Organic maturation of the Algarve Basin (southern Portugal) and its bearing on thermal history and hydrocarbon exploration

• The Mesozoic rocks of the Algarve Basin (Portugal), lie within the oil window.
• Peak oil generation occurred in late Cretaceous time.
• Exhumation in the onshore is 2000–2500 m and occurred prior to Miocene times.
• The probable age for the removed cover is Upper Cretaceous-Paleogene.
• Palynology indicates major hiatuses in the offshore part of the Algarve Basin.

The Algarve Basin, southern Portugal is under-explored in terms of petroleum geology. Organic maturation 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®.A palynostratigraphic study of two offshore wells was also completed, showing thick marl–limestone 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 0.52–0.7%Rr in the Lower Cretaceous to 1–1.1%Rr in the Upper Triassic–Hettangian. Miocene rocks that unconformably overlie the Mesozoic strata are immature (0.42–0.47%Rr). Maturation levels increase with increasing age of the strata, indicating that burial was the main process controlling maturation. Thick marl–limestone 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 exposure 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 2000–2500 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 Cretaceous–Early 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.