Research paperGeochemical characterization of oils and their correlation with Jurassic source rocks from the Lusitanian Basin (Portugal)
Introduction
In the Lusitanian Basin, located in western-central Portugal (Fig. 1), non-commercial oil accumulations were discovered in wells drilled in the north and central sectors, and oil shows were found in wells drilled in the southern sector (Baptista, 2002).
Except for data in unpublished reports (e.g. Core Lab, 1983, Beicip-Franlab, 1996), there is only one work available (Spigolon et al., 2010) about oil-source rock correlation in the Lusitanian Basin. According to that study, the oils recovered from Lower Jurassic reservoirs in the northern sector were generated from organic matter from the Lower Jurassic, whereas organic matter from Cabaços Formation (Fm.) (middle Oxfordian; Kullberg and Rocha, 2014a) was the source rock for oils found in Upper Jurassic reservoirs in the central sector of the basin (Fig. 2). As to the origin of the oil shows of the southern sector, recovered in Upper Jurassic reservoirs (UPEP Archives; Fig. 2), nothing can be said, since they were never studied.
Prior published works (Oliveira et al., 2006, Silva et al., 2010, Silva et al., 2011, Silva et al., 2012, Silva et al., 2015, Duarte et al., 2010, Duarte et al., 2012, Duarte et al., 2013, Spigolon et al., 2011, Correia et al., 2012, Poças Ribeiro et al., 2013, Gonçalves et al., 2015, Silva and Duarte, 2015) recognized that the potential source rocks of the oils found in the Jurassic of the Lusitanian Basin occur as follows: in the northern sector, in the Coimbra Fm. (lower-upper Sinemurian; Duarte et al., 2014a), Polvoeira Member (Mb.) of Água de Madeiros Fm (upper Sinemurian-lower Pliensbachian; Duarte et al., 2014b) and in the Marly limestones with organic-rich facies (MLOF) Mb. of the Vale das Fontes Fm. (lower-upper Pliensbachian; Duarte and Soares, 2002); and in the central sector, in levels equivalent to the Moleanos Mb. of Santo António-Candeeiros Fm. (Callovian; Azerêdo, 2007), Cabaços Fm. and Montejunto Fm (middle-upper Oxfordian; Kullberg and Rocha, 2014a) (Fig. 2). In the southern sector (e.g. Barreiro region), despite geochemical studies (Spigolon et al., 2011, Gonçalves et al., 2014), no source rocks have been identified yet.
In order to contribute towards recognizing the oil groups accumulated in the Jurassic of the Lusitanian Basin, stable carbon isotope and gas chromatography coupled with mass spectrometry analyses were performed on nine oil samples recovered in Coimbra Fm., Água de Madeiros Fm., Montejunto Fm. and Abadia Fm (lower-upper Kimmeridgian; Kullberg and Rocha, 2014b) and two oil shows samples recovered in Montejunto Fm. (Fig. 2), gathered from the main discoveries reported in Baptista (2002). Representative samples of the different potential source rocks were collected from sections with oil source potential, according to previous works. Total organic carbon (TOC) and Rock-Eval pyrolysis analyses were performed on twenty-five rock samples from Coimbra, Água de Madeiros, Vale das Fontes, Santo António-Candeeiros, Cabaços and Montejunto formations (Fig. 2).
After this procedure, stable carbon isotope and gas chromatography coupled with mass spectrometry analyses were performed on the organic extracts of the potential source rocks. The results obtained with these techniques were compared with those from the analyzed oils using the cluster analysis, in order to establish the oil-source rock correlations of the various admitted petroleum systems in the Lusitanian Basin.
Section snippets
Geological setting
The tectono-stratigraphic evolution of the Lusitanian Basin reflects major controlling factors and multi-stage history typical of ocean margin rift-related basins, in this case in relationship with the opening of the North-Atlantic Ocean (Wilson et al., 1989, Pinheiro et al., 1996, Rasmussen et al., 1998, Pereira and Alves, 2012). In this framework, the Jurassic of the Lusitanian Basin can be characterized by the succession of two 1st-order cycles (Fig. 2) (Wilson et al., 1989): Upper
Oil and oil shows samples
Oil samples of 14A-1 and MR-W9 wells, located in northern sector of the Lusitanian Basin, of Bf-1, TV-4, Ba-1, Se-1, Ald-6, FR-1 and Ab-1a wells, located in central sector of the Lusitanian Basin, and oil show samples of Br-1 and Br-4 wells, located in southern sector of the Lusitanian Basin (Fig. 1) were studied.
The 14A-1 oil was recovered in fractured marly limestones from the top of the Coimbra Fm., between 2375.00 and 2404.00 m (UPEP Archives) (Table 1).
The MR-W9 oil was recovered in
Biodegradation
The absence of linear alkanes, pristane and phytane in Ald-6, FR-1 and Ab-1a (Fig. 3D) oils, indicate that they were biodegraded after accumulation (Chosson et al., 1992), attaining at least level 5 of biodegradation in the Peters and Moldowan (1993) scale. The C29ααα(S/S + R) steranes ratio > 0.56 in FR-1 and Ab-1a oils (Fig. 4B–C and Table 3) also provide further evidence of biodegradation, level 6 in the Peters and Moldowan (1993) scale. The presence of biodegradation in Ald-6, FR-1 and
Conclusions
In the northern sector of the Lusitanian Basin, the oil of 14A-1 well was generated and accumulated in the Coimbra Fm., while that of MR-W9 well had its origin in that unit, but migrated and accumulated in the Água de Madeiros Fm. Of the oils belonging to this sector, the one from 14A to 1 was formed under a higher maturation level.
In the central sector, the oil of Bf-1 well occurs in the unit (base of Montejunto Fm.) where it was generated. The other oils, TV-4, Ba-1, Se-1, Ald-6, FR-1 and
Acknowledgements
The authors thank Galp Energia, for its financial support; Mohave Oil and Gas Corporation; UPEP/ENMC and LNEG, for allowing access to the data on exploratory wells; LGQM/UERJ for the geochemical analyses; Maria Virgínia Alves Martins by the technical support during the cluster analysis; and two anonymous reviewers for the helpful comments that improved the manuscript. Marco Brito also thanks Capes for the doctoral scholarship received.
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