Undiscovered Oil and Gas in Argentina's San Jorge Basin: the 2017 USGS Assessment

A U.S. Geological Survey assessment from 2017, published as Fact Sheet 2017–3043 by Christopher J. Schenk, Tracey J. Mercier, Sarah J. Hawkins, Marilyn E. Tennyson, Kristen R. Marra, Thomas M. Finn, Phuong A. Le, Michael E. Brownfield, Heidi M. Leathers-Miller and Cheryl A. Woodall.

Using a method built on the geology of the rocks, the U.S. Geological Survey (USGS) estimates that the San Jorge Basin Province of Argentina holds a mean of 78 million barrels of undiscovered, technically recoverable continuous oil and 8.9 trillion cubic feet of gas.

How the basin formed

The San Jorge Basin took shape through several phases of stretching of the crust, beginning in the Permian–Triassic with the first oblique rifting of South America away from Africa and ending with the final continental breakup in the late Early Cretaceous. That stretching created east–west to northwest–southeast normal faults, which outline a deep central basin with shallower margins to the north and south.

• Fill: during Late Jurassic–Early Cretaceous extension and the sagging that followed, mostly non-marine river and lake sandstones, siltstones and shales were laid down — including the province's main petroleum source rocks.
• Later squeezing: Neogene compression from Andean subduction inverted some of the old faults in a narrow band, forming the roughly north–south San Bernardo fold belt.

The San Jorge Basin Province and its four assessment units. Base map from the National Park Service. USGS.

Two petroleum systems, four assessment units

The D-129 Lacustrine Total Petroleum System is defined by thermally mature, organic-rich lake shales of the Hauterivian–Aptian D-129 Formation, with total organic carbon up to 4 weight percent and up to 2,000 metres thick. It contains a shale oil and a shale gas assessment unit. The model: some oil or gas stayed in the D-129 shales after the rest migrated into conventional traps.

The Callovian–Aptian Lacustrine Composite Total Petroleum System covers possible tight gas sourced from the lake shales of the Callovian–Berriasian Aguada Bandera Formation and the D-129 Formation; total organic carbon reaches 4 weight percent and the shale interval is up to about 600 metres thick. It contains the Eastern and Western San Jorge Tight Gas units. The model: gas trapped in low-permeability river, alluvial and lake-margin sandstones of the Neocomian synrift Matasiete Formation and the overlying Mina del Carmen Formation.

Well drainage areas, estimated ultimate recoveries and success ratios were guided by U.S. shale-oil, shale-gas and tight-gas analogues.

Results

The estimated means are:

• oil: 78 million barrels, with an F95–F5 range from 17 to 180 million barrels;
• gas: 8,893 billion cubic feet, from 1,802 to 20,869 billion cubic feet — 3,440 billion (39 percent) shale gas and 5,453 billion (61 percent) tight gas;
• natural gas liquids: 67 million barrels, from 13 to 165 million barrels.

F95 means a 95 percent chance of at least the amount shown, and the other columns are read the same way. Each unit was given a probability of 1.0, and the results are fully risked; for gas accumulations, all liquids are counted as natural gas liquids.

Sources

• Christopher J. Schenk and others, Assessment of Continuous Oil and Gas Resources in the San Jorge Basin Province, Argentina, 2017, U.S. Geological Survey Fact Sheet 2017–3043. https://doi.org/10.3133/fs20173043 — the map, both tables and the results recovered from the PDF, which the imported page had lost.
• The fact sheet draws on Fitzgerald and others (1990), Foix and others (2012), Legarreta and Villar (2011), Macdonald and others (2003), Paredes and others (2013), Rodriguez and Littke (2001) and Sylwan (2001).
• Rewritten in hubnx's own words.