The Vale das Fontes Formation in the Alcobaça area (Lusitanian Basin, Portugal): organic petrology characterization and thermal maturation A Formação de Vale das Fontes na área de Alcobaça (Bacia Lusitânica, Portugal): caracterização petrológica e maturação térmica

The Lower Jurassic of the Lusitanian Basin (Portugal) is characterized by marly–limestone sedimentary deposits, rich in organic matter (OM). This paper focuses on the study of the Vale das Fontes Formation (Pliensbachian) in the Alcobaça area. Its objectives are the petrographic characterization of the OM and evaluation of the thermal maturation based in organic petrographic analysis, such as vitrinite (%Rr) and solid bitumen reflectances and the identification of maceral groups, and organic geochemistry analyses, such as total organic carbon (TOC) and insoluble residue (IR). The petrographic analysis shows that OM belongs mostly to a type II kerogen. The mean random vitrinite reflectance ranges between 0.56–0.70 %Rr. The interpretation of solid bitumens reflectance and their optical characteristics allowed to distinguish three different families. The solid bitumen family with the lowest reflectance, which values range between 0.39–0.65 %Rr, was classified as grahamite. The solid bitumen families with reflectance values between 0.72–0.85 %Rr and 1.04–1.61 %Rr were classified as impsonites. The vitrinite reflectance indicates that the OM is thermally mature. TOC values range between 0.55–2.91 wt.%, pointing out a fair to very good content of organic carbon. IR values lower than 50% confirms the presence of carbonate sediments.