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The U.S. Geological Survey estimates a mean of 8.3 trillion cubic feet of undiscovered, technically recoverable tight gas in the Zona Glauconitica of the Magallanes Basin Province, in southern Chile. The figure comes from the USGS programme that assesses unconventional oil and gas (shale oil and gas, tight oil and gas, and coalbed gas) in priority geologic provinces worldwide.
The rock
The Zona Glauconitica is an informally named unit, 50 to 150 meters thick, of low-permeability sandstone and siltstone laid down as the sea advanced, rich in the mineral glauconite. Its sediment came from the Andes to the west.
The basin's Lower Cretaceous Total Petroleum System includes several source-rock layers of marine Type II organic matter, up to 6 weight percent. The oil and gas they generated went three ways:
- up into the Zona Glauconitica;
- nowhere, staying in the source rocks;
- down into the Springhill Sandstone, the main conventional reservoir for more than 200 oil and gas fields in the eastern basin.
The model
The assessment assumes oil and gas formed in the underlying Lower Cretaceous organic-rich shales and moved up into the tight siltstones and sandstones of the Zona Glauconitica. Mostly oil migrated; it was later cracked to condensate and gas inside the tight reservoir, possibly because of high heat flow on the Springhill Platform. More than 10 wells have tested and produced gas and condensate from the unit, evidence of a workable unconventional accumulation. Recovery per well, drainage areas and success rates were taken from U.S. tight-gas reservoirs used as analogues.
| Input | Minimum | Mode | Maximum | Mean |
|---|---|---|---|---|
| Potential production area (acres) | 50,000 | 750,000 | 2,300,000 | 1,033,333 |
| Drainage area per well (acres) | 60 | 80 | 180 | 107 |
| Success ratio (%) | 70 | 80 | 90 | 80 |
| Recovery per well (billion cubic feet) | 0.6 | 1.0 | 1.6 | 1.025 |
The results
- Gas: a mean of 8,271 billion cubic feet, ranging from 2,458 (a 95% chance of at least that much) to 16,290 (a 5% chance).
- Liquids: a mean of 83 million barrels, ranging from 23 to 172.
The wide range reflects real geologic uncertainty, especially about where the unit's boundaries lie.
The basin may hold other unconventional resources, such as shale gas and shale oil in the Lower Cretaceous source rocks and tight gas in deepwater sandstones of the same age as the Zona Glauconitica, but these were not assessed.
Sources
- Christopher J. Schenk, Ronald R. Charpentier, Janet K. Pitman and others, Assessment of unconventional tight-gas resources of the Magallanes Basin Province, Chile, 2015, U.S. Geological Survey Fact Sheet 2015–3085. https://pubs.usgs.gov/publication/fs20153085
- The original's location map, drawn on a licensed commercial base map, is not reproduced here.
Licencia: CC0 1.0 (dominio público) · Adaptado de pubs.usgs.gov
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