Geochemical correlation of pyrobitumen fills with host mid-Cretaceous Black Flysch Group (Basque-Cantabrian Basin, western Pyrenees)

Abundant syndepositional pyrobitumen fills occur within Albian hydrothermal vent carbonates of the organic-rich Black Flysch Group in the Basque-Cantabrian Basin, indicating that these carbonates were charged with oil during Albian times. A carbon isotopic study of pyrobitumens and samples of candidate oil generating rocks (Jurassic and Albian host rock units) as well as Carboniferous-sourced condensate was undertaken to test an Albian source hypothesis for the pyrobitumens. On the basis of similar isotopic ratio values, the pyrobitumens show a strong positive correlation with the host Black Flysch Group. This is supported by Stahl-type curves and a Sofer plot, which show that the pyrobitumens best match the host Albian deposits, whereas the Jurassic source rock and the Carboniferous sourced condensate plot separately. This new Albian pyrobitumen-source correlation demonstrates the existence of an oil generating rock unrecognised till now in the Basque-Cantabrian Basin – the mid-Cretaceous Black Flysch Group.