Petroleum Systems

 

Petroleum generation

 

Significant quantities of petroleum were generated in the Lusitanian and Porto basins, as shown by the numerous surface manifestations and evidence found in exploration wells. In the Algarve Basin, the oil shows found are less significant despite the fact that, in 2 of the 5 wells drilled to date, gas and/or oil shows were detected. As for the Alentejo Basin and the remaining 5 outer basins (in deep-offshore), nothing can be said since they were never drilled, but there is no reason to doubt the existence of a petroleum system in these basins as well.

 

At least two main petroleum systems in the Portuguese basins can be evidenced. One with Paleozoic source rocks, Upper Triassic (or more recent) reservoir rocks, and Lower Jurassic (or more recent) seal rocks; and the other with Mesozoic source rocks and reservoir rocks and Mesozoic and/or Cenozoic seals. The first "Paleo-Mesozoic Petroleum System" and the second "Meso-Cenozoic Petroleum System".

 

Source GPEP: Stratigraphic synopsis showing the main source rocks and reservoirs

 

The Paleo-Mesozoic Petroleum System:

 

Geochemical analyzes of samples of Paleozoic rocks collected from onshore Portugal revealed that marine sediments from the Carboniferous, outcropping in the Alentejo and Algarve, and black claystones from the Silurian, outcropping on the northeast edge of the Lusitanian Basin, may have been source-rocks for this petroleum system. We believe that these rocks that outcrop onshore extend below the Meso-Cenozoic basins, forming part of what, until recently, was called the Paleozoic basement.

 

Analyzes carried out on the black Silurian claystones - the most promising source rock - revealed TOC (Total Organic Carbon) values ​​between 0.5 and 1.5%, with some samples reaching 4%, and mean vitrinite reflectance values of 0.7%, which puts them right inside the oil window. On the other hand, the analysis of marine sediments of the Carboniferous showed TOC values ​​around 0.5%. Although a large part of these samples are overmature, with mean vitrinite reflectance values ​​of around 3%, some of the samples showed mean vitrinite reflectance values ​​between 1.5 and 2.5%, that puts them in the gas window - between wet-gas and dry-gas.

 

Source GPEP: Silurian rocks (Praia do Murração, Visean age)

 

This petroleum system was the object of investigation by Mohave Oil and Gas Corporation through the Aljubarrota wells. The Aljubarrota-2 well had as main objective, the Silves formation, constituted by clastic rocks with reservoir capacity and that Mohave believed could have commercial gas accumulations. Drilling was not a commercial success, despite having found good evidence of gas in this reservoir, but it had the merit of demonstrating the potential of this Paleo-Mesozoic petroleum system. Drilling also tested gas (350 Mcf/day) in the Jurassic fractured carbonates of the Brenha formation that may have originated in the Paleozoic source rocks.

 

The Meso-cenozoic Petroleum System

 

Source-rocks

 

Deep-sea marine paper shales (Source-Rock “Brenha”), rich in oil-producing organic matter (oil prone), were identified in the Lower Jurassic (Upper Sinemurian to Lower Toarcian - corresponding to the base of the Brenha formation) in the North of the Lusitanian Basin, either in boreholes or outcrops. Light oil, with low sulfur content, recovered drillstem tests in this area, is believed to have been generated by these source rocks. Geochemical analyzes of samples from the North of the Lusitanian Basin revealed source rock thicknesses between 140 and 190 m, TOC values ​​between 0.2 and 5.8% and mean vitrinite reflectance values ​​between 0.7 and 2.0%, which places these samples in the oil-gas window. These clays rich in organic matter seem to have been deposited, in considerable thicknesses, in the main depocenters in a reducing environment, while outside these depocenters they are not found or have negligible thicknesses. Source rocks of approximately the same age and lithology were found in wells drilled in the Porto Basin; these are likely to be richer and better developed outside the structures.

 

Source GPEP: Brenha formation, in Água de Madeiros

 

In the South Lusitanian Basin the sedimentary sequence of Upper Jurassic includes oil prone source rocks from the Upper Oxfordian (Source-Rock “Cabaços”). These source rocks occur as deep marine environment massive limestones and coastal to lacustrine bituminous limestones, either in boreholes or in outcrops, the latter being better developed in synclines. Geochemical analyzes of samples from the southern Lusitanian Basin (Torres Vedras - Montalegre) revealed source rock thicknesses between 20 and 110 m, TOC values ​​up to 3% and maturation levels that vary rapidly from immature to overmature. These rocks from the Cabaços formation (Source Rock “Cabaços”) are probably responsible for the various surface signs - seeps and impregnations - observed, as well as for the oil found in many of the wells drilled in this basin. The oil recovered in drillstem tests on Upper Jurassic sandstones in the Moreia-1 exploration well will likely have been generated by this source-rock.

 

Source GPEP: Cabaços Formation, Rocha Forte Quarry

 

Many other potential generator intervals have been identified. Among these are the intercalations of marls and black clays rich in organic matter (Dagorda formation), in Hettangian evaporites, and also the carbonates of Lower Sinemurian that overlap them. This sequence as generation potential for oil and gas in almost all places where it is exposed and where penetrated by wells, particularly in the central, deeper region of the Lusitanian Basin. However, its thickness and richness, in the observed places, only allows classifying it as a marginal to reasonable source-rock.

 

Source GPEP: Dagorda formation, black marls in A-da-Gorda outcrops

 

In the Algarve Basin, in the 5 wells drilled to date, only source rocks with low potential for gas and oil were observed in the immature section of the Neogene. Marginal source rocks, mainly gas prone, were also found in one of these drill-holes in the Lower to Middle Cretaceous section. Despite the poor quality of these results, the limited number of wells allows us to admit, by analogy with what happens in the Lusitanian Basin, the existence of richer and better developed source rocks in other places of this extensive and underexplored basin, in particular in the rocks from the oldest Mesozoic (Jurassic and Triassic) only penetrated by 1 exploration well.

 

In the remaining deep-offshore basins, with the exception of some ODP-DSDP drillings ("Ocean Drilling Program" - "Deep Sea Drilling Project") (see Map of ODP-DSDP Drillings), there is practically no direct information on quality, age, thickness and extent of the source rocks. We can, however, speculate that, taking into account the paleogeographic evolution of the Iberian plate, several source rocks may exist and have fed these outer basins. It also seems reasonable to believe that the Paleozoic source rocks, mentioned above, which appear to lie beneath these basins, may have generated petroleum. It is also believed that, both in the Jurassic and the Cretaceous, rich levels of organic matter were deposited in these basins. This is, moreover, proven by the DSDP Leg 47B (Site 398 - Vigo Submarine Mountain), ODP Leg 103 (Site 638 and 641 - Galicia basin) and ODP Leg 149 (Site 897 - Iberian Abyssal Plain) wells that recovered black clays from the Lower to Middle Cretaceous, rich in organic matter, in addition to a thin level in the Cenomanian-Turonian with values ​​of up to 13% organic carbon, 90% of which is derived from marine sources. Although the organic matter in some of these wells is immature, Site 897 contains Aptian-Albian black claystones that appear to be mature.

 

Reservoir rocks and seals

 

On outcrops around the Lusitânica, Alentejana and Algarvia basins, the coarse, clsatic and reddish sediments of the Upper Triasic age (Grés de Silves), which constitute the first deposits in these basins, have moderate to good reservoir rock characteristics. However, its grain and porosity generally decrease towards the interior of the basins and, in some wells that penetrated these sediments, they are shown as mediocre reservoirs. It should, however, be taken into account that we are talking, for the most part, of wells drilled in basement high blocks that could constitute, already at the time of the deposition of these sediments in the Triasic, small reliefs that would diverge from these places the assemblage of material. Thus, it is believed that the development of better reservoirs could occur along distribution channels that crossed all the basins. The extensive and sometimes thick sequence of Hettangian evaporites, which covers these sediments, constitutes the sealing rock for these Upper Triasic reservoirs.

 

Source GPEP: Silves formation, near Coimbra and in Algarve

 

The first carbonates deposited over the evaporitic sequence are limestone and dolomitic limestone from the Sinemurian (Coimbra formation) and include intervals, with vacuolar and fracture porosities and reasonable permeabilities.

With the exception of restricted fractured reservoirs, no other Lower to Middle Jurassic age reservoirs are known. The exception may be in the Algarve basin, where limestones and vacuolar dolomites, with porosities of up to 11%, were observed in boreholes and may be better developed elsewhere.

Carbonated reefs, mainly from the Upper Jurassic, are common in the Lusitanian Basin. Reasonable to good reservoirs are found locally in the Upper Jurassic of this basin, which can be either carbonate reefs or coastal clastics from the Kimeridgian to the Tithonian. The presence of reef structures with similar ages is assumed in the Porto basin.

 

Source GPEP: Candeeiros formation, near Leiria

 

In Early Cretaceous, the sands and poorly consolidated conglomerates that extend with a more or less constant thickness (300 to 400 m) over almost the entire Lusitanian Basin, have porosities of up to 35% and constitute an excellent reservoir (Torres Vedras formation). The sealing rocks for these reservoirs can be claystones intercalated in the clastic sequence or the Cenomanian marly limestones and marls. Seeps and oil impregnations occur in outcrops of these sandstones in places where salt diapirs pierce these sediments, namely in the northern onshore of the Lusitanian Basin.

 

Source GPEP: Torres Vedras formation, outcrops near Torres Vedras

 

Although there are no known reservoirs in the Cenozoic of the Porto and Lusitanian Basins, the Algarve Basin has good Miocene reservoirs, in sands with porosities of up to 35%, crossed by several wells. Sandy limestones of the same age can also constitute a good reservoir, with average porosities of up to 15% in this basin. By analogy, in the outer basins which, like the Algarve, have considerable thicknesses of Cenozoic sediments, it is natural that there are also good reservoirs.

 

Traps

 

Petroleum traps are stratigraphic and/or tectonic (or structural) structures that allow their accumulation. Both types exist in Portuguese basins, but probably more than 90% of the exploration wells drilled to date in Portugal, had structural traps as their objective. This probably reflects the fact that structural traps are easier to identify in seismic than stratigraphic traps. Another cause may have to do with the fact that structural traps often have greater accumulations of petroleum than stratigraphic traps, and at the time when most wells were drilled in Portugal, in the 70s and 80s, concessionaires were looking for larger petroleum fields.

 

Source GPEP: Structural/stratigraphic traps exemples drilled in Portugal

 

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).