USGS Assessment: Undiscovered Oil and Gas of Portugal's Lusitanian Basin (2016)

The Lusitanian Basin Province and its three assessment units. Base: National Park Service. USGS.

Using a geology-based method, the USGS estimates that 121 million barrels of oil and 212 billion cubic feet of gas (mean estimates) are undiscovered and technically recoverable in the Lusitanian Basin Province of Portugal.

The basin

• Where: about 20,000 square kilometers along Portugal's western margin.
• Origin: one of the extensional basins formed as Mesozoic Tethyan and Atlantic rifting began, before the north-central Atlantic Ocean opened.
• A complicated history: four phases of Mesozoic extension, two of Cenozoic compression, and salt movement left it complexly structured and split into several subbasins.
• Source rocks: organic-rich, thermally mature shales of the Lower Jurassic Brenha Formation, and possibly Silurian shales in the north.

How the oil and gas got there

Conventional (Paleozoic–Mesozoic Composite Total Petroleum System; Lusitanian Basin Conventional Reservoirs assessment unit):

• Oil and gas from Lower Jurassic source rocks formed in the Cretaceous and migrated into Jurassic carbonate and Cretaceous clastic reservoirs.
• Silurian source rocks may have generated oil, possibly in the Late Jurassic, mostly cracked to gas and trapped in Triassic rift sandstones under Jurassic shale seals.
• The main risk: oil and gas may have leaked away or moved after Cenozoic compression and structural inversion. North of the Nazaré Fault, compression was weaker, so risk may be lower there than to the south.

Continuous (Lower Jurassic Total Petroleum System; Brenha Formation Shale Oil and Shale Gas units):

• Oil and minor gas formed in the Brenha source rocks in the Cretaceous, with some retained in the shale through the two phases of compression. The units were outlined using thermal maturity, source-rock thickness and organic carbon data.
• The main risk: whether recoverable oil or gas stayed in the source rocks after compression.
• Analogs: well drainage areas, ultimate recoveries and success ratios come from U.S. shale-oil and shale-gas plays.

Results (mean estimates, with F95–F5 range)

F95 is a 95 percent chance of at least that amount. The assessment-unit probabilities were 0.5 (conventional), 0.6 (shale oil) and 0.5 (shale gas), which is why every range starts at zero.

Sources

Based on Assessment of Undiscovered Oil and Gas Resources of the Lusitanian Basin Province, Portugal, 2016, by the Lusitanian Basin Province Assessment Team (Christopher J. Schenk and others), USGS Fact Sheet 2016–3086, U.S. Geological Survey (DOI), citing Rasmussen and others (1998), Alves and others (2003), dos Reis and others (2014), Uphoff (2005) and McWhorter and others (2014); a work of the United States government in the public domain. The map and results table come from the fact sheet's PDF. More: USGS Energy Resources Program.