The Mesozoic–Cenozoic organic facies in the Lower Tagus sub-basin (Lusitanian Basin,Portugal): Palynofacies and organic geochemistry approaches |
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| Institution: | 1. Centro de Geologia da Universidade do Porto, Rua Campo Alegre 687, 4169-007 Porto, Portugal;2. Laboratório de Palinofácies & Fácies Orgânica (LAFO), Departamento de Geologia, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira, 274, prédio do CCMN, sala J1020, Campus Ilha do Fundão, Cidade Universitária, CEP 21.949-900 Rio de Janeiro, RJ, Brazil;3. Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal;1. School of Energy Resources, China University of Geosciences (Beijing), Beijing, 100083, China;2. School of Geosciences, Yangtze University, Wuhan, 430100, Hubei, China;3. Department of Geoscience, University of Calgary, Calgary, Alberta, T2N 1N4, Canada;4. National Energy Heavy Oil Exploitation R&D Center, Panjin, Liaoning, 124010, China;1. Bio5 Institute, University of Arizona, Tucson, Arizona;2. Geisinger Health Systems, Danville, Pennsylvania |
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| Abstract: | Studies of the Mesozoic and Cenozoic sequence crossed by the Barreiro-4 borehole provide an improved understanding of the organic matter deposited in the Lower Tagus sub-basin (Lusitanian Basin, Portugal) and the implications for the potential source rock and depositional environment. This study focused on 43 samples (Middle Jurassic to Neogene) that were subjected to palynofacies and organic geochemistry analyses (Total Organic Carbon, Rock-Eval pyrolysis and molecular biomarker analysis). The palynofacies data indicate that the sequence contains mainly phytoclasts (non-opaque phytoclasts). However, the Middle Jurassic samples are dominated by Amorphous Organic Matter (AOM). Continental and/or marine palynomorphs are present in all the samples. The Cretaceous samples are characterized by small amounts of kerogen that have high contents of solid bitumen. The Total Organic Carbon (TOC) content is generally less than 1 wt.%. The Rock-Eval S1 and S2 parameters vary from 0.01 to 3.50 mgHC/g rock and 0.15 to 34.03 mgHC/g rock, respectively, with the highest values corresponding to the Cretaceous samples. The hydrogen index (HI) and oxygen index (OI) values range from 35 to 552 mgHC/g TOC and 4 to 180 mgHC/g TOC, respectively. The Tmax values range from 416 to 437 °C. The biomarker analysis showed that n-alkanes with 15–30 carbon atoms are present and usually have a unimodal distribution with a predominance of low to medium molecular weight compounds. The CPI values range between 0.63 and 3.65, and the pristane/phytane ratios vary between 0.48 and 1.64, indicating alternation of oxic–anoxic conditions along the sequence. The distribution of terpanes shows small amounts of tricyclic and tetracyclic terpanes in most of the samples (except for some Cretaceous samples) and a predominance of pentacyclic terpanes. The amount of 17α (H),22,29,30-trisnorhopane (Tm) usually exceeds the amount of 18α (H),22,29,30-trinorneohopane (Ts). The 20S/(20S + 20R) and αββ/(ααα + αββ) ratios of the C29 steranes generally have values below the range of equilibrium, indicating an immature stage of the OM. |
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| Keywords: | Organic matter Biomarkers Rock-Eval pyrolysis Solid bitumen |
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