II CENTRAL & NORTH ATLANTIC CONJUGATE MARGINS CONFERENCE

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1 ISBN: Volume III p II CENTRAL & NORTH ATLANTIC CONJUGATE MARGINS CONFERENCE Contribution to the knowledge of petroleum generative potential of Late Sinemurian Pliensbachian of the Lusitanian Basin - northern sector (Portugal) Silva, F. (1) ; Duarte, L.V. (2) ; Oliveira, L.C. (3) ; Rodrigues, R. (4) & Comas-Rengifo, M.J. (5) (1) Galp Exploração e Produção Petrolífera, S.A., R. Tomás da Fonseca, Torre A, Lisboa, Portugal. franciscosilva@galpenergia.com (2) Departamento de Ciências da Terra, IMAR-CMA, Faculdade de Ciências e Tecnologia da Universidade de Coimbra, Coimbra, Portugal. lduarte@dct.uc.pt (3) Petrobras/RH/UP/ECTEP, R. Ulisses Guimarães, 565, Rio de Janeiro, Brasil. lcveiga@petrobras.com.br (4) Dep. Estratigrafia e Paleontologia, Faculdade de Geologia, UERJ, Rio de Janeiro, Brasil. rene@uerj.br (5) Departamento-UEI de Paleontología, UCM-CSIC, José Antonio Novais, 2, Madrid, Spain. mjcomas@geo.ucm.es ABSTRACT The Late Sinemurian Pliensbachian interval of the Lusitanian Basin is characterized by marly limestone deposits sometimes with organic-rich layers. The study of these levels in outcrops located in the northern part of the Lusitanian Basin using some geochemical parameters such as total organic carbon (TOC) and Rock-Eval pyrolysis, allowed the definition of its petroleum generative potential with high stratigraphic accuracy. There are two stratigraphic intervals particularly rich in organic matter, which are positioned in the Polvoeira Member of Água de Madeiros Formation and in the Marly limestones with organic-rich facies member of the Vale das Fontes Formation. These intervals are characterized by a high frequency of TOC values greater than 1% and/or several high values that can reach about 10%. The Rock-Eval pyrolysis parameter S2 is frequently above 10 mg HC/g rock, with highest value of mg HC/g. The values of HI obtained for these intervals, very often larger than 150 mg HC/g TOC, show potential for generation of oil and gas-oil. KEYWORDS: Rock-Eval pyrolysis, petroleum generative potential, Lower Jurassic, Portugal. 1. Introduction The occurrence of organic-rich marls in the Lower Jurassic of the Lusitanian Basin (Portugal) has always been considered a potential oil generative rock, as confirmed by GPEP s most inclusive study of organic geochemistry (GPEP, 1996), which utilized a great number of samples of exploration wells and some samples of the outcrops of the Lusitanian and Porto basins. On the other hand, the organic geochemical and petrological study conducted by Dias (2005) in samples collected only in wells of the onshore basin discard this interval after finding only small levels of total organic carbon (TOC) within the interval. However, in the last years, based on a detailed stratigraphic framework (Duarte & Soares, 2002), some works/papers have been published focusing on the organic geochemical analysis of the late Sinemurian lower Toarcian of the Lusitanian Basin (e.g. Duarte et al., 2010; Oliveira et al., 2006; Silva et al., 2006, 2007). Through TOC and Rock-Eval pyrolysis analysis, this work presents a geochemical characterization of organic-rich sediments of the Água de Madeiros and 256

2 Vales das Fontes formations (late Sinemurian Pliensbachian; Duarte & Soares, 2002) that crop out in the northern sector of the Lusitanian Basin (FIG. 1). This work complements the evaluation of the petroleum generative potential by Oliveira et al. (2006) in the Peniche sector, with widening of the study to include the Água de Madeiros Formation, and adds a perspective of lateral variability with respect to the Vale das Fontes Formation. FIG. 1 Geological map of Lower Jurassic in Lusitanian Basin and stratigraphic chart with positioning of studied Sinemurian Pliensbachian outcrops: 1) Figueira da Foz; 2) Montemoro-Velho; 3) Coimbra; 4) Mealhada; 5) Anadia. 2. Material and methods The sampling was performed almost exclusively with dark gray and/or laminated marls, occasionally several samples per layer, where interesting vertical variations of facies occur. A total of 136 samples were collected for organic geochemical analysis of TOC and Rock-Eval pyrolysis. Every rock sample was first submitted to acidification with HCl to remove carbonate content and determine the insoluble residue (IR), then pyrolysed with Leco- SC444 equipment. The CO 2 resulting from the organic matter combustion in oxidant atmosphere was measured and results calculated in initial weight percentage (wt. %). The samples with TOC content greater than 1% were submitted to Rock-Eval pyrolysis, following the procedures of Espitalié et al. (1977). Four parameters were obtained: S1, S2, S3 and Tmax. The S1 and S2 parameters refer to the registered peaks of hydrocarbon release during heating up to 550ºC, expressed in milligrams (mg) of hydrocarbons (HC) per gram (g) of rock. The S3 parameter, expressed in mg CO 2 /g rock, is the quantity of CO 2 generated during the temperature increase of up to 390ºC. The Tmax parameter refers to the temperature that occurs at the maximum rate of S2 hydrocarbon generation. From these, new parameters derived include the Hydrogen Index (IH = S2/TOC x 100; in mg HC/g TOC) and Oxygen Index (OI= S3/TOC x 100; in mg CO 2 /g TOC), both used to classify the type of organic matter in terms of hydrocarbon generative potential (see Espitalié et al., 1977, 1985a,b, 1986; Peters, 1986). 3. Results and discussion In the group of outcrops studied, 136 samples were collected for TOC analysis, 47 of which analyzed by Rock-Eval pyrolysis. TOC content above 1% are present in each studied member (see Silva et al., 2006, 2007; Duarte et al. 2010), although the richest section of Marls and limestones with Uptonia and Pentacrinus (MLUP) with a maximum content of 3.79% are not shown in FIG

3 FIG. 2 Vertical distribution of IR, TOC, S2 and HI content of Vale das Fontes Formation in Figueira da Foz and Água de Madeiros in Montemor-o-Velho. The Rock-Eval pyrolysis reveals that the studied 8 m of Polvoeira member (PM) contains very high values of the S2 parameter, with mg HC/g rock and HI of mg HC/g TOC (FIG. 2), demonstrating type I and type II organic matter. On the other hand, there are layers rich in TOC but low S2. The study of this interval in S. Pedro de Moel in a more distal sector of the basin shows a larger and richer thickness of organic-rich levels (Duarte et al., 2010). The lack of Rock-Eval pyrolysis results does not reveal the real potential of MLUP. The most marly and organic-rich part of the member with TOC maximum of 3.79 wt% was sampled in the Montemor-o-Velho (Duarte et al., 2010) but was not analysed. The section shown here was obtained in more calcareous portion located stratigraphically above (FIG. 2). Few but very rich organic layers in the Lumpy marls and limestones member (LML) were found. The highest values of S2 parameter were mg HC/g rock in 258

4 Figueira da Foz (FIG. 2) and in Coimbra, with HI values of mg HC/g TOC and , respectively. These values are indicative of type II organic matter. In the Marly limestones with organic-rich facies member (MLOF), 26 samples were studied using Rock-Eval pyrolysis, with medium value of the S2 parameter reaching 7.14 mg HC/g rock in Figueira da Foz, 3.83 mg HC/g rock in Coimbra and 4.31 mg HC/g rock in Mealhada, and a maximum HI of mg HC/g TOC. The data presented are correlated with those of Oliveira et al. (2006) obtained in Peniche, but with lower values in every parameter. 4. Conclusions The data presented reveal that within the late Sinemurian - Pliensbachian of the Lusitanian Basin there exist at least two intervals with high hydrocarbon generation potential which correspond roughly to the Polvoeira and MLOF members. The PM contains organic-rich levels of Type I and Type II, and are therefore the most oil prone (see Espitalié et al., 1977; Peters, 1986). In the MLOF, the organic-rich levels of type II and type III are very frequent along the member (up to 28 m) throughout the basin, making it an interval of reference with potential to generate oil and gas. The remaining members of the Vale das Fontes Formation also present layers of high TOC content, which does not exclude the possibility a bigger importance in others parts of the Lusitanian Basin. Acknowledgements The authors thankfully acknowledge the laboratory work with organic geochemical analysis performed at Petrobras (Cenpes / PDEXP / GEOQ) and UERJ (Brazil), and Galp E&P for the general support References Dias, A. J. G., (2005) - Reavaliação do potencial petrolífero do onshore da Bacia Lusitaniana, Portugal. Departamento de Geologia Faculdade de Ciências, Universidade do Porto.142 p. (PhD thesis). Duarte, L.V., Soares, A.F. (2002) - Litostratigrafia das séries margo-calcárias do Jurássico Inferior da Bacia Lusitânica (Portugal). Comunicações do Instituto Geológico e Mineiro, 89, Duarte, L.V., Silva, R.L., Oliveira, L.C.V., Comas-Rengifo, M.J., Silva, F. (2010) - Organic-rich facies in the Sinemurian and Pliensbachian of the Lusitanian Basin, Portugal: Total Organic Carbon and relation to transgressive-regressive facies cycles. Geologica Acta, 8 (3), Espitalié, J.; Deroo, G.; Marquis, F. (1985a,b) - La pyrolyse Rock-Eval et ses applications: partie 1& 2. Revue de l'institut Français du Pétrole, Paris, v. 40, n.5 and 6, & Espitalié, J.; Deroo, G.; Marquis, F. (1986) - La pyrolyse Rock-Eval et ses applications: partie 3. Revue de l'institut Français du Pétrole, Paris, v. 41, n.1, pp Espitalié, J.; Laporte, J. L.; Madec, M.; Marquis, F.; Leplat, P. & Aulet, J. (1977) - Méthod rapide de caractérizationdes roches méres, de leur potential pétrolier et de leur aligré d' éolution. Revue de l'institut Français du Pétrole, Paris, v. 32, n. 1, pp Gabinete para a Pesquisa e Exploração de Petróleo (GPEP) (1996), Porto and Lusitanian Basins, Geochemical Analysis BEICIP-FRANLAB: Contribution to the MILUPOBAS Project, 9 vols., Lisbon. Oliveira, L.C., Rodrigues, R., Duarte, L. V. & Lemos V.B. (2006) - Avaliação do potencial gerador de petróleo e interpretação paleoambiental com base em biomarcadores e isótopos estáveis de carbono da seção Pliensbaquiano - Toarciano inferior (Jurássico Inferior) da região de Peniche (Bacia Lusitânica, Portugal). Boletim de Geociências da Petrobras, Rio de Janeiro, v. 14, n. 2, pp Peters, K., (1986) - Guidelines for evaluating petroleum source rocks using programmed pyrolyses. Bulletin of the American Association of Petroleum Geologists, Tulsa, 70(3), pp Silva, F., Duarte, L.V., Oliveira, L.C., Comas-Rengifo, M.J. & Rodrigues, R., (2006) - A Formação de Vale das Fontes no sector norte da Bacia Lisitânica (Portugal): caracterização e avaliação 259

5 preliminar de Carbono Orgânico Total. VII Congresso Nacional de Geologia. Livro de resumos, vol. II, Évora, pp Silva, F., Duarte, L.V., Oliveira, L.C., Rodrigues, R., Comas-Rengifo, M.J., (2007) - Caracterização do Carbono Orgânico Total e pirólise Rock-Eval no intervalo Sinemuriano superior-pliensbaquiano do sector norte da Bacia Lusitânica (Portugal). In: Gomes, E.P., Alencoão, A.M. (eds.). Vila Real, Proceedings of VI Congresso Ibérico de Geoquímica,