By the USGS Arabian Peninsula Assessment Team: Christopher J. Schenk, Tracey J. Mercier, Cheryl A. Woodall, Marilyn E. Tennyson, Thomas M. Finn, Michael E. Brownfield, Kristen R. Marra, Phuong A. Le, Ronald M. Drake II and Scott A. Kinney
Using a geology-based method, the U.S. Geological Survey estimated undiscovered, technically recoverable continuous oil and gas in the Lower Silurian Shale Total Petroleum System of the Arabian Peninsula: a mean of 4.6 billion barrels of oil and 561 trillion cubic feet of gas.
The six assessment units (AUs), three in the north and three in the south. USGS.
How the source rock formed
- Early Paleozoic: the Arabian Peninsula was the eastern end of a broad, north-facing passive margin stretching across North Africa from Morocco.
- Early Silurian: as the Ordovician ice cap retreated, organic-rich marine shales — the basal shales — filled north-trending paleochannels (Le Heron and Craig, 2008; Hayton and others, 2017a).
- Middle Carboniferous: the paleo-Tethys Ocean closed and Pangea assembled, bringing uplift and erosion across North Africa and the Middle East (Konert and others, 2001; Ruban and others, 2007; Faqira and others, 2009) — up to 2,800 meters in the north, but only a few hundred meters in the southern basins, more at their edges (Cantrell and others, 2014; İnan and others, 2017).
The source rock: lower Silurian shales are among the most prolific petroleum source rocks in the world (Ulmishek and Klemme, 1990; Lüning and others, 2000). Here they hold up to 20 weight percent organic carbon, have hydrogen indices up to 600 milligrams of hydrocarbon per gram of organic carbon, are up to 70 meters thick, and are mature for oil and gas over wide areas.
A gamma-ray log through the Qusaiba Formation in Saudi Arabia, showing the organic-rich basal shale — the main Silurian source rock (modified from Jones and Stump, 1999). USGS.
North and south: two histories
The assessment weighs how differently the north and south evolved, and estimates the oil and gas still held in the shales and neighboring tight reservoirs (Al Duhailan and others, 2014; Hayton and others, 2017b).
| North (of the Al Batin Arch) | South | |
|---|---|---|
| Oil generated | possibly from the Devonian; by the lower Carboniferous, oil in the deeper basin had cracked to gas — a complex process, perhaps involving hydrothermal fluids (Hayton and others, 2017b) | peak oil in the Late Jurassic, peak gas in the Late Cretaceous |
| The uplift | up to 2,800 meters of uplift and erosion in the mid-Carboniferous let gas expand, lowered pressure, fractured the rock, and may have let oil and gas escape; it also stopped further maturation | came before maturation, which is about the same age as the gentle Mesozoic folds that trapped the region's conventional fields |
| Result | oil and gas held in the shales since the mid-Carboniferous | nearly continuous burial from the Permian into the Tertiary kept oil and gas better sealed in the shales and tight sandstones (Hayton and others, 2010; Faqira and others, 2011; Al Duhailan and others, 2014) |
| Units | Lower Silurian Shale Oil, Shale Gas, and Ordovician–Devonian Tight Gas | Lower Silurian Shale Oil, Shale Gas, and Ordovician–Permian Tight Gas |
The units were drawn by shale thickness, organic carbon and thermal maturity.
A southwest–northeast cross section of the southern Arabian Peninsula: the Silurian source rock extends across much of it, above possible Ordovician sandstone reservoirs (modified from Konert and others, 2001). USGS.
The estimates
For all six units, the mean estimates are:
- Oil: 4,614 million barrels — about 4.6 billion (F95–F5 range 960 to 10,924).
- Gas: 561,022 billion cubic feet — about 561 trillion cubic feet (126,964 to 1,240,387). About 94 percent of it is in the southern units.
- Natural gas liquids: 3,056 million barrels (613 to 7,209).
F95 means a 95-percent chance of at least that much; F5, a 5-percent chance.
| Assessment unit | Oil, mean (million barrels) | Gas, mean (billion cubic feet) |
|---|---|---|
| Southern Lower Silurian Shale Gas | — | 420,198 |
| Southern Ordovician–Permian Tight Gas | — | 108,986 |
| Northern Ordovician–Devonian Tight Gas | — | 16,314 |
| Northern Lower Silurian Shale Gas | — | 12,601 |
| Northern Lower Silurian Shale Oil | 2,471 | 1,562 |
| Southern Lower Silurian Shale Oil | 2,143 | 1,361 |
More: USGS Energy Resources Program.
Sources
Based on Assessment of Continuous Oil and Gas Resources in Lower Silurian Shales of the Arabian Peninsula, 2019, by the Arabian Peninsula Assessment Team, USGS Fact Sheet 2019–3070, U.S. Geological Survey; a work of the United States government in the public domain. The three figures and the results by assessment unit are taken from the fact sheet's PDF, which the import had left out; the import had also run a list of citations into one sentence, restored here.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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