Research paper Characterization of the source rocks of a paleo-petroleum system (Cameros Basin) based on organic matter petrology and geochemical analyses Silvia Omodeo-Sal  e a, * , Isabel Su  arez-Ruiz b , Jos  e Arribas a , Ram  on Mas c , Luis Martínez d , Maria Josefa Herrero a a Departamento de Petrología y Geoquímica, UCM, IGEO (UCM-CSIC), Madrid, Spain b Instituto Nacional del Carb on, INCAR-CSIC, Oviedo, Spain c Departamento de Estratigrafía, UCM, IGEO (UCM-CSIC), Madrid, Spain d EOST, Universit e de Strasbourg, Strasbourg, France article info Article history: Received 29 January 2015 Received in revised form 4 January 2016 Accepted 4 January 2016 Available online 5 January 2016 Keywords: Source rocks Cameros Basin Hydrothermalism Paleo-petroleum system Organic petrography abstract The Cameros Basin is a paleo-petroleum system formed in the north-eastern Iberian Peninsula. The basin formed during the Mesozoic Iberian Rift and was later inverted during the Alpine orogeny. Hydrothermal events took place during the post-extensional and inversion stages, producing an important impact on the thermal history of the basin. In order to determine the source rock of the petroleum system of the basin, organic matter characteristics, the petroleum generation potential and the maturity of the basin infill are determined by means of petrology and geochemical analyses. Several organic rich units of the stratigraphic record of the basin are potential source rocks for hy- drocarbons generation, although their characteristics differ depending on their location. Organic matter content in the northern sector is scarce and limited to vitrinite, inertinite and solid bitumen particles. The residual total organic carbon (TOC < 1%), the low hydrocarbon potential (S2 < 0.3 mg HC/g rock, HI < 50 mg HC/g TOC), the mature to overmature thermal stages (%Ro from 1.7 to 4.6) and the presence of solid bitumen particles indicate that hydrocarbons have been already generated by these rocks. In contrast, the southern sector of the basin is characterized by abundant organic matter remnants (TOC from 2 to 17%) and immature to early oil-window thermal conditions (0.38e0.75%Ro), indicating a high hydrocarbonpotential for these rocks (S2 from 11 to 123 mg HC/g and HI values from 23 to 715 mg HC/g TOC). The different evolution of the source rocks in the basin is the result of the combination of differential subsidence rates, which determine different thermal histories and of the circulation of hydrothermal fluids in the northern-central sector during the evolution of the basin. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Source-rock characterization is an important risk-reduction tool that is essential to ensure an effective search in oil and gas pro- specting, especially in the case of basins with complex thermal evolutions and those affected by hydrothermalism (Tissot and Welte, 1984; Magoon and Dow, 1994). Source rock characteriza- tion includes the type and amount of organic matter contained in the rocks and their hydrocarbon generation potential. In the case of mature-to-overmature deposits, these data can be difficult to obtain. This paper presents an interesting case-study that is not commonly reported in the literature: a paleo-petroleum system (the Cameros Basin petroleum system) where the original source rocks are currently in an overmature thermal stage. For this pur- pose, the petrographic and geochemical “signature” of a source rock that has already completely generated and expelled hydro- carbons is reconstructed. The knowledge obtained could provide the key to determining whether the exploitation of a petroleum system is feasible. * Corresponding author. Current address: Direction G eosciences, IFPEN, Rueil Malmaison, France. E-mail address: silvia.omodeo@geo.ucm.es (S. Omodeo-Sal e). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo http://dx.doi.org/10.1016/j.marpetgeo.2016.01.002 0264-8172/© 2016 Elsevier Ltd. All rights reserved. Marine and Petroleum Geology 71 (2016) 271e287