period in the formation of
"final" structures and in the maturation of organic material for hydrocarbon generation
(Paleozoic plays and local Mesozoic synformational environments; Zalan, 1987). The final
configuration of subbasins, structural highs and platforms in the Chaco basin is in place at the
end of the thermotectonic South Atlantic cycle.
Best explored is the evolution of the Pirity subbasin: an initial thermal arching of the Paleozoic
platform in the central and north-and southeastern Chaco exposed and eroded presumably
some of the older sediments. Intense en echelon faulting in northeast-southwest and north-
northeast-south-southwest directions caused differential vertical and horizontal movements
that generated a tectonically controlled asymmetric graben structure, limited by Paleozoic
shoulders. Local basic magmatic activity accompanied the event.
The northwest flank of the subbasin exhibits increased faulting and younger Paleozoic
sediments (Devonian to Carboniferous), while the southeast flank was less affected by faulting
(flexures) and is composed of Ordovician sediments (coinciding with the Paleozoic subsidence
history). Intense minor block faulting throughout the subbasin accounts for numerous small-
scale structures in onlapping sediments (subtile arches and flexures) that form structural traps
for possible hydrocarbon accumulations.
The sedimentary infill is mostly continental with periodic marine influence from the southwest
that extended to the Acosta-1 well (Palo Santo Formation). Predominant sediments are alluvial
fan, fluvial, and lacustrine deposits, with minor eolian sequences. The marine influence is
characterized by carbonates, clastics, and evaporites.
The strongly heterogeneous continental sedimentation with marine and magmatic influence
(maximum thickness about 4000 meters), caused local unconformities and lateral facies
changes that form stratigraphic traps and seals.
Toward the northeast (Filadelfia area) the Pirity subbasin looses its expression and recorded
only minor sedimentation. The structural control persisted and reflect a rift propagation into the
Bahía Negra platform and the forming Pantanal basin.
4th phase: Middle Eocene-Quaternary
The uplift of the Andean ranges to the west of the Chaco basin originated from compressional
tectonics (Andean cycle, 50-35 Ma) generated an important source for sediments, and
excluded any possible marine influence from the west.
Effects of structural and magmatic reactivation are recorded in areas of inverted tectonics at
the Asunción and San Ignacio blocks around the exposed crystalline basement in
southeastern Paraguay as the uplift of aulacogene rift segments and local nephelinitic
magmatism (49-40 Ma).
Subbasins and highs in the Chaco basin, that were established during the Mesozoic, became
covered by thick and extensive continental sediments (Chaco Formation). Minor readjustment
structures within Mesozoic sediments could have been caused by Tertiary tectonics
accompanied by local basic-alkaline magmatism.
Structural control along major tectonic lineaments remains notably effective until today: soil
type and drainage distribution in the Bahia Negra area toward the northeast, and weak
earthquakes in the Asunción area toward the southeast. Neotectonic features control the
sedimentary response that is observed within recent formations (recent undifferentiated
deposits), and the Chaco becomes a wide Quaternary plain (Figures 2 and 5).
PHANEROZOIC STRATIGRAPHY OF THE PARAGUAYAN CHACO
The Phanerozoic sedimentary section in the Paraguayan Chaco ranges from Eocambrian to
recent deposits (Figure 4). Crucial to the understanding of the Phanerozoic history are the
outcrops in the northern Chaco (Gómez, 1986), along the Paraguay river and
hydrocarbon/groundwater exploration efforts. Outcrop and well data from eastern Bolivia
(Tucavaca, Roboré, and Santiago de Chiquitos), the Subandean belt in Bolivia and Argentina,
as well as hydrocarbon exploration data from the Argentinian Chaco and abundant geological
data from eastern Paraguay, further contribute to the understanding of the Phanerozoic history
of the Paraguayan Chaco (Figures 4, 6 and 7).
Little is known about the Precambian basement in the Chaco and correlations are suggested
from the Río Apa / Río Tebicuary subcratons (eastern Paraguay) or the Brazilian shield
(eastern Bolivia). No exploration well reached the crystalline basement in the Chaco basin.
Paleozoic Itapucumí Group (Latest Proterozoic to Cambrian)
Outcrops occur along the Paraguay river in the area of Puerto La Victoria. Eocambrian rocks in
the main Chaco basin are likely to be present at greater dephts than drilled so far. The
eocambrian sediments are correlative to the Corumbá Group (Mato Grosso do Sul, Brazil) and
the Tucavaca Group (eastern Bolivia; Figure 8; Wiens, 1986).
The Itapucumí Group might be subdivided into a clastic basal trangressive unit (mudstones,
sandstones, and conglomeratic-arkosic sandstones; 2-25 meters thick) and a major calcareous
sequence (Wiens, 1990). Carbonates consist of bituminous, laminated limestones, alternated
with abundant oolitic and conglomeratic layers that are separated by argillaceous-pyritic
shales. Magmatic and tectonic events caused widespread recrystallization and dolomitization,
reaching marble and chert grades.
The sediments were deposited on a transgressive continental platform, in shallow and warm
marine environments. A thickness of 250-400 meters is reported (Wiens, 1986).
Biostratigraphic dating of algal remnants (Aulaphycus lucianoi n.sp. and Collenia sp.; Beurlen
and Sommer, 1957; Correa et al., 1979) suggests a Cambrian age, while Scyphozoae
remnants (Corumbella werneri n.sp. and Cloudina waldei n.sp.; Hahn et al., 1982; Hahn and
Pflug, 1985) seem to reflect Latest Proterozoic to Cambrian ages (Aceñolaza et al., 1989).
Cerro León Group (Early Ordovician to Silurian)
Outcrops of this group can be observed in the Cerro León area (northern Chaco; Wiens,
1991), as well as in the Cordillera de los Altos area (eastern Paraguay). Only one exploration
well in the Carandaity subbasin penetrated Lower Ordovician sediments (Parapití-1 well;
Figures 9 and 10).
Clebsch (1991) intended a stratigraphic subdivision into three formations based on the
Parapiti-1 well. Wiens (1990) proposed two formations based on the La Paz-1 well, including
Lowermost Devonian quartzitic sandstones. Because of the very scant palynological and
paleontological information, an overall picture of the section is missing.
Tentative correlations suggest that the Cerro León Group corresponds to the Kirusillas and El
Carmen formations in eastern Bolivia, the San Benito to Tarabuco formations in the
Subandean belt, the Pirané Formation in the Argentinian Chaco, and to the Caacupé and
Itacurubí groups in eastern Paraguay.
The stratigraphic sequence reflects a transgression-regression cycle on a passive continental
margin from a protopacific sea to the west, that covered the complete area of the Paraguayan
Chaco basin and the Paraná basin. Therefore, while the deepest Lower Ordovician layers in
the Chaco basin reflect dark marine shales and siltstones with Lingula fragments (Vistalli,
1989) , in the Cordillera de los Altos (eastern Paraguay) a transgressive conglomeratic
sandstone unit with common Skolithus is correlative. Although, a basal conglomeratic to sandy
sequence is expected to underlie the shales in the Chaco basin, but this has not been reported
The Ordovician shales (La Paz Formation, Wiens, 1990; Don Quixote Formation, Clebsch,
1991) in the northwestern Chaco are black, pyritic to limonitic, fissile, very micaceous, and
interbedded with siltstones and sandstones. A calm marine environment is suggested.
Penetrated thickness reached 172 meters (Don Quixote-1 well).
In a transitional contact medium grained gray siltstones with shale intercalations have been
reported (upper La Paz Formation, Wiens, 1990; Siracuas Formation, Clebsch, 1991) that
contain abundant mica and some pyrite. The depositional environment remains marine
lagoonal, starting whit a regressive cycle. Drilled thickness reached 356 m (Don Quixote-1
The initiated marine regression continued throughout the Silurian (uppermost La Paz
Formation and Santa Rosa Formation, Wiens 1990; Nueva Asunción Formation, Clebsch,
1991). Massive sandstones become predominant with dark shaly interbeds. The sandstones
are gray to whitish, fine grained, somewhat micaceous and strongly recrystallized, while shaly
levels are characterized by high pyrite and some Crinoid stem contents (Pennzoil, 1972).
Wolfart (1961) reports Arthrophycus, Brachiopods, and Gastropods from quartzites, fine
grained sandstones and gray mudstones in upper levels of the Cerro León massif
(Llandoverian). Penetrated thicknesses vary from 135 meters (Don Quixote-1 well) to 335
meters (Parapití-1 well).
The Ordovician-Silurian Cerro León Group connects through the Paraguayan Chaco eastward
(López-1 and Orihuela-1 wells) into the San Pedro low (Asunción-1 and Asunción-2 wells;
Milani and Daemon, 1992; Wood and Miller, 1991), and toward the main Paraná basin. The
isopach map of the lower Paleozoic sequence and stratigraphic correlations confirm a regional
marine influence throughout the Chaco and Paraná basins in a general subsiding environment
with exposed Precambrian-Eocambrian basement in the Río Apa and Río Tebicuary
San Alfredo Group (Early to Late Devonian)
Extensive Devonian outcrops can be observed in the Cordillera de San Alfredo and at the
western flanks of the Cerro León massif (Lagerenza high). Almost all exploration wells in the
western and northern Chaco (Carandaity and Curupaity subbasins) penetrated Devonian
sections, as well as groundwater surveys throughout the northern Chaco (PNUD, 1991;
Figures 11, 12 and 14).
The San Alfredo Group correlates with the Santa Rosa and Roboré formations and including
the Los Monos and Iquiri formations in eastern Bolivia (López et al., 1982) and Argentina. It
corresponds to the Furnas and Ponta Grossa formations in the Brazilian Paraná basin and to
identified Devonian sections (unnamed so far) in the Asunción-1 and Asunción-2 wells (San
Pedro low, eastern Paraguay; Milani and Daemon, 1992).
A generally transgressive sea connects the existing Chaco and Paraná basin through the San
Pedro low. If this connection although recovers the central Chaco uplift (e.g., as continental
sedimentation; López-1 and Orihuela-1wells) is not confirmed satisfactorally.
Thickest Devonian systems developed in the Curupaity subbasin (2000 meters) and the
Carandaity subbasin (3500 meters); Figure 12. The section can be subdivided in a lower sandy
formation and an upper monotonous shaly unit, which grades upwards into sandier levels.
The lower San Alfredo Group (Santa Rosa Formation; Fernández Garrasino and Cerdan,
1981; Wiens 1990; Clebsch, 1991) indicates coastal to very shallow marine environments.
White to gray quartzites and sandstones, with intercalated siltstones and shales, sometimes
very micaceous or dolomitic / hematitic and strong crossbedded to massive bedded,
characterize the heterogeneous sedimentary facies (0-300 meters). The presence of
Leiospheres and Chitinozoans in exploration wells in the northern and western Chaco
(Pennzoil, 1972) are indicative of marine environments. Fossil casts with Corals, Bryozoans
and Crinoids at the Cerro León massif support a marine environment. Wolfart (1961)
characterized Favosites sp., Leptocoelia flabellites, Chonetes falklandicus, and Tentaculites
stubeli as Early Devonian indicators at Fortin Aroma and the Lagerenza high. Palynological
information of the central Chaco uplift (López-1 well) are interpretative; spores at the
Lagerenza high (Wiens, 1990) support the strong changing lateral sedimentary facies
(basinward becoming more marine and shaly ; toward the central Chaco uplift sandier coastal
Postdepositional transformation into tight quartzites and shales, and lateral sedimentary
variations result in variable porosities and synformational unconformities (important
hydrocarbon reservoir factors).
The upper San Alfredo Group (Los Monos and Iquiri formations or Limoncito Group; López,
1982) is represented by thick homogeneous, fossiliferous, dark gray to black, shallow-marine
shales that show an upward-transitional regressive sequence from marine to continental
depositional conditions. The extent of the shaly upper San Alfredo Group is restricted to the
Carandaity and Curupaity subbasins, with the easternmost record in the La Paz-1 well (0-3200
The thick shales developed (distal conditions) and the uniform paleontological content confirm
an active shallow depocenter with matching sediment supply in the western Paraguayan
Chaco (Carandaity subbasin) during the Middle to Late Devonian. Reported Tentaculites,
Brachiopods and Crinoids (Harrington, 1956) indicate shallow-marine facies.
Postdepositional tectonic and thermal influences affected the Devonian shales from almost
immature conditions (Carandaity and Curupaity subbasins; upper units) to incipient
metamorphic features (Lagerenza and Fuerte Olimpo highs) that are important hydrocarbon
source parameters in exploration targets.
The uppermost regressive heterogeneous sediments (Iquiri Formation; López, 1982; thickness
0-300 meters) with similar fossiliferous records indicate a pronounced discordance with Lower
Carboniferous coastal to continental sequences. The sediments correspond to similar
depositional conditions as the lower section of the San Alfredo Group (lateral facies changes,
large variations in lateral continuity, continental influence etc.), representing the conclusion of
the Devonian transgression-regression cycle in the Paraguayan Chaco basin.
Palmar de las Islas Group (Early Carboniferous to Early Permian)
Important Carboniferous outcrops are reported in the northern Chaco on the western
Lagerenza high and in the area from Palmar de las Islas to Adrian Jara. The favorable
hydrocarbon and groundwater reservoir conditions of the Carboniferous clastics resulted in
numerous exploration wells throughout the northwestern Chaco (Carandaity and Curupaity
subbasins and adjacent highs), distributed along the Bolivian border (Figures 12, 13 and 14).
Correlative rocks are sections from the Saipurú to the San Telmo formations in Bolivia (López,
1982; Sanjines, 1982), the Coronel Oviedo Group in eastern Paraguay, and the Itararé Group
in the Brazilian Paraná basin.
Sedimentary conditions are changing from shallow-coastal marine to continental environments
with glacial influence. A heterogeneous sedimentary section of up to 1600 meters thick
developed on a well-defined erosional unconformity on top of the underlying Devonian San
The Lower Carboniferous subsystem is referred to as the San José Formation (Saipurú to
Itacuami formations; López, 1982): rapid lateral changing sedimentary facies with only local
continuity are characteristic. Basal whitish to gray sandstones with dark shaly lenses become
greenish argillaceous to varvic upward, including reddish diamictitic to tillitic horizons. While
coastal marine influence is observed in the basal section, continental glacial environments are
interpreted for the upper levels. Local unconformities and the varying thickness (0-800 meters;
Carandaity subbasin) indicate heterogeneous depositional conditions.
The Upper Carboniferous subsystem (Figure 14) to Lower Permian sequence includes the
Cabrera Formation (Tarija to San Telmo formations; López, 1982): in a transitional contact with
the underlying San José Formation a sandstone sequence is recorded. Local basal
conglomerates relate to local unconformities. The main sandstones are whitish, medium
grained, friable, and massive. Fluvial crossbedding is noticed upward with increasing shaly and
silty levels. The fair hydrocarbon reservoir conditions are indicative, considering as well lateral
porosity and permeability variations (traps). The Cabrera Formation sediments are best
developed in the Curupaity subbasin (0-1300 meters thickness).
The age relations of the Palmar de las Islas Group is generally lithostratigraphic, as only few
and scattered fossils have been reported from lower Upper Carboniferous subsystem red beds
Mesozoic-Cenozoic Adrián Jara Formation (Late Jurassic to Middle Eocene)
Friable, poorly sorted sandstones with intercalated conglomeratic and argillaceous levels are
deposited in the Curupaity subbasin (northern Chaco; Figures 2 and 15). Similar outcrops of
the Adrián Jara Formation are observed as remnants in the northwest corner of the
Paraguayan Chaco (top of the Cerro Cabrera and the Cordillera Guaraní). Exploration wells
confirmed the unit in the northern Chaco; undefined red beds in the central and southeastern
Chaco could be correlative.
The section correlates with the Cajones up to the Tacurú formations in eastern Bolivia and
shows similarities to the Misiones, Botucatú, Rivera, Tacuarembó formations in the Paraná
basin. Maximum thickness reaches 400-600 meters.
The formation consists of reddish to yellowish sandstones, medium to fine grained, moderately
sorted, parallel massive bedded to pronounced crossbedded, with ferrugineous and carbonatic
cement. Porosity is good. Intercalated are heterogeneous conglomeratic levels with washout
pebbles from underlying units. Interbedded mudstones are multicolored, soft, and impure. The
sedimentary pattern indicates a continental fluvial to eolian environment.
The Adrián Jara Formation overlies the Carboniferous Cabrera Formation discordantely and is
capped by an unconformity at the base of the Cenozoic Chaco Formation. Isolated microfloral
and pollen in the lower section (Toro-1 well), indicating Triassic to Late Permian ages, seem to
represent reworked material. Considering the regional sedimentation characteristics, the
deposition of the Adrián Jara Formation could have been started as early as Late
Permian/Early Triassic, but definitely since Late Jurassic (Mesozoic structural reorganization).
Pirity Subbasin Stratigraphy (Late Jurassic to Middle Eocene)
Mesozoic extensional tectonics (South Atlantic cycle, 230-65 Ma) marked a reorganization of
tectonic elements in the Paraguayan Chaco basin. Paleozoic basins (Curupaity and
Carandaity subbasins) become more stable, while Mesozoic extension establishes the Pirity
and Pilar subbasins, and structurally controls the Bahia Negra platform (Figures 15 , 16 and
Most information is available from hydrocarbon exploration efforts in the Pirity subbasin
(southwestern Chaco). No outcrops of the stratigraphic section exist on Paraguayan territory.
Mesozoic rift tectonics formed the asymmetric graben system of the Pirity subbasin with
pronounced normal faults on the northern flank (Boqueron high) and a structural flexure on the
southern flank (Presidente Hayes uplift).
Lithological correlations, paleontological information, and absolute age determinations on
contemporaneous magmatites, permit a stratigraphic division into three major formations and
local subdivisions (Salfity et al., 1981; Figure 16).
Berta Formation (Late Jurassic to Late Cretaceous)
The lowermost Mesozoic sediments in the Pirity subbasin correspond to the early stage of
continental graben fill (YPF, 1984; Figure 16) with heterogeneous sources and variable
depositional environments (alluvial fans, braided rivers, some lacustrine and eolian influence).
Substantial lateral thickness changes occur in the Berta Formation (graben fill), reaching a
maximum of about 3000 meters.
The section correlates with the Pirgua subgroup (Orán basin, northern Argentina), the
Palacios Formation (Asunción and San Ignacio blocks, eastern Paraguay), and the Mariano
Boedo Formation (Las Breñas subbasin, northern Argentina); Figures 6 and 18. Correlations
with certain levels of the Adrian Jara Formation (northern Chaco) or the Misiones Formation
(eastern Paraguay) lack clear stratigraphic control, but may be valid at some units.
The Berta Formation consists of reddish brown fine grained sandstones with intercalated
mudstones. Some calcareous and anhydritic cement is present. Fossils have not been
reported in the Berta Formation in the Paraguayan Chaco. However, palynological analysis of
the adjacent Argentinian sections indicate a Late Cretaceous age (Coniacian; Carle et al.,
1991). Radiometric ages (128 ± 5 Ma - 126 ± 3.5 Ma K/Ar, Galliski and Viramonte, 1985) from
oxidized amigdaloidal to massive basaltoid magmatites within the sedimentary section suggest
an Early Cretaceous age (Valanginian) for the formation.
The magmatites indicate a certain differentiation from basal olivinic andesites, andesites to
leucoandesites in association with volcanic agglomerates, volcanic breccias, and pyroclastics
(Carle et al., 1991). A geochemical change upward from moderately sodic to potassic
composition has been indicated for these alkalic magmatites. Their emplacement occurred
along northwest-southeast and northeast-southwest fracture trends (tectonic lineaments of the
Palo Santo Formation (Late Cretaceous to Early Paleocene)
While the Pilar subbasin and the Bahia Negra platform recorded exclusively a continental
graben fill (equivalent to the Berta Formation) due to their proximity to basement highs, a
marine transgression from the southwest in the Pirity subbasin was responsible for the
deposition of the Palo Santo Formation during Latest Cretaceous to Earliest Paleocene (Figure
The Palo Santo Formation is commonally subdivided into three subformations, based on the
northern Argentinian stratigraphy (Moreno, 1970).
Lower Palo Santo Formation (Lecho Formation, northern Argentina; Turner, 1959): The
formation constitutes basal transgressive sandstones throughout the Pirity subbasin that partly
cover the Paleozoic flanks. The light colored friable sandstones are easily distinguished from
reddish sediments of the Berta Formation . They are whitish to light gray and pinkish colored,
massive, somewhat arkosic, fine to coarse grained, and with some carbonatic-dolomitic
cement. Quartz overgrowths and the presence of dolomite are the result of diagenetic
processes and may locally reduce the good primary porosity (reservoir potential). Grain size
seems to be smaller in the deeper parts of the Pirity subbasin and larger towards the margins,
reflecting substantial terrigeneous influence. The depositional environment corresponds to a
braided stream or fluvial and alluvial fan type sedimentation under shallow coastal influence.
The stratigraphic control includes seismic and lithostratigraphic correlations with northern
Argentinian sequences, where sparse fossils (Reyes and Salfity, 1973; Bonaparte et al., 1977)
and absolute age determinations on basic magmatites at the base of the correlative Lecho
Formation (70 ±5 Ma K/Ar; Carle et al., 1991) indicate the earliest deposition at Late
Cretaceous time (Campanian-Maastrichtian).
Maximum thickness in the deeper parts of the Pirity subbasin is 200-220 meters.
Middle Palo Santo Formation (Yacoraite Formation, northern Argentina; Turner, 1959): The
basal transgressive unit (lower Palo Santo Formation) grades into a sequence that reflects a
major Maastrichtian-Paleocene sea that transgressed from southwest to northeast into the
Pirity subbasin (Figure 17).
Above the basal sandstone, littoral clastics and carbonates alternate with fluvial and eolian
sandstones. A shallow carbonate-muddy plain is formed with periodic sedimentation in shallow
depocenters leaving wide areas to dry up and undercome terrestrial deposition (clastics,
shales and evaporites). Upward a widespread predominantely carbonate-muddy facies is
recorded, in internal depocenters with minor desiccation periods and sedimentation (Gómez
Omil et al., 1989). The vertical depositional characteristics can be observed as well in the
lateral distribution. The comparison of well sections in the Pirity subbasin between Berta-1 and
Palo Santo-1 (north-south line), as well as between Carmen-1 and Nazareth-1
(southwest-northeast line) , indicates finer grained to carbonaceous-dolomitic cemented
sequences in deeper parts, and somewhat coarser clastics and less dolomitic matrix in
Poor biostratigraphic parameters confirm the depositional environment: some Ostracodes in
the Berta-1 well (Paleocene) indicate shallow-marine conditions, as well as do nearshore to
swamp type palynomorphs in the Carmen-1 well. Eastern wells (Gloria-1 and Nazareth-1) did
not report reliable paleontological content, showing clastic sequences that confirm continental
sedimentary conditions. In northern Argentina (Moroni, 1982), Foraminifers and Ostracodes
(upper Maastrichtian) in deeper parts of the Orán basin indicate shallow-marine environments
(Mendez and Viviers, 1973), that laterally change into fresh-water conditions.
Therefore, a restricted carbonate basin appears to be responsible for the sedimentation of the
middle Palo Santo Formation. A restricted calm shallow sea of low salinity transgressed from
the southwest, without a permanent connection to the open sea, reaching as far as just west of
the Gloria-1 well (pinch out line; Figure 17). Depositional environments vary from brackish
lagoonal to fluvial and eolian conditions laterally (e.g. Acosta-1 or Nazareth-1 wells). The
overall fine lamination of the sediments, alternating compositions and graded bedding indicate
low-energy deposition and minor sea-level fluctuations.
The thickness measured in wells averages 200-550 meters.
Upper Palo Santo Formation (Olmedo Formation, northern Argentina; Moreno, 1970): This
formation indicates a strong evaporatic environment with precipitation of halite, gypsum, and
dolomite beds in a typical lagoonal evaporation plain. Fluvial sandstones at the margins
graded into silty / argillaceous muds, saline clayey muds with gypsum nodules, and halite
levels (saline member; Figure 17) towards the center (Gómez Omil et al., 1989).
Only the Carmen-1 and Anita-1 wells reported halite beds, the other wells in the Pirity
subbasin recorded transitions to marginal areas.
Therefore, the upper Palo Santo Formation marks for the regression of the restricted
Maastrichtian - Paleocene sea in the southwestern Paraguayan Chaco, indicating desiccation
under warm and dry conditions.
Paleontological control is sparse. The few amorphous organic samples and lithofacial
parameters support the change toward continental conditions.
Average thickness is between 100 and 230 meters.
Santa Barbara Formation (Early Paleocene to Middle Eocene; Pascual, 1978)
After deposition of the Palo Santo Formation in the Pirity subbasin, a widespread lacustrine to
fluvial plain evolved. Predominant reddish brown mudstone with some evaporites and
carbonates/dolomites deposited within local depocenters throughout the subbasin (Figure 16).
Lower Santa Barbara Formation (Mealla Formation, northern Argentina; Moreno, 1970):
Reddish brown and minor greenish mudstones are predominant, disappearing towards
northeast. They are intercalated with fangolites, marls, and thin layers of red sandstones, inclu-
ding calcareous silty shales and traces of nodular gypsum and dolomite. Palynological studies
in the Palo Santo-1 well suggest a Paleocene age and deposition on a saline mud flat within
an interior shallow basin characterized by fluvial influence under very low energy conditions
and desiccation (Gómez Omil et al., 1989). Thicknesses reach up to 800 meters.
Middle Santa Barbara Formation (Maíz Gordo Formation, northern Argentina; Moreno, 1970):
Claystones, siltstones, fangolites and marls, with intercalated calcareous levels and sporadic
sands, form a unit of 190 meters thick sediments in the northeastern and southwestern areas
of the Pirity subbasin. They are predominantly reddish brown and in parts greenish gray. The
deposition took place in an internal lake with slowly changing extension. Pollen analysis from
the Palo Santo-1 well indicate a Paleocene age (Millioud, 1975).
Upper Santa Barbara Formation (Lumbrera Formation, northern Argentina; Moreno, 1970):
The formation corresponds to a wide areal extension of alluvial, lacustrine and sporadic fluvial
sediments that overlap the flanks of the Pirity subbasin and invade adjacent subbasins and
highs. The true extent is unknown.
Brick-red claystones and fangolites, with thin intercalations of fine grained sandstones and
siltstones conform a monotonous section in the Pirity subbasin. Gypsum levels, calcareous
nodules and cement are reported. Characteristic is the "green horizon" as a regional strati-
graphic marker (Gómez Omil et al., 1989). It represents an interval of gray green to grayish
green, and reddish calcareous shale with oolitic limestone stringers, thinning from 10 meters
in the Carmen-1 well to 2.5 meters in the Nazareth-1 well.
The formation may reach thicknesses up to 1100 meters. According to lithostratigraphic
correlations to the Orán subbasin in northern Argentina (Pascual, 1978; Quattrocchio, 1980),
the age is Early to Middle Eocene.
The depositional environment of the upper Santa Barbara Formation consists of a continental
alluvial plain in a lacustrine setting with freshwater influx ("green horizon") and reduced fluvial
Chaco Formation (Middle Eocene to Early Pleistocene)
By the Middle Eocene the Paraguayan Chaco becomes tectonically stable. Subbasins as a
result of Mesozoic rift tectonics and even structural highs are covered by sediments (Figures
16 and 18).
The uprising Andes in the west (Andean cycle, 50-35 Ma) represent the most important
sediment source during the deposition of younger Cenozoic sequences in the Paraguayan
Chaco basin. Extensive thick continental sediments cover all of the Chaco from the west. The
average thickness is around 1050 meters at the Bolivian border and 500-800 meters in the
southeastern Pilcomayo river area.
The lower Chaco Formation (Middle Eocene to Late Pliocene) represents a continuous
continental depositional cycle (Mingramm et al., 1979) which is interrupted in the southeastern
Chaco by a short-lived marine transgression (Middle Miocene; Russo et al., 1979). In the basal
section, whitish to yellowish, medium to coarse grained sands with conglomeratic lenses
predominate over reddish brown slightly calcareous mudstones. The depositional environment
changes from continental fluvial-eolian conditions in the northwest (Berta-1 and La Paz-1
wells) to lagoonal sediments in the southeast (Palo Santo-1 well, Tranquitas Formation; Russo
et al., 1979).
The basal section interfingers southeastward with gray-greenish mudstones (Palo Santo-1
well) that contain characteristic shallow and low-energy marine microorganisms (Millioud, 1975;
Russo et al., 1979), halite horizons, and gypsum concentrations. The marine incursion from
the Atlantic Ocean (Mingramm et al., 1979; Chebli et al., 1989) reached as far as to the
Boquerón high and central Chaco uplift, where it borders younger Cenozoic deposits from the
west as in the Andean foreland region. The marine deposits correlate with the Argentinian
Paraná Formation (Russo et al., 1979).
The continental sedimentation of reddish brown sands, mudstones and some conglomeratic
beds continues later on throughout the Chaco. Typical are halite and gypsum concentrations
(e.g., along the Lagerenza high, northern Chaco; Wiens, 1990) in wide mud plains with strong
desiccation characteristics, along with fluvial and eolian deposits. The section correlates with
the Chaco Formation in northern Argentina (Russo et al., 1979).
The upper Chaco Formation (Late Pliocene to Early Pleistocene) constitutes an intimately
alternating sequence of fine grained brownish to yellowish sandstones, clayey to sandy,
grayish to grayish-green siltstones, and claystones. Carbonate and gypsum nodules and
lenses are frequently observed. Westward some fluvial conglomeratic beds may occur. The
sediments coarsen toward the west and claystones are predominant eastward, reflecting the
sediment source from the Andes. These sediments contain the most important aquifers in the
Paraguayan Chaco (Tullstrom, 1973; Godoy, 1990). Sedimentation and drainage conditions
are reflected in the water element variation: pronounced sodium bicarbonates in the west,
sodium sulfates in the central Chaco, and sodium chlorides in the east (Osterbaan, 1988).
The upper Chaco Formation represents a continental plain with fluvial (braided streams,
"internal deltas", alluvial fans etc.), lagoonal (mud flats, saline lakes etc.), and eolian (loess,
dunes etc.) deposits. Thickness decreases eastward from 800 to 250 meters (foreland basin
Quaternary ( Early Pleistocene/Holocene to recent)
During the Quaternary the Chaco basin accumulated heterogeneous sediments with strongly
changing environments. The lower units are generally sandy and coarsen toward west.
Glyptodontia, Lestodontia, and Megatheria (Hoffstetter, 1978; Presser and Crossa, 1984), as
reported from shallow drainage valleys, indicate an Early to Middle Pleistocene age.
Freshwater vertebrates (Bertoni, 1939; Herbst and Santa Cruz, 1985) in clays are related to
shallow-lacustrine deposition during the Late Pleistocene.
The Holocene climate changed to humid condition: eolian dunes of the Chaco Formation in
the western Chaco are stabilized by vegetation. Subordinated braided rivers and
anastomosing streams in the central Chaco transport alluvial material and eroded dune sands
accompanied by mud flow plains as reworked loess. Wide inundation plains with abundant
organic matter built up in the eastern Chaco (Herbst and Santa Cruz, 1985).
The most recent geological history (since 8000-6000 years BP) is marked by relative dryer
climatic conditions: eolian transport (sands and loess) and soil generation is characteristic.
River systems (campos) and mud plains (montes) dry up, brackish to saline lakes with sporadic
rivers envolve. Inundation plains (palmares) become only periodically flooded.
HYDROCARBON EXPLORATION POTENTIAL
Promising areas for hydrocarbon exploration in the Paraguayan Chaco are related to
Paleozoic and Mesozoic marine shales and carbonates (Figure 20). No commercial
hydrocarbon deposits have yet been discovered, due to the lack of long-term integrated
exploration programs. The Paraguayan Chaco remains widely unexplored (Wiens, 1990).
Paleozoic marine shales and carbonates reach thicknesses up to 2500 and 3600 meters in
the Carandaity and Curupaity subbasins, and cover adjacent highs in a differential transcurrent
setting. Immature organic-rich intervals are observed in the undisturbed centers of the basins
(Katarina-1 well) that are related to the upper sequences. Middle and lower sections in the
basins and the flanks of isolating highs reveal mature conditions, well into the oil generation
phase (Don Quixote-1 well). The increasing maturity with depth is marked by decreasing gas
wetness with depth. Similar conditions are reported along structural highs, where increasing
geothermal gradients (Figure 21), tectonic fracturing with associated magmatism, and strongly
transformed sediments by incipient metamorphism, indicate highly mature organic facies
Hydrocarbon seeps are observed in the Tucavaca area (eastern Bolivia) in lowermost
Phanerozoic limestones and shales of the Itapucumí Group. Gas indications from outcrops are
reported from contemporaneous bituminous sediments in northeastern Paraguay (Figure 19)
and Goias (Brazil).
Major hydrocarbon potential is related to Upper Devonian shales of the upper San Alfredo
Group and to a lesser extent to the shaly sections of the Lower Ordovician Cerro León Group
(producing fields in Argentina and Bolivia). Important dry gas blows (Mendoza-1 and Mendoza-
2 wells) and good oil shows (Toro-1 well) support the hydrocarbon potential in Carboniferous
channel sandstones. High-gravity oil is to be expected near the top of the sections in the
interior of the subbasins with wet gas, condensate, and even dry gas in the lower sections.
Only gas may be concentrated along structural highs (methane in slates or sandstones).
Mesozoic marine sediments of the Palo Santo Formation form a locally important hydrocarbon
play in the Pirity subbasin under viable conditions (Schlumberger, 1987). Gas and oil
indications within the Berta and Palo Santo Formations (producing fields in northern Argentina)
suggest a potential in smaller oil concentrations within the subbasin (Fernandez Garrasino,
Generation parameters, such as distribution of sedimentary facies, local magmatism,
paleomorphology, and geothermal gradients guide the hydrocarbon potential.
The regional distribution of geothermal gradients in the Paraguayan Chaco (calculated on
surface temperatures of 37.7
Celsius) delineates contours that outline the main geotectonic
units (Figures 3 and 21), indicating that the sedimentary maturity was strongly influenced by
the tectonic evolution. Structural highs may have gradients of up to 0.7-0.9
meters (Lagerenza and Fuerte Olimpo highs) suggesting magmatic activity taking place along
these tectonically favorable emplacement areas as confirmed by aeromagnetics and surface
control (Cerro León massif; Toro-1 well, with fractures and magmatites = high thermal gradient;
Gato-1 well, without magmatites = lower thermal gradient). Structural highs are related to
highly mature organic conditions and generally include very tight reservoirs.
The subbasins, however, record low geothermal gradients and show mature organic
conditions toward the base of the sedimentary sequence. Structural deformation within the
basins is less important. Here, paleomorphology, lateral sedimentary facies changes, and
unconformities are important parametes.
Therefore, the hydrocarbon potential in the Paraguayan Chaco reveals several Paleozoic
targets in the western and northern Chaco, based on the sedimentary and diagenetic
distribution, and the tectonic, magmatic, and thermal history. Mesozoic plays are concentrated
in the southwestern Chaco and are controlled by sedimentary facies, magmatism and
More than 3600 meters of upper San Alfredo Group shales are reported in the Curupaity
subbasin (Figure 14). They represent an important hydrocarbon source (oil show at Toro-1
well, 0.33-2.13% TOC). Slaty shales in the Toro-1 well indicate the overmature organic facies
along the Fuerte Olimpo high, which is highly fractured, injected by magmatites and reaches
geothermal gradients up to 0.59
Celsius/100 meters. As a result, targets are present at the
flanks of the high where oil generation may have taken place. In the interior of the Curupaity
subbasin organic material is expected to be largely immature.
Hydrocarbon migration updip may have resulted in concentrations of gas in sandstones of the
upper Cerro León Group and clastics of the lower San Alfredo Group . These reservoirs were
subjeted to strong diagenesis (quartzites in outcrops and wells) that distroyed porosity and
permeability. Fracture zones remain interesting targets. Important reservoirs are present in
sandy horizons within the upper San Alfredo Group, as they are surrounded by source rocks,
but depending on favorable permeability or fracturation.
The best reservoir rocks in the Paleozoic Curupaity subbasin are sandstones within the Early
Carboniferous San José Formation and the Late Carboniferous Cabrera Formation (channel
sands). The heterogeneity of these sandstones deserves attention as variable lateral
permeability generates intraformational traps and seals. Good permeability and porosity with
less diagenetic influence in Carboniferous sandstones form the main target zones in the
Excellent reservoir conditions are present in the Adrián Jara Formation (Late Jurassic to
Middle Eocene). Mesozoic sandstones may unconformably overlie shales of the San Alfredo
Group. A regional seal may be absent, therefore lowering the trapping potential.
Silts and shales along the unconformities at the top of the San Alfredo Group and argillaceous
levels at the top of the Cabrera Formation represent viable stratigraphic seals.
Structural traps are present, specifically as the result of Mesozoic tectonic reactivation (normal
faults, fracturing, magmatites, etc.).
Devonian shales (upper San Alfredo Group), exceeding 2500 meters in thickness, represent
the best hydrocarbon source in the Carandaity subbasin (Figure 12).
Targets consist of intercalated, uppermost Devonian sandstones (upper San Alfredo Group)
and extensive sandstones of the Carboniferous Palmar de las Islas Group (channel sands).
The heterogeneity of sandstone permeability and porosity requires careful exploration. Special
interest is given to areas along the flanks of isolating structural highs where mature organic
facies have been reported (0.5% TOC; Don Quixote-1 well). Very mature conditions are related
to the highs (Mendoza-1 and Mendoza-2 wells) that have important gas shows, but the
quartzitic sandstones and fracture systems show low permeability and porosity. Immature
conditions are revealed in the interior of the subbasin (Katerina-1 well). Eocene unconformities
(Chaco Formation) act as seals, and structural traps are favored along the Lagerenza and
Boquerón highs and toward the central Chaco uplift.
Shaly units of the Ordovician Cerro León Group (0.5% TOC) may represent a second source
rock in the Carandaity subbasin. Viable targets would consist of Upper Ordovician and Silurian
tight sandstones of the upper Cerro León Group in the interior of the subbasin where oil
generation is reached, but where low permeability and porosity is present. Towards the flanks
and highs only gas would be produced, due to the influences of strong transformation.
Fracture zones might be promising.
Main hydrocarbon targets in the Pirity subbasin are Upper Cretaceous marine source rocks of
the Palo Santo Formation (Figures 16 and 22). Geochemical analysis revealed oil generation
conditions whitin shales and carbonates of the middle Palo Santo Formation (Carle et al.,
1991). Reservoir rocks are represented by sandstones in the middle Palo Santo Formation or
the underlying lower Palo Santo and Berta formations. Hydrocarbon migration is controlled by
local paleomorphological or magmatic highs, and lateral sedimentary facies changes or
interfingering (Figures 22 and 23). Migration updip into structures and reservoirs outside the
main source area seem to be less likely. Brecciated magmatites that are intercalated within the
Berta and Palo Santo formations also represent possible reservoirs for localized hydrocarbon
accumulations (Palmar Largo field; northern Argentina).
Effective seals are present in the upper Palo Santo and Santa Barbara formations or may be
set by structural control.
A second hydrocarbon play in the Pirity subbasin is related to the Devonian shales of the
upper San Alfredo Group that form a source that is restricted to fault blocks along the
northwestern flank (Devonian is not present at the southeastern flank). Cretaceous and
Tertiary clastics unconformably overlie the shales and represent viable reservoirs. Overlying
sediments and rift tectonic features in the Pirity subbasin form potential traps.
San Pedro low (western part)
An important Phanerozoic succession in the San Pedro low extends into the Paraguayan
Chaco from the western Paraná basin (Figure 19). Lower Ordovician, Silurian and Permian
source rocks are to be expected in hydrocarbon generation conditions. Reservoirs and seals
within surrounding Paleozoic sections are present. No exploration surveys have been caried
out in the area.
Bahia Negra platform
Oil seeps, gas indications and high bituminous-fossiliferous contents in sediments of the
lowermost Phanerozoic Itapucumí Group (Figures 3, 8 and 19) sign for a hydrocarbon source
potential in the northeastern Chaco (Bahia Negra platform). However, strong recrystallization,
incipient metamorphism, and dolomitization may have reduced the potential. Structural control
is observed along the Paraguay river and the Fuerte Olimpo high, indicating viable traps.
Possible reservoirs may be intraformational fracture zones, zones with leaching effects (karst),
and sandstone intercalations. Interbedded shales provide perfect seals. The target remains
unexplored and can only be indicated tentatively.
Pilar subbasin (northern part)
Ordovician-Silurian marine clastic sediments surrounding the Río Tebicuary subcraton (eastern
Paraguay) form a Paleozoic platform in the southeastern Chaco. They are covered by
Cretaceous to Paleocene continental sediments that accumulated in asymmetric graben
systems, established during Mesozoic rifting (Figures 6 and 18). Early Silurian Vargas Peña
shales are indicative hydrocarbon source rocks in the subbasin. Reservoirs and seals are
present in Mesozoic sediments and structural deformations represent good traps. Significant
hydrocarbon accumulations may be only local in the Pilar subbasin.
No explorations efforts are registred on Paraguayan territory.
1. The Chaco basin in Paraguay was formed on a continental crust as a direct and
subsequent result of the Eocambrian tecto-orogenetic Brasiliano cycle (680-450 Ma).
The basement reflects a complex structural block system with preferential fracture zones
oriented north 45
west and north 50
east. The tectonic-sedimentary evolution
of the Phanerozoic Chaco basin is intimately related with later structural reactivations
along these tectonic lineaments.
2. The Chaco basin indicates four distinct superimposed subsidence phases during the
Phanerozoic that were controlled by weak tectonic pulses during Ordovician- Devonian
(first phase) and Carboniferous-Permian (second phase), extensional rift tectonics during
Late Jurassic / Early Cretaceous-Middle Eocene (third phase), and calm subsidence
during Middle Eocene-Quaternary (fourth phase). The phases are separated by periods
of erosion, non deposition or low sedimentation rates.
3. The Phanerozoic stratigraphic section in the Paraguayan Chaco ranges from Latest
Proterozoic / Cambrian to recent deposits. The crystalline basement has not been
reached by exploration surveys.
4. The stratigraphic subdivision (based on outcrops and hydrocarbon / groundwater
exploration surveys) starts with Uppermost Proterozoic to Cambrian marine clastics and
limestones (Itapucumí Group) that rest unconformably on crystalline basement. An
erosional surface underlies Lower Ordovician-Silurian to Upper Devonian marine shales
and sandstones (Cerro León and San Alfredo groups) that are unconformably overlain
by Lower Carboniferous to Lower Permian shallow marine to continental clastic deposits
with glacial influence (Palmar de las Islas Group). The Paleozoic sequence remains
registred in its full expression in the western and northern Chaco. The basement in
crystalline subcratons is observed only in eastern Paraguay.
Upper Jurassic to Middle Eocene continental clastics form the basal Mesozoic sediments
in the Carandaity and Curupaity subbasins (Adrian Jara Formation). The first continental
sedimentary fill and related magmatites in asymmetric graben systems as a result of
Cretaceous rift tectonics (Pirity and Pilar subbasins; Berta Formation), are correlative and
are disposed in a sharp angular discordance to Paleozoic sediments.
A Late Cretaceous to Early Paleocene marine transgression with clastic and carbonate
sediments is recorded in the Pirity subbasin (Palo Santo Formation); that evolves into a
widespread continental lacustrine to fluvial plain (Santa Barbara Formation) during the
Early Paleocene to Middle Eocene.
In a pronounced unconformity Middle Eocene to Early Pleistocene continental deposits
are recorded as foreland basin sediments throughout the Chaco basin, that were
sourced from the uprising Andes to the west (Chaco Formation), and interrupted in the
southeastern Chaco by a local marine incursion (Middle Miocene, Paraná Formation in
Undifferentiated recent Quaternary deposits in the Chaco basin (Early
Pleistocene/Holocene to recent) are controlled by climatic changes in an extensive
5. There is a promising hydrocarbon potential in the Paraguayan Chaco basin that is related
to Paleozoic and Late Cretaceous to Paleocene shales and carbonates. A major
hydrocarbon favorability depends on source rocks from Upper Devonian shales (upper
San Alfredo Group) in the Carandaity and Curupaity subbasins. Oil generation is
recorded in middle and lower portions of the subbasins interior and along the flanks of
isolating highs. Increasing geothermal maturity with depth and on the structural highs
indicate a certain gas and condensate potential.
Main reservoir rocks include heterogeneous Carboniferous sandstones (channel sands;
Palmar de las Islas Group) with good permeability and porosity. Viable reservoir
conditions are present in the Mesozoic Adrián Jara Formation sandstones and Devonian
Seals include the source rock, argillaceous levels at the top of Carboniferous reservoirs,
and Tertiary unconformities above Mesozoic reservoirs.
Structural traps may be present as the result of Mesozoic tectonic reactivation.
6. Marine sediments of the Upper Cretaceous Palo Santo Formation are targets in the Pirity
subbasin, where oil generation has been reported. Migration is controlled by local paleo-
morphological and magmatic highs, and strong lateral sedimentary facies changes and
interfingering. Reservoirs are present in the source formation, in sandstones of the Lower
Palo Santo Formation, in clastic sediments of the Berta Formation (Upper Jurassic to
Upper Cretaceous), and in brecciated Cretaceous magmatites as a combined
Effective seals are reported within the overlying Santa Bárbara Formation (Early Tertiary),
accompanied by local structural traps.
7. Along the southeastern flank of the Boquerón high, Upper Devonian shales are block
faulted during Mesozoic rift tectonics and are unconformably overlain by Cretaceous and
Tertiary clastics (reservoirs) and shales (seals). The play represents a viable target for
8. Uppermost Proterozoic - Cambrian limestones and shales (Itapucumí Group) and Lower
Ordovician shales (lower Cerro León Group) are characterized by high geothermal
maturity and may generate gas. As a result, the lower sedimentary units within the
Curupaity-Carandaity subbasins and the Bahia Negra platform form a secondary
potential for hydrocarbons.
9. Lower Ordovician, Silurian and Permian sediments at the western extension of the San
Pedro low may have generated oil.
Continental and marine Paleozoic and Mesozoic sediments stay for the unexplored
hydrocarbon potential in the Pilar subbasin.
10. No commercial hydrocarbon accumulations have been discovered so far in the
Paraguayan Chaco basin. This is due to the lack of long-term integrated exploration
programs. The Chaco basin in Paraguay remains widely unexplored, but offers
interesting and viable hydrocarbon targets.
I like to express my acknowledgments to H.J. Welsink and H.J. Tankard for critical suggestions
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indications; to M.P. de Pella and A.J.L. de Di Sopra for typing the manuscript; to J.R. Britez
Urdapilleta for the technical design of the illustrations; and J.D. Ray and R. Diana for the
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