The Porto Basin (NW Portugal) – a preliminary re-assessment of its evolution and petroleum potential

Abstract

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 Mesozoic infill is around 2.5 to 4 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 work is funded by IEFP and PARTEX Oil & Gas, as part of a Master's thesis at the Lisbon University supported by a PETROBRAS/GALP/PARTEX scholarship.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015