The U.S. Geological Survey (USGS) assesses the world's undiscovered oil and gas using a method based on geology. For the offshore Salt Basin area of Morocco it estimated mean undiscovered, technically recoverable resources of 3.8 billion barrels of oil and 20.7 trillion cubic feet of gas.
Location of the Offshore Salt Structures Assessment Unit off Morocco. U.S. Geological Survey.
How the basin formed
- Triassic rifting. As Africa began to pull away from North America, rift basins opened off Morocco, forming horsts and grabens trending northeast–southwest and north–south. By the Late Triassic up to 5 km of clastic sediment had filled them, topped by as much as 1.5 km of salt.
- Seafloor spreading began in the Sinemurian. Thermal subsidence and rising seas built broad carbonate platforms beside deep-water basins, which lasted into the Early Cretaceous; organic-rich calcareous shales settled in the basins.
- Burial. Continued subsidence drowned the platforms and pushed clastic sediment into deep water as slope and basin-floor systems — potential reservoirs. The added weight heated Triassic and Jurassic source rocks enough to generate oil and gas, and set the Triassic salt moving, possibly forming traps.
- Collision. In the Late Cretaceous and Paleogene, the opening South Atlantic turned Africa counterclockwise as it moved north, bringing it into collision with Europe and closing the western Tethys Ocean. Triassic rifts, including the Atlas rifts on land, were inverted. Erosion sent more sediment offshore in the Paleogene and Neogene, forming reservoirs and perhaps burying Cenomanian–Turonian source rocks deeply enough to generate oil and gas. Salt kept moving through the Neogene, which may have let some oil and gas escape.
Source rocks
The USGS defined a Mesozoic Composite Total Petroleum System fed by several organic-rich shales:
| Shale | Organic content (TOC) | Hydrogen index (mg HC/g TOC) | Thickness | Notes |
|---|---|---|---|---|
| Triassic synrift lake shales (below the salt) | — | — | — | no geochemical data |
| Lower Jurassic basinal calcareous shales | up to 8 wt% | up to 511 | up to 100 m | Type II marine; the most viable source rock offshore |
| Middle Jurassic shales | up to 6 wt% | up to 260 | under about 2 m | mixed marine/terrestrial; may be too thin |
| Oxfordian shales | up to 6 wt% | up to 200 | under 10 m | |
| Cenomanian–Turonian shales | up to 10 wt% | up to 500 | up to 50 m |
Modeling suggests the Triassic, Jurassic and Lower Cretaceous rocks may have begun generating oil and gas in the Late Cretaceous, and the Cenomanian–Turonian shales in the Neogene.
Where it could be trapped
The Offshore Salt Structures Assessment Unit covers oil and gas from all these sources that migrated into structural and stratigraphic traps. Possible reservoirs include:
- Triassic river, delta and lake sandstones, perhaps sealed by salt;
- Lower Jurassic to Lower Cretaceous reefs on carbonate platform margins, karst zones, debris flows off the platforms, and slope-channel and basin-floor sandstones;
- sandstones along the flanks of salt diapirs, sealed by salt, and basin-floor, slope-channel and salt-withdrawal-basin sandstones sealed by shale.
Neogene compression folded and faulted the rocks, which may have let trapped oil and gas move again or escape.
Key inputs
| Minimum | Median | Maximum | Mean | |
|---|---|---|---|---|
| Number of oil fields | 1 | 60 | 180 | 63.8 |
| Number of gas fields | 1 | 40 | 120 | 42.5 |
| Oil field size (million barrels) | 5 | 10 | 6,000 | 59.7 |
| Gas field size (billion cubic feet) | 30 | 60 | 36,000 | 358.4 |
The assessment unit's probability was set at 0.99.
Results
Fully risked estimates, where F95 means a 95% chance of at least that amount and F5 a 5% chance:
| Resource | F95 | F50 | F5 | Mean |
|---|---|---|---|---|
| Oil (million barrels) | 1,014 | 3,193 | 8,518 | 3,776 |
| Gas (billion cubic feet) | 4,952 | 16,825 | 50,363 | 20,743 |
| Natural gas liquids (million barrels) | 244 | 842 | 2,576 | 1,047 |
The means round to 3.8 billion barrels of oil, 20.7 trillion cubic feet of gas and 1.0 billion barrels of natural gas liquids.
Sources
- U.S. Geological Survey, Morocco Assessment Team (Schenk CJ, Mercier TJ, Woodall CA, et al.): "Assessment of undiscovered conventional oil and gas resources in the offshore Salt Basin area of Morocco, 2021," Fact Sheet, with input data in Schenk (2023).
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
1
0
0
0
Comments
