Characterization of asphaltic oil occurrences from the south- eastern margin of the Basque-Cantabrian Basin, Spain

P. M a r í n e t a l . G e o l o g i c a A c t a , 1 2 ( 4 ) , 3 2 7 3 4 2 ( 2 0 1 4 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 1 4 . 1 2 . 4 . 4 Asphaltic oils from SE Basque-Cantabrian Basin, Spain 328 reported by several authors (Ramírez del Pozo, 1969; Pujalte, 1977; Salamon, 1982; Rat, 1988; Grafe and Wiedmann, 1993; Cámara, 1997; Gómez et al., 2002; Capote et al., 2002; Barnolas and Pujalte, 2004). In the 1960s, occurrence of Lower Cretaceous tar sands in the south–western part of the Basque-Cantabrian Basin led to the discovery of the Ayoluengo onshore oil field. Its structure consist of a Late Jurassic–Early Cretaceous salt-related extensively faulted anticline, whose main reservoirs are Upper Jurassic–Lower Cretaceous siliciclastic sandstone beds with porosities ranging from 15% to 25% and permeabilities up to 1000mD (Sanz, 1967; Álvarez de Buergo and García, 1996; Quesada et al., 1997). Geochemical studies focusing on biomarker oil-to-source rock correlations (Quesada et al., 1997; Beroiz and Permanyer, 2011) showed that the Ayoluengo oil is derived from a Pliensbachian–Toarcian shale source rock. These Lower Jurassic organic-rich shales stand out in the whole Liassic sequence by having total organic carbon (TOC) content around 4%, high RockEval S2 values (S2≤21mg HC per gram of rock) and hydrogen indices (HI) greater than 400mg HC per gram of organic carbon (Quesada et al., 1997; Quesada et al., 2005; Beroiz and Permanyer, 2011). Similar facies have also been reported in the North Sea and the Paris Basin and proved there as effective source rocks (Cooper and Barnard, 1984; Herron and Le Tendre, 1990; Huc, 1990). Additionally, δ13C isotopic studies suggest that Lower Cretaceous tar sands outcropping near the Ayoluengo oil field (e.g. Basconcillos del Tozo and Zamanzas locations) have the same origin as the Ayoluengo oil (Beroiz and Permanyer, 2011). Furthermore, oil shows are common throughout the Basque-Cantabrian Basin. For instance, pyrobitumens in Albian carbonates in the northern margin of the basin have been studied by Agirrezabala et al. (2008), who concluded that they correlate with a previously unrecognised source rock in the basin, i.e., the Middle Albian–Lower Cenomanian Black Flysch Group (mid-Cretaceous), whose TOC content is around 1%. In the south-eastern part of the Basque-Cantabrian Basin (Álava sector) bitumen-impregnated Upper Cretaceous Campanian and Maastrichtian carbonates and sands occur, outcropping beside Triassic diapiric structures. Studies of these hydrocarbons were carried out by Dorronsoro et al. (1994) and García Sánchez (1994), though no correlation with the western hydrocarbons of the basin has been carried out until now. Therefore, this paper reports on hydrocarbons sampled in exhumed reservoirs near the Maestu and Peñacerrada diapirs in order to: i) characterize their biodegradation and maturity level, ii) suggest a depositional environment and source rock, and, above all, iii) to determine their genetic resemblance to hydrocarbons of the Ayoluengo oil field. For this purpose, a geochemical study including quantification of saturated and aromatic hydrocarbons, as well as resins and asphaltenes (SARA), stable carbon isotope analyses and gas chromatography-mass spectrometry have been carried out.