Chaco Basin

From SEG Wiki
Jump to navigation Jump to search

The Chaco basin is a large foreland basin of the Andean ranges, located between the west of the Andean belt, northeast of the Brazilian shield, east of the Paraná basin, and south of the Pampa basins with an area of approximately 240 thousand sq km as you can see in Figure 1. The Chaco basin is structurally simple with a variety of (mainly vertical) normal faults primarily located in Paraguay. It has a thick covering of terrestrial sediments, specifically, fine sands and clays with underlying older rocks. Over 2 Bcf and 50,000 bopd have been produced from this basin and it is estimated that the Chaco basin could hold at least 4 Bbo. Much of the oil and gas production in the Chaco basin occurs in the Bolivia and Argentina areas of the basin rather than in Paraguay, as Paraguay has faced more drilling challenges in the past.[1]

Figure 1 Map of South America showing the location of the Paraguayan Chaco basin and its regional geologic setting. [2]

History of The Basin

The term “Chaco” comes from the Quechua word, “hunting grounds” as it had flat open grasslands. The present knowledge of the Chaco basins stratigraphic and tectonic evolution is primarily the result of hydrocarbon and groundwater exploration. Exploration of the Chaco basin began in the 1900s, however, there was not much success in the beginning. It was not until the 1940’s that the first seismic was acquired by Union Oil. Union Oil drilled around five wells from 1947 to 1949. There was a ten-year down period of drilling where only a total of seven successful wells were drilled from the 1950s to the 1960s. Not many successful efforts occurred through the time of the basin exploration. However, it was not till President Energy made the basins, and Paraguay’s, first major oil discovery in 2014.[3] This proved that the Chaco basin had an active petroleum system and that there were oil and gas-bearing Paleozoic sands. Additionally, in 2013 the US Energy Information Administration (EIA) forecasted unproven wet-shale gas recoverable resources for Paraguay to be around 75 Tcf.

Geological Setting

The tectonic style of the Chaco basin is that of northwest/northeast oriented structural faults of the Eocambrian Brasiliano cycle. The Brasiliano event is what started the Phanerozoic Chaco basin as carbonate and clastic sequences of the Eocambrian group were deposited.[4] The thickness of the sedimentary rocks is over 6000 meters including strata of Devonian, Carboniferous, Permian, Triassic, Cretaceous, and Cenozoic. During Phanerozoic, the substance phases were controlled weak tectonics during the Ordovician-Devonian phase and the Carboniferous-Permian phase, extensional rifts tectonics in the late Jurassic to early Cretaceous-Middle Eocene, and the calm substance during the Eocene-Quaternary phase. All the phases of the Phanerozoic time were separated by erosional unconformities or by low sedimentation rates.

Geological Risks

There are some risks when it comes to the Chaco basin, especially when you examine the ten-year drilling hiatus back in mid-1900s when there was not much success from the basin. Many companies have failed at exploiting the area and even seismic studies, such as the Exxon seismic study in the 1980s. Much of the risk associated with the basin is the uncertainty of the hole conditions of the wells. Years of unsuccessful exploration and drilling were partly due to the unreliable areas of drilling. Fear of deteriorating hole condition and high risk of hole collapse caused hesitation from drillers.

Chaco Subbasins

Figure 2 The location and surrounding areas of the Chaco basin and subbasins.[1]

The Chaco basin is composed of four subbasins. The basins located within the Chaco are the Carandayty, Curupayty, Pirity, and Pilar. As you can tell from Figure 2, The Carandayty subbasin is in the west, the Curupayty subbasin is in the north, the Pirity subbasin in the southwest, and the Pilar subbasin in the south. The Pirity and the Pilar subbasins are attributed to the extension on the Mesozoic, and the Carandayty and the Curupayty subbasins are Paleozoic depocenters.[4] There were three main geological events that occurred in the basin. The first event was the deposition of the Paleozoic sedimentary section with a more eastern source area. The second was the rifting of the Paleozoic section during the Cretaceous that formed the subbasin of Pirity. Third, the deposition of continental and marine sediments took place in a foreland basin setting. The Carandayty and the Curupayty subbasins were considered to be the most prospective as almost half of the wells drilled in the Chaco basin have been drilled in the Carandayty subbasin. The Pirity subbasin has been attracting interest for years are is extends into Argentina, producing a number of fields in the Cretaceous rift. The Pilar subbasin is considered to be like a pull-apart basin as it has steep basin bounding faults with intrusions related to the early Andean progeny.

Petroleum Systems

Source Rock and Migration

Targets for hydrocarbon exploration depend on the presence of source rocks. The main source rocks include Silurian Kirusillas Formation and the Devonian Los Monos and Icla formations. The Devonian, which is considered to be the main source rock for the conventional gas fields in the Tarija Basin foothills of southeast Bolivia, seems to have shale gas potential in the northwestern area of Paraguay (Figure 3). The gas window in the basin is reported to be about 2km deep. [5] Another source rock is the Puesto Guardian Member in the lower area of the U. Cretaceous Yacoraite Formation. This source rock’s peak hydrocarbon maturation and charge is estimated to have occurred over 30 million years ago, with the current maturity in the oil window. The Early Ordovician Cerro León Group shaly units can also be identified as a secondary source rock. The main migration routes are linked to failure and fractured systems.

Figure 3 Regional cross-section across the Chaco Basin showing thick and mostly flat-lying source rocks.[5]

Paleozoic marine shales and carbonates can reach a thickness of around 2000 to 3500 m in the Carandayty and Curupayty subbasins. Therefore, the Devonian upper San Alfredo Group shales in the Carandayty subbasin are over 2500 m thick and are an exceptional source rock. This contributes to the fact that the Carandayty subbasin is considered to be more gas-prone, while a slightly lower source rock temperature indicated that the Curupayty subbasin is likely to contain oil. In the Curupayty subbasin the upper San Alfedo shales represent important hydrocarbon targets. An oil show in the subbasin, Toro-1 well, has TOC values between .03 and 2.1 wt. %.[6] The Toro-1 consists of slatey shales which indicate over-mature conditions, where the rocks are highly fractured.

Traps and Seals

Structural traps are present, especially as a result of Mesozoic tectonic reactivation. Some accumulations in the Tertiary, where the reservoirs are short-lived fluvial sandstones, possibly have a strong stratigraphic influence in the entrapment. Seals include source rock, argillaceous levels of the top of Carboniferous reservoirs, and Tertiary unconformities above Mesozoic reservoirs. Effective seals are reported with the Early Tertiary, Santa Bárbara Formation, along with local structural traps. Eocene shale-draped unconformities act as seals along the Lagerenza high, Boquerón high, and towards the Central Chaco uplift, referred to in Figure 4.[5]

In Paraguay, the shale gas develops within an impermeable clay layer compressed by the superposition of sediments accumulated in large basins and for a long time, such as Silurian and Devonian (Paleozoic), which are transformed into shale gas. Due to the high impermeability of the rocks, the gases are trapped inside them, without the possibility of migrating towards geological traps or deposits. This situation requires complex extrusion processes compared to a conventional hydrocarbon drilling that currently represent very high costs and in many cases are impractical.

Figure 4 Area of the Devonian Los Monos Formation.[5]


The best quality reservoir rocks are the lower Carboniferous San José Formation and Upper Carboniferous Cabrera Formation channel sandstones. Good permeability and porosity with less influence in Carboniferous sandstones create the targeted zones in the Curupayty subbasin. Such sandstones are similar to some of the producing reservoir characteristics that also occur in the Upper Jurassic-middle Eocene Adrian Jara Formation. Because the Devonian Los Monos Formation is so thick in the Chaco it can be assumed that it is organically rich. There is not extensive faulting, therefore indicating that little of the area is sterilized due to the complexity of the structure. The shale matrix consists primarily of brittle materials such as calcite and dolomite and there is some clay present, but it is not as common. [7]

Future Petroleum Potential

Several factors contribute to the Chaco basin’s attractiveness for oil and gas exploration and development. Some of these factors are growing markets for hydrocarbon products, especially increased oil and gas prices. Additionally, improved infrastructure in the Chaco basin and positive changes in the political climate contribute to the basin’s attractiveness. While there is increasing potential for hydrocarbons in the Chaco’s, some pitfalls are likely to occur. Such pitfalls are seen through the high operating costs and the indigenous groups that would have to be considered, which could result in more time-consuming processes.[8] Overall, the Chaco basin is thought to have significant unconventional hydrocarbon potential, as its benefits outweigh the costs. There are approximately 1,100 MMbo remaining recoverable reserves of the Chaco basin which places the basin in an extremely prospective position.[9] Additionally, the shale potential of Paraguay is estimated to be the fifth largest resource hub in South America. [10] In order to extract great quantities of shale gas the most promising zones will have to be drilled, requiring the application of vertical and horizontal drillings, fracturing and water flooding. The search and eventual exploitation of unconventional hydrocarbons is largely stimulated by the method of hydraulic fracking.

Petroleum and Facility Engineering

Until President Energy's 2014 discovery of the Lapacho well in the Pirity subbasin, Paraguay imported all of its fuel to cover the domestic demand of around 27,000 barrels/day. [11] Since the discovery it proved the basin to be a viable source of oil that could fuel Paraguay's domestic consumption. However, the discovery means that it would take a couple of years for this to happen as there is a lack of infrastructure and cost of mobilizing equipment in Paraguay to process the distribute the fuel. President Energy's extensive 3-D seismic survey proved to be the key aspect to finding and establishing productive drilling prospects.


  1. 1.0 1.1 Whaley, J., 2017, Oil in the Heart of South America, Vol. 14, No.4,, accessed November 15, 2021.
  2. Celso Nazario Velázquez Ibarrola "Potential of Unconventional Hydrocarbons in the Paraguayan Chaco: Carandayty Sub-Basin Case." IOSR Journal of Environmental Science, Toxicology and Food Technology (IOSR-JESTFT) 12.7 (2018): 65-77.
  3. Bloomberg. (2014, October 20). President energy finds oil in Paraguay's Chaco basin. Retrieved November 16, 2021, from
  4. 4.0 4.1 Wiens, F. (1995). Phanerozoic tectonics and sedimentation in the Chaco basin of Paraguay, with comments on hydrocarbon potential. Petroleum Basins of South America. doi:10.1306/m62593c8
  5. 5.0 5.1 5.2 5.3 Kuuskraa, V., Stevens, S. H., & Moodhe, K. D. (2013). Technically recoverable shale oil and shale gas resources: An assessment of 137 shale formations in 41 countries outside the United States (pp. VII-1-VII-19) (United States, Energy Information Administration, U.S. Department of Energy). Washington, DC: Independent Statistics & Analysis. Retrieved November 16, 2021, from
  6. Wiens, F., 1995, Phanerozoic Tectonics and Sedimentation of The Chaco Basin, Paraguay. Its Hydrocarbon Potential: Geoconsultores, 2-27, accessed November 15, 2021;
  7. Alfredo, C., & Kuhn, C. (1991, August 01). The geological evolution of the Paraguayan Chaco. Retrieved November 15, 2021, from
  8. White, K., 2016, Paraguay Draws Industry Attention, Vol. 11, No. 6,, accessed November 15, 2021.
  9. Zhang, Lei, Zhang, Guangya, Liu, Lei, and Xi Chen. "Geological Conditions of Tight Oil and Gas in Chaco Basin." Paper presented at the SPE/AAPG/SEG Unconventional Resources Technology Conference, Denver, Colorado, USA, August 2014. doi:
  10. Gómez, C. (2013, July 12). The promise of oil in Paraguay. Retrieved November 17, 2021, from
  11. EIU Digital. (214, November 10). The Economist Intelligence Unit. Retrieved November 17, 2021, from

See Also