Organic maturation of the Algarve Basin (southern Portugal) and its bearing on thermal history and hydrocarbon exploration Paulo Fernandes a, * , Bruno Rodrigues a , Marisa Borges a, b, 1 , Vasco Matos a, 2 , Geoff Clayton c a Universidade do Algarve, CIMA, Campus de Gambelas, 8005-139 Faro, Portugal b LNEG, Rua da Amieira, 4465-965 S. Mamede Infesta, Portugal c Department of Geology, Trinity College, University of Dublin, Dublin 2, Ireland article info Article history: Received 25 August 2012 Received in revised form 17 June 2013 Accepted 18 June 2013 Available online 27 June 2013 Keywords: Algarve Basin Mesozoic Portugal Vitrinite reflectance Maturation Thermal history Hydrocarbons abstract The Algarve Basin, southern Portugal is under-explored in terms of petroleum geology. Organic matu- ration levels and the thermal history of this basin have been ascertained, together with their implications for future exploration. Maturity was determined using vitrinite reflectance and spore fluorescence/ colour. The succession was extensively sampled (158 onshore samples and 20 samples from two offshore wells). Thermal history was modelled using 1D PetroMod Ò . A palynostratigraphic study of two offshore wells was also completed, showing thick marlelimestone sequences of Middle and Upper Jurassic age. Hiatuses were identified in the offshore wells within the Jurassic section and between the Jurassic and the Miocene sections. The Mesozoic rocks of the Algarve Basin lie within the oil window. Vitrinite reflectance ranges from 0.52e0.7%R r in the Lower Cretaceous to 1e1.1%R r in the Upper TriassiceHettangian. Miocene rocks that unconformably overlie the Mesozoic strata are immature (0.42e0.47%R r ). Maturation levels increase with increasing age of the strata, indicating that burial was the main process controlling maturation. Thick marlelimestone sequences of the Middle Jurassic (Callovian) and the Upper Jurassic (Oxfordian) are the main source rock intervals. The Miocene successions of both offshore wells contains reworked vitrinite and palynomorphs with maturation levels similar to the Mesozoic rocks of the basin, suggesting expo- sure and erosion of these rocks during Miocene times. Palaeogeothermal gradients in the onshore Algarve Basin range from 52  C/km to 24.7  C/km with pre- Miocene exhumation estimated at 2000e2500 m. The probable age for the removed cover is Upper Cretaceous to Lower Palaeocene(?) and peak temperatures in the Mesozoic rocks were attained during latest CretaceouseEarly Palaeocene times. Modelled palaeotemperatures suggest that the Jurassic entered the oil-window at the beginning of the Cretaceous with peak oil generation in late Cretaceous time. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The Algarve Basin is a Mesozoic to Cenozoic sedimentary basin located in southern Portugal, cropping out along the entire south coast from Cape Saint Vincent in the west to the Guadiana river on the PortugueseeSpanish border in the east (Fig. 1). The basin ex- tends offshore, where it has been recognized in seismic lines and 5 hydrocarbon exploration wells (Fig. 1). The Mesozoic geology of the Algarve Basin is, therefore, better known onshore than offshore and comprises sedimentary rocks ranging in age from Upper Triassic to Lower Cretaceous, constituting a succession over 3 km thick in the depocentre of the basin in the central Algarve. The development of the Algarve Basin is related to consecutive phases of rifting associated with the break-up of Pangaea. Its origin, location, sedimentary record and evolution during the Mesozoic are closely related to a series of basins that developed during the opening of the North and Central Atlantic Ocean, including, for example, the Lusitanian Basin in central Portugal, the Porcupine Basin in offshore western Ireland and the Scotian, Whale and Jeanne d’Arc basins in offshore eastern Canada (Wilson et al., 1989). Although the main geological features of the Algarve Basin have been summarised in several publications (e.g. Manuppella et al., 1988; Terrinha et al., 2013), none of these discuss organic maturation * Corresponding author. Tel.: þ351 919707488. E-mail address: pfernandes@ualg.pt (P. Fernandes). 1 Current address: PetroStrat Ltd., Tan-y-Graig, Parc Caer Seion, Conwy, Wales LL32 8FA, UK. 2 Current address: Petrobras, Portugal, Lagoas Park Edifício 11 e 1  Norte 2740- 270 e Porto Salvo, Oeiras, Portugal. Contents lists available at SciVerse ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2013.06.015 Marine and Petroleum Geology 46 (2013) 210e233