Sedimentary Basins

 

The new seismic and gravity data acquired by TGS-NOPEC allowed for a better understanding of the onshore and shallow offshore sedimentary and also allowed to recognize new basins developed in deeper waters.

 

It is possible to group these Meso-Cenozoic basins into two groups:

 

Interior basins - located in the interior region of the continental margin and often extending to onshore:

Porto Basin

Lusitanian Basin

Algarve Basin

Outer basins - located in deep waters to the west and south of the former:

Inner Basin of Galicia

 Peniche Basin

Alentejo basin

Sagres Basin

Gulf of Cadiz Basin

 

Source GPEP: Meso-Cenozoic sedimentar basins

 

All these basins were created by the tectonic processes that led to the opening of the North Atlantic Ocean.

 

Of these basins, the Porto Basin, located further north, has a length of 100 km and extends beyond the Portuguese-Spanish maritime border. The basin is developed exclusively offshore and covers an area of ​​about 2150 km² - up to 200 m bathymety - or 2800 km2 - up to 1000 m bathymetry - in Portuguese waters. The Mesozoic sediments deposited in this basin can reach 8 km in thickness and are covered by a thin layer of Cenozoic sediments.

 

The Lusitanian Basin, located south of the Porto basin, is the largest of the Portuguese interior basins and extends from onshore to offshore with an area of ​​about 22000 ². The sedimentary record that fills it, about 6 km thick, is similar in age to that of the Porto basin but presents a greater thickness of sediments from the Jurassic, compared to those from the Cretaceous. As with the Porto Basin, here too the thickness of the Cenozoic sediments is small.

 

The Algarve Basin, with about 8500 km², is located in the south of the country and extends onshore and offshore, more or less parallel to the coastline. This basin continues to the east with the name of Cádiz Basin, already in Spanish waters. The sedimentary depth up to the Carboniferous base can exceed 7 km and the filling is, also here, constituted by sediments from the Upper Triassic to the Cenozoic. The relative thickness of the Cenozoic and in particular the Neogene sediments is greater than the thicknesses observed in the two interior western basins described above.

 

The evolution of the outer basins is not so well known as most data covering this area is only recently available. The total area of ​​the outer basins is still unknown. The most recent data acquired provides an idea of ​​the shapes of these basins, but their boundaries are still uncertain. The same can be said for sediment thickness and sediments age. However, we can state that the thickness of the Cenozoic sedimentary record seems significantly thicker than what is observed in the interior basins of the western margin, which allows us to think about a promising future for the research of these basins.

 

Other studies are also encouraging, regarding the national petroleum potential, such as the studies of the conjugated margins. Before the geological events that triggered the opening of the Atlantic, the regions where these basins were formed, namely those on the west coast, were geographically very close to the regions of the current Grand Banks, in Newfoundland, Canada. The fact that the Portuguese sedimentary basins have a common initial genesis and present geological similarities with the basins on the other side of the Atlantic, namely Canada, where the generation of oil and gas was revealed and where its production occurs, for example, in Jeanne D'Arc Basin is very promising. As far as the Algarve margin is concerned, the Algarve Basin, in Portuguese waters, constitutes the geological continuity to the West of the Cádiz Basin, in Spanish waters. as is known, natural gas has been produced for more than two decades, raising the probability that gas reservoirs will also be identified in the Algarve Basin, whose geological modeling also indicates the possibility of the occurrence of petroleum generation, as well as of their migration and their accumulation in geological structures.

 

All the necessary ingredients for a favorable petroleum system in which there may be commercial petroleum accumulations, occur in Portugal: there are good source rocks that have reached the generation stage, good reservoirs adequately covered by sealing rocks and an abundance of structures that provide good traps. It was just not possible to find all the ingredients in the right combination yet, despite the numerous studies and investigation carried out in the country.

 

In fact, the data acquired over 8 decades of exploration allowed the identification of several geological levels potentially generating oil and gas in the Jurassic and the Cretaceous, having been recognized two main petroleum systems in the interior Portuguese basins: the Paleo-Mesozoic Petroleum System and the Meso-Cenozoic Petroleum System.

 

Through this data, hydrocarbons shows were revealed in 59 onshore wells and in 7 offshore wells and in the tests carried out it was possible to recover hydrocarbons in 24 onshore wells and in 2 offshore wells. However, it is important to note that 89% of the onshore wells are more than 50 years old and that of the 27 offshore wells, 85% were carried out more than 30 years ago. This shows the difficulty of understanding geology of great complexity such as the Portuguese continental shelf, without new geological and geophysical data acquisition operations continued, especially at the level of the outer basins, where well drilling has never been done, which makes it very difficult to conclude on the quality, age, thickness and extension of any source rocks and reservoir rocks.

 

However, there is no reason to doubt the existence of petroleum systems in these basins as well, taking into account their paleogeographic evolution, as well as the thick sedimentary sequences of the Mesozoic and Cenozoic, especially in the Peniche and Alentejo Basins, inferred from seismic exploration surveys, carried out between 2008 and 2015, which also provided good indications of the presence of hydrocarbons and allowed the identification of geological structures, whether of a tectonic or stratigraphic nature, promising and sufficiently attractive for the continuation of the exploration.

 

Thus, despite the fact that some exploration has been carried out over several decades in the interior sedimentary basins, it can be considered that these are still undervalued. Even the Lusitanian Basin, the most researched of the Portuguese basins, with a density of wells in the order of 2.4 per 1,000 km², is a good example. As for the outer basins, in which data, essentially indirect, were acquired only in recent years, and where no wells were drilled, they continue to constitute an opportunity for investigation, and the continuation of exploration work is essential to increase knowledge of the its petroleum potential and resources, as well as the geological infrastructure of the basins.

 

For more information “Livro Verde sobre a prospeção, pesquisa, desenvolvimento e produção de hidrocarbonetos em território nacional”, prepared by the former National Entity for the Fuel Market, E.P.E. (ENMC), current National Entity for the Energy Sector, E.P.E. (ENSE) (See Documentation/Disclosure - Publications).