The Porto Basin (NW Portugal) a preliminary re-assessment of its evolution and
petroleum potential
João Casacão (1); Nuno Pimentel (2); Cláudia Paiva Silva (1),
Rui Pena dos Reis (3)
1 Partex Oil & Gas (jcasacao@partex-oilgas.com, cvsilva@partex-oilgas.com)
2 University of Lisbon (pimentel@fc.ul.pt), 3 University of Coimbra (penareis@dct.uc.pt)
The Porto Basin is a Meso-Cenozoic rift-type basin, located in the northwest Iberian
margin, a typical non-volcanic passive margin. This offshore basin connects to the Galicia
Basin (to the N), and the Lusitanian and Peniche Basins (to the S and SW). The basin
presents a general NNW fault-bounded trend, divided by conjugate ENE-WSW structures
into two tectonic domains. The Porto Basin has been explored in the past, with several 2D
seismic surveys (1973-2002) and five shallow offshore wells (1975-1995). However, due to
poor hydrocarbon occurrences, its regional evolution and petroleum systems have not been
fully addressed since then, except for its post- breakup sequences (Alves, 2003; Soares et
al., 2012). In this abstract we present a synthesis of the tectono-sedimentary history of this
area, as well as some preliminary results concerning maturation modeling and its impact on
petroleum potential.
Five well-reports have been consulted and studied (Lu-1, 5A-1, Cv-4, Li-1 and To-1Z),
including wire-line and geochemical data, of which only one Lu-1 reached the Upper Triassic
evaporites and 5A-1 reached the Paleozoic basement. The total thickness of the Meso-
Cenozoic infill is around 2.5 to 6.5 km. Upper Triassic redbeds (Silves Fm.) and evaporites
(Dagorda Fm.) seem to be present along the basin (less than 1 km thick), although
apparently there are no diapiric structures. The Jurassic formations are irregularly present
along the wells (around 0.5 to 1.5 km thick) probably due to differential block subsidence
and/or erosional unconformities. The Cretaceous overburden is also identified on the wells,
and its regional distribution is fairly regular (1 km to over 2.5 km thick). The Cenozoic cover
is usually absent (except 5A-1 well, 130 m thick), which can indicate that it was either
affected by late erosion, due to the tectonic inversion during the Alpine compression, or it
was not deposited at all. Seismic interpretation from several 2D surveys covered the
continental shelf and also the distal part, immediately after the continental slope. Based on
the identification of erosional truncations and tectono-stratigraphic correlation with the
studied wells, the main identified unconformities are Callovian (top of the limestone Esturjão
Fm.), Berriasian (top of the limestone Linguado Fm.) and Cenomanian (top of the dolomitic
Cacém Fm.).
The main seismo-stratigraphic packages were identified and correlated with the main
tectonic and rifting events that controlled the basin’s evolution. Based on well reports,
petroleum systems have been defined and characterized in different places of the basin.
Geochemical data from Lu-1, Cv-4 and 5A-1 wells indicate that most of the samples have
type-III and type-II kerogen (gas- and oil-prone), while TOC values range from 0.5 to 1.6
wt%. Most well reports characterize several units with good reservoir properties. In the Lu-
1 well, there were both light-oil and gas shows, in different reservoirs, while in the To-1Z
there were poor oil and gas shows. The main source-rock for oil and gas is considered to
be Lower Jurassic marly shales, but Upper Jurassic oil and Late Paleozoic basement gas
cannot be excluded.
Backstripping, burial evolution and heat-flow variations were defined to set the boundary
conditions for thermal modelling using PetroMod 1D. Preliminary results show that, in some
wells, units with potential as source-rocks entered the oil and even the gas window,
generating hydrocarbons for identified potential reservoir units. The final results of this
project will identify the most promising sectors and contribute to the re-evaluation of the
area’s petroleum potential, thus contributing for future exploratory approaches.
Acknowledgements This project is part of a Master’s Thesis at the University of Lisbon. The authors would like to thank
Partex Oil & Gas and UPEP for their technical and professional support, as well as permission to publish the interpreted
seismic lines. A special note to IEFP financial support and PETROBRAS / GALP / PARTEX scholarship.
Lisbon, May 2015
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