By the USGS Mongolia and China Basins Assessment Team: Christopher J. Schenk, Tracey J. Mercier, Janet K. Pitman, Thomas M. Finn, Cheryl A. Woodall, Kristen R. Marra, Phuong A. Le and Heidi M. Leathers-Miller
Using a geology-based method, the U.S. Geological Survey estimated undiscovered, technically recoverable continuous (unconventional) shale-oil resources in five basins of Mongolia and China: the East Gobi, Nyalga, Tamtsag-Hailar, Erlian and Yingen.
The five assessment units (AUs). USGS.
How the basins formed
- Rifting: in the Middle Jurassic and the Late Jurassic–Early Cretaceous, the crust was pulled apart into a regional system of extensional basins, filled with nonmarine sediment (Changsong and others, 2001; Meng, 2003; Meng and others, 2003; Wilhem and others, 2012).
- Sag: after rifting stopped, a thermal sag phase made room for more sediment. Source rocks matured locally and oil migrated.
- Squeeze: in the Late Cretaceous, regional compression, uplift and erosion interrupted the sag, inverted many of the rift structures, and largely stopped the source rocks from maturing further.
What was assessed
Each basin is a group of subbasins. To be assessed for shale oil, a subbasin's self-sourced shale needed:
- Type I or Type II kerogen;
- over 2 weight percent total organic carbon;
- over 15 meters of organic-rich source rock;
- to be within the thermal window for oil or gas.
Heat is the limit: in many subbasins, the source rock never got hot enough to make oil, and those were left out. The assessment covers only the subbasins interpreted, from published information, to have entered the oil window.
The source rocks are organic-rich deep-water lake shales (Traynor and Sladen, 1995; Graham and others, 2001; and others):
- mainly Type I kerogen, with minor Type III;
- up to 10 weight percent organic carbon;
- hydrogen index up to 600 milligrams of hydrocarbon per gram of organic carbon;
- up to 700 meters thick.
The USGS defined five total petroleum systems, each with one shale-oil AU, covering the subbasins thought to hold thermally mature source rock. The uncertainty in each unit's productive area reflects three unknowns: how far the source rocks extend within the grabens, how mature they are, and how much oil stayed put after the compression. Well drainage areas, success ratios and estimated ultimate recoveries come from geologic analogs in the United States.
The estimates
For all five AUs, the mean estimates are:
| Mean | F95–F5 range | |
|---|---|---|
| Shale oil | 1,121 million barrels (MMBO) — about 1.1 billion barrels | 0 to 2,941 MMBO |
| Associated gas | 674 billion cubic feet (BCFG) | 0 to 1,789 BCFG |
| Natural gas liquids | 2 million barrels | 0 to 7 million barrels |
F95 means a 95-percent chance of at least that much; F5, a 5-percent chance.
By assessment unit:
| Assessment unit | AU probability | Oil, F95–F5 (MMBO) | Oil, mean (MMBO) | Gas, mean (BCFG) |
|---|---|---|---|---|
| Erlian Basin Upper Aershan–Lower Tengger Shale Oil | 0.9 | 0–1,335 | 515 | 309 |
| Tamtsag-Hailar Basin Nantun Shale Oil | 0.9 | 0–845 | 326 | 196 |
| East Gobi Basin Zuunbayan Shale Oil | 0.9 | 0–414 | 158 | 95 |
| Yingen Basin Bayingebi Shale Oil | 0.8 | 0–266 | 94 | 57 |
| Nyalga Basin Khuren Dukh Shale Oil | 0.7 | 0–81 | 28 | 17 |
About 75 percent of the oil is in two basins: the Erlian (46 percent) and Tamtsag-Hailar (29 percent).
More: USGS Energy Resources Program.
Sources
Based on Assessment of Undiscovered Continuous Oil Resources of the East Gobi, Nyalga, Tamtsag-Hailar, Erlian, and Yingen Basins of Mongolia and China, 2018, by the Mongolia and China Basins Assessment Team, USGS Fact Sheet 2018–3040, U.S. Geological Survey; a work of the United States government in the public domain. The map and the results by assessment unit are taken from the fact sheet's PDF, which the import had left out; the import had also cut and reordered two paragraphs, restored here.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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