Geochemical correlation of oil from the Ayoluengo field to Liassic black shale units in the southwestern Basque-Cantabrian Basin (northern Spain)

A study of the Liassic sequence in the Basque-Cantabrian Basin has shown the presence of organic-rich (TOC, HI and S2 up to 8.7%, 760 and 56.5 mg/g, respectively) Pliensbachian-early Toarcian black shales that constitute the only feasible source rock for Ayoluengo oil. The stratigraphical and sedimentological data show that these black shales developed into a major transgressive sequence of hemipelagic Lotharingian-Toarcian facies, which suggests the presence of anoxic or near anoxic bottom water conditions in troughs during their deposition. The δ13C composition, and a detailed study of the linear, branched, isoprenoid, steroid and hopanoid aliphatic hydrocarbons in these shales, marls and the Ayoluengo oil confirms this hypothesis. Specifically, the °13C values of the total solvent extract of the thicker shale (No. 2) and the oil are −29.93 and −29.88‰, respectively; also a close similarity in the relative compositions of steranes and hopanes is observed between these samples. The distributions of these compounds exhibit several distinct features. The steranes in this shale and crude oil are characterized by a depletion of C28 vs C27 and C29 homologues (24 vs 32 and 44%, respectively), which is consistent with the carbon number sterane distributions generally observed among Upper Paleozoic to Liassic rock-sourced oils. Furthermore, hopanes and steranes are characterized by their high relative content of rearranged molecules. Diasteranes (13β(H),17α(H)- and 13α(H),17β(H)- series) are the dominant compounds among the steranes and significant amounts of 18α(H)-17α-methyl-28-norhopanes (C27, C29 and C30 homologues) and 17α(H)-15α-methyl-27-norhopanes (C30 homologue) are found in the hopanoid hydrocarbons. Conversely, the proportion of these rearranged molecules in other sediment samples from the same formation decrease with increasing carbonate content. Rearrangement is generally related to catalytic effects due to clay minerals, which is in agreement with the lithology of the source rock attributed to the Ayoluengo oil.