HYDROCARBON HABITAT OF THE SEDANO TROUGH, BASQUE-CANTABRIAN BASIN, SPAIN

This paper analyzes the hydrocarbon habitat and potential of the Sedano trough in the SW sector of the Basque-Cantabrian Basin (northern Spain). The study is based on regional geological data, geochemical analyses, basin modelling simulations and play analysis techniques, and attempts to quantify by volumetric resource appraisal the volume of hydrocarbons generated, expelled and migrated from the main Sedano trough depocentre. A Lower Jurassic shale source rock has been identified and is responsible for the oil at Ayoluengo field, for the oil shows at the Polientes and Tozo wells, and for the Zamanzas and Basconcillos de Tozo tar sands which outcrop at the NE and SW margins of the Sedano trough respectively. Thermal history modelling indicates that petroleum generation and expulsion from the Lower Jurassic source rock started in the Sedano trough in the Early Cretaceous, with the main oil generation phase occurring in latest Cretaceous to Paleogene times. GC, GC-MS and isotopic analyses of oils, tar sands and source rock extracts from the Sedano trough indicate good correlations between the Lower Jurassic source rock and the Ayoluengo oil, and tar sands from the basin margins. Petroleum plays and traps are abundant and are a result of a complex polyphase geological history. They can be grouped into:(i) early salt-induced structural plays; (ii) later structural plays associated with a mid-Tertiary compressional phase; and (iii) stratigraphic plays within the Upper Jurassic - Lower Cretaceous siliciclastic succession. A volumetric resource appraisal of the Lower Jurassic source rock indicates that a total of 11 billion bbl of oil could have been generated and expelled in the Sedano trough, and around 880 million bbl of oil have migrated into potential traps in 15 identified drainage areas. This results in a generation-accumulation efficiency of 7 %. Undiscovered resources have been estimated at 154 million bbl of oil, indicating that there is still moderate undiscovered hydrocarbon potential in the area.