C02 Workshop

Source Rocks of the Onshore Kwanza Basin: A New Geochemical Approach Cristina Rodrigues1,2, Zélia Pereira3, Paulo Fernandes4, Domingos Cunha5, Fernando Gonçalves5, Ligia Barroso5, Sheila Barata1, Pedro Claude N´sungani6, Júlia Carvalho1, Manuel João Lemos de Sousa2 1 Halliburton, Rua Kwame N’Krumah, 217/221, Ed. Metropolis, 7ª, Sagrada Familia, Luanda, Angola 2 FP-ENAS, Universidade Fernando Pessoa, Praça 9 de Abril, 349, 4249-004 Porto, Portugal 3 LNEG, Rua da Amieira, Ap. 1089, 4466-901 S. Mamede Infesta, Portugal 4 CIMA-Centro de Investigação Marinha e Ambiental, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal 5 Sonangol, Rua 1º Congresso do MPLA nº 8/16 – C. Postal 1316 – Luanda, Angola 6 Departamento de Geologia, Universidade Agostinho Neto, Avenida 4 de Fevereiro nº 71, Luanda, Angola

Abstract The Onshore Kwanza Basin can be faced as a complementary sector in the Oil Scenario, considering the new crisis in the Oil Market. In fact, bearing in mind the already known resources, the Onshore can be considered the next promising exploration area, which is less expensive and the oil production would be quicker than expected in the offshore areas. The main purpose of this paper is to present a pioneer surface organic geochemistry study developed at a regional scale by Sonangol to characterize and identify potential source rocks. The 1/250 000 scale geological maps (Sonangol/Total 1987 and Sonangol/Obrangol 2015) were used to support the sampling programme developed. To implement this work were involved field sampling and laboratorial analyses. A number of 800 outcrop samples were collected in the entire basin, which were strategically selected considering the stratigraphic framework. The methods and analyses selected for the present study were palynostratigraphy, optical organic maturation and organic geochemistry (TOC, Pyrolysis, carbon stable isotopes, gaseous chromatography/mass spectrometry). Results suggest that some shale dominated layers of Quifangondo, Cunda/Gratidão, Teba and Cabo Ledo Formations, when compared to the rest of the analysed formations, present the highest potential source rocks, in specific areas of the basin.

First EAGE/ASGA Workshop on Petroleum Exploration 2-4 October 2017, Luanda, Angola

Introduction The Kwanza Basin, located in the central part of the Angolan coast, is one of the several basins within the West African Aptian Salt Basin, formed during the course of the South Atlantic Ocean opening. The rifting event of the Angolan margin began from south during the Late Jurassic to Early Cretaceous, which allowed the formation of the Aptian salt basin (Guiraud et al 2010, Brognon and Verrier 1966, Burwood 1999, Hudec and Jackson 2004). The region, nowadays, known as Gulf of Guinea was the last part of the African coast to be submitted to the opening of the South Atlantic Ocean, allowing the development of a confined anoxic sea from the Walvis Chain up to the North of Africa (Late Albian to Turonian). In Late Turonian, considering that continental margins were still separating, oxidizing conditions were developed between African and South American continents. The Kwanza Basin shows features archetypal of Atlantic-type Marginal Sag Basins filled with Meso-Cenozoic deposits, due to a complex geodynamic evolution, which can be characterized by three main stages, namely: Pre-rift, Syn-rift and Post-rift (Brownfield and Charpentier 2006, Hudec and Jackson 2004). The Kwanza Basin is separated into inner (onshore) and outer (offshore) sub-basins. However the two sub-basins are split into small-basins by basement structural highs, produced by major transform faults, where Aptian salt is usually thin or absent (Brownfield and Charpentier 2006, Hudec and Jackson 2004). The basement structural highs are Flamingo, Ametista, and Benguela Platforms combined with the Aptian salt migration, produced during the basement shortening and uplift, played a major role in the Kwanza Basin configuration. In fact, the independent evolution of both onshore and offshore basins resulted in quite different depositional environments but time-equivalent units. This study deals with the Onshore Kwanza Basin only, so a consensual stratigraphic summary (Figure 1) (Brognon and Verrier 1966, Brownfield and Charpentier 2006, Guiraud et al 2010) is presented. Pre-rift deposits (Jurassic) contain a terrigenous detrital sequence which was deposited in continentaldeltaic environments. Syn-rift sequence (Neocomian to Barremian or perhaps early Aptian) is represented by fluvial deposits and consists of conglomerates and coarse-grained sandstones (Red Cuvo Formation), lacustrine deposits consisting of shales, organic-rich shales, finegrained sandstones and siltstones (Grey Cuvo Formation), and conglomerate, red sandstone, claystone, and dolomite from transitional environment (Cuvo and Chela Formations), which embody the end of rifting. Post-rift deposits (Late Aptian to present-day) are represented mainly by marine strata, nevertheless continental conditions dominated from Pliocene to Pleistocene. The Loeme Formation (Late Aptian) represents the first marine transgressive deposits and, contains a carbonate sequence deposited in marine-lagoonal environment. The transitional carbonate-evaporities cycles from Late Aptian to Albian (Binga and Tuenza Formations) overlay the Loeme Formation. From Albian to Maastrichian widespread organicrich marine mudstones and marls (Quissonde to Teba Formations) developed laterally and above the carbonate units. Cenozoic times wereas dominated by progradational marine conditions. The Paleocene (Rio Dande Formation) and Eocene (Cunga/Gratidão Formation) are represented by regressive sandstones and siltstones, turbidites, and deep-marine deposits. During the Miocene (Quifangondo Formation), the marine conditions persisted and deposits are mainly represented by poorly sorted material and locally channel-filled sandstones and turbidities are identified. Finally, Pliocene and Pleistocene consists of continental terrigenous detrital sequence. The main objective is to present a general assessment of the outcrops source rock potential, considering specific stratigraphic intervals and lithofacies, in order to improve the maturation model and refine the stratigraphy of the onshore Kwanza Basin. This work involved palynostratigraphy and the assessment of organic maturation, supported by organic petrology and geochemistry techniques. The results presented in this paper are one of the tasks that have been developed in the Geochemical Project of the Onshore Kwanza Basin conducted by Sonangol EP.

First EAGE/ASGA Workshop on Petroleum Exploration 2-4 October 2017, Luanda, Angola

Figure 1 Stratigraphic column and summary of tectonostratigraphic events in the Inner Kwanza Basin (Guiraud et al 2010). Material and Methods In the last decades, an abundant data set of the potential source rocks of the Onshore Kwanza Basin was been systemically collected. Although, the current maturation model has some uncertainties, mainly due to the basin complex geodynamics (Brognon and Verrier 1966, Brownfield and Charpentier 2006, Burwood 1999, Burwood et al 1992, Duval et al. 1992, Hudec and Jackson 2004, Jackson and Hudec 2009, Jackson et al. 2000, Schiefelbein et al. 2000). This study involved field work, with the sampling of 800 rock samples in outcrops (mainly shales and marls) using a systematic methodology, supported by the Geological Maps (Sonangol/Total 1987 and Sonangol/Obrangol 2015) and regional Kwanza Basin key parameters, such as: the tectonic-sedimentary framework, the stratigraphic sequence, and the geographic distribution, in order to make the samples set representative of the complex geology of the basin. Standard palynological laboratory procedures were employed in the extraction and concentration of the palynomorphs. The slides were examined with transmitted light, with a BX40 Olympus microscope equipped with an Olympus C5050 digital camera. Mean random vitrinite reflectance in oil immersion (%Ro) was the vitrinite reflectance (VR) parameter chosen for maturation assessment of the shale and marls samples because the mounting techniques used provide non-oriented vitrinite particles. VR measurements on all samples were made using an Olympus BX 51 microscope equipped with a black and white digital camera. The greyscale (8-bit) digital images of vitrinite particles were analysed using a MatLab routine. This routine is a graphical tool that runs within the MIRONE suite and calibrates the scale of 256 grey levels with standards of known reflectivity. The reflectance values of the standards used were: 0.428 %, 0.595 %, 0.897 %, 1.314 %, 1.715 %, 3.15 % and First EAGE/ASGA Workshop on Petroleum Exploration 2-4 October 2017, Luanda, Angola

5.37 %. VR was measured in incident light with a wavelength of 546 nm and immersion oil with a refractive index of 1.518. Organic geochemistry analyses of TOC (Total Organic Carbon), Pyrolysis (Rock-Eval) Biomarkers and Stable Carbon Isotopes were performed by GEO-Data according to NCS DC 73025, IFP 160000, and NBS 22/NBS 21 standards, respectively; Gas Chromatography/Mass Spectrometry was carried out by ACTLABS according to ASTM designation of E-1618, which analysis requires a preparation of 0.5 g of sample in order to achieve whole extracts without any fractionation. Results The following results produced, up to now, by Sonangol embody a first study of the Regional Geochemical Project of the Onshore Kwanza Basin, only. The integration of all the analytical results was done and the interpretation of the results was analysed taking into account each formation throughout the entire basin. Palynostratigraphic studies and the source rocks potential assessment were not accomplished in some of the stratigraphic units, due to the lack of amount and well-preserved organic matter and to the insufficient number of representative outcrops, which should be culminated in future works. Taking into consideration the results obtained from the samples set collected during the field sampling programme, it is possible to reach some pertinent results. Data obtained from Binga and Mucanzo Formations suggest kerogen type II, II/I, II/III and due to sampling locations, it means, basement proximity, do not show positive maturation indices. A preliminary age of Late Albian was suggested for the analysed samples collected in the Quissonde Formation (a lateral facies variation of Mucanzo Formation), supported by dinoflagellates cysts and pollens. The Cabo Ledo Formation comprehends mainly kerogen type II, I/II, III and the samples analysed show immaturity/early oil window stage. A terminal Late Maastrichtian age was suggested for samples analysed in the Teba Formation, according to dinoflagellates; and the optical and geochemical maturation techniques suggest for the analysed samples kerogen type II, I/II, II/III, and the maturation parameters show, depending on sampling locations, immature and mature data. The Rio Dande Formation is characterized by kerogen type II, II/I, II/III and, as seen in the previous formation, show immature and mature samples depending on sampling location. The dinoflagellate cysts and rare pollens identified in the samples collected in the Gratidão Formation indicated a Late Chattian age; those samples are characterized by kerogen type II, II/I, I/II, II/II, and show immature and mainly mature indices. The Cunga Formation results suggest kerogen type II, I/II, II/III, and mainly mature samples. The analysed samples from the Quifangondo Formation suggested a Late Burdigalian age, according to a well preserved dinoflagellates association; the organic maturation assessment suggests kerogen type II, II/III, II/I, and mainly mature samples (peak oil window). Conclusions and final remarks This study was based on the tectonic-sedimentary framework and the analytical data produced from samples collected in outcrops throughout the entire Onshore Kwanza Basin. Nevertheless, this preliminary assessment of the already available Onshore Kwanza Basin Geochemical Database allows us to reach the following conclusions: a) the organic maturation does not increase with the increase age of the stratigraphic unit; in fact, samples analysed from the older stratigraphic units present the lower maturation values, and are assigned to samples collected near the border of the sedimentary basin. Lower maturation values can be justified by the low subsidence rates induced by the competent basement proximity, and due to the location of samples collected (fault footwall blocks forming structural highs); b) the stratigraphic units showing the highest source rock potential among all the analysed stratigraphic units are Quifangondo, Cunga, Gratidão, Teba and Cabo Ledo Formations. However, this is only valid in some specific areas of Kwanza Basin, since petroleum potential is highly dependent on the geodynamic model; c) Formations presenting low source rock potential is mainly related to oxidizing depositional conditions since they did

First EAGE/ASGA Workshop on Petroleum Exploration 2-4 October 2017, Luanda, Angola

not favour the preservation of organic matter, and we suggest that in future studies these should be targeted for resampling; d) the lower and upper limits of the analysed stratigraphic units must be confirmed in future studies, as well; e) furthermore, the integration of the results obtained from this work with the subsurface pre-existing data will be a crucial element to enhance the presented interpretation. Acknowledgements The authors are grateful to Sonangol EP, Halliburton (Landmark), Previsão Oil, Fundação Fernando Pessoa/Universidade Fernando Pessoa (FPENAS Research Unit), LNEG (Laboratório Nacional de Energia e Geologia), UALG (Universidade do Algarve) and GEOData for the excellent collaboration to produce this study. References Brognon, G.P. and Verrier, G.R. [1966] Oil and Geology in Cuanza Basin of Angola. Bulletin of the American Association of Petroleum Geologists, 50(1), 108-153. Brownfield, M.E. and Charpentier, R.R. [2006] Geology and Total Petroleum Systems of the West-Central Coastal Province (7203), West Africa. U.S. Geological Survey Bulletin 2207-B, 1-52. Burwood, R. [1999] Angola: source rock control for Lower Congo Coastal and Kwanza Basin petroleum systems. In: Cameron, N.R., Bare, R.H. and Clure, V.S. (Eds) The Oil and Gas Habitats of the South Atlantic. Geological Society, London, Special Publications, 153, 181194. Duval, B., Cramez, C. and Jackson, M.P.A. [1992] Raft tectonics in the Kwanza Basin, Angola. Marine and Petroleum Geology, 9, 389-404. Guiraud, M., Buta-Neto, A. and Quesne, D. [2010] Segmentation and differential post-rift uplift at the Angola margin as recorded by the transform-rifted Benguela and oblique-toorthogonal-rifted Kwanza basins. Marine and Petroleum Geology, 27, 1040–1068. Hudec, M.R. and Jackson, M.P.A. [2004] Regional restoration across the Kwanza Basin, Angola: Salt tectonics triggered by repeated uplift of a metastable passive margin. American Association of Petroleum Geologists Bulletin, 88(7), 971-990. Jackson, M.P. and Hudec, M. [2009] Interplay of Basement Tectonics, Salt Tectonics, and Sedimentation in the Kwanza Basin, Angola. American Association of Petroleum Geologists International Conference and Exhibition – Cape Town, Article 30091. Schiefelbein, C.F., Zumberge, J.E., Cameron, N.C. and Brown, S.W. [2000] Geochemical Comparison of Crude Oil Along the South Atlantic Margins. In: Mello, M.R. and Katz, B.J. (Eds.) Petroleum systems of South Atlantic margins. American Association of Petroleum Geologists Memoir, 73, 15-26. Sonangol/Total [1987] Carte Geologique du Bassin du Kwanza, Angola. (1:250 000). Sonangol/Obrangol [2015] Carta Geológica da Bacia do Kwanza, Angola. (1:250 000).

First EAGE/ASGA Workshop on Petroleum Exploration 2-4 October 2017, Luanda, Angola