Unconventional Oil in Iraq's Jurassic Sargelu Formation: A 2014 USGS Assessment

The Sargelu Formation is a Jurassic layer of marine shales in Iraq, one of the main source rocks for the conventional oil fields of the Arabian Peninsula. It was laid down in a relatively deep basin short of oxygen and holds up to 10 percent by weight of sulfur-rich (Type IIS) organic matter. The U.S. Geological Survey assessed how much oil and gas may still be held in the shale itself — oil that stayed behind after the rest migrated, cracked or degraded — as unconventional, or continuous, resources. Iraq's conventional resources were assessed separately in 2012.

The estimate

F95 means a 95 percent chance of at least that amount. The figures are undiscovered, technically recoverable volumes, not what would be economic to produce.

How the USGS judges a shale

The method rests on how wells perform, allowing for geological variation within a reservoir. Outside the United States, a shale counts only if it:

1. holds more than 2 percent total organic carbon by weight;
2. lies in the right thermal maturity window for oil or gas;
3. has more than 15 metres of organic-rich shale;
4. contains Type I or II organic matter.

These criteria can shrink the assessed area considerably compared with maps drawn at 1 percent organic carbon.

The geological model

Oil and gas formed in the Sargelu's deep, oxygen-starved shales during the Late Cretaceous. Some stayed in the source rock; some migrated long distances; some moved again during the late Tertiary Zagros mountain-building, as Sargelu-sourced oil found in some Zagros-age structures shows. At the most likely value, about 30 percent of the assessment area is thought to hold good source rock little disturbed by later movements, where oil might remain. Evaporites of the overlying Upper Jurassic Gotnia Formation probably form a partial seal. Shales of the Naokelekan Formation were left out, as they may be a separate system.

Well performance, drainage areas and success ratios are borrowed from U.S. shale-oil and shale-gas analogues.

Sources

• Schenk, C.J., Pitman, J.K., Charpentier, R.R., Klett, T.R., Gaswirth, S.B., Brownfield, M.E., Leathers, H.M., Mercier, T.J., and Tennyson, M.E., 2015, Assessment of unconventional oil and gas resources in the Jurassic Sargelu Formation of Iraq, 2014: U.S. Geological Survey Fact Sheet 2015–3006. https://pubs.usgs.gov/publication/fs20153006
• Figures: U.S. Geological Survey (map base from the U.S. National Park Service).
• Rewritten in hubnx's own words.