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 sho wn the presence of organic-rich (TOC, HI and S-2 up to 8.7%, 760 and 5 6.5 mg/g, respectively) Pliensbachian-early Toarcian black shales that constitute the only feasible source rock for Ayoluengo oil. The strat igraphical and sedimentological data show that these black shales deve loped 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 delta( 13)C composition, and a detailed study of the linear, branched, isopre noid, steroid and hopanoid aliphatic hydrocarbons in these shales, mar ls and the Ayoluengo oil, confirms this hypothesis. Specifically, the delta(13)C values of the total solvent extract of the thicker shale (N o. 2) and the oil are -29.93 and -29.88 parts per thousand, respective ly; also a close similarity in the relative compositions of steranes a nd hopanes is observed between these samples. The distributions of the se compounds exhibit several distinct features. The steranes in this s hale and crude oil are characterized by a depletion of C-28 VS C-27 an d C-29 homologues (24 vs 32 and 44%, respectively), which is consisten t with the carbon number sterane distributions generally observed amon g Upper Paleozoic to Liassic rock-sourced oils. Furthermore, hopanes a nd steranes are characterized by their high relative content of rearra nged molecules. Diasteranes (13 beta(H), 17 alpha(H)- and 13 alpha(H), 17 beta(H)- series) are the dominant compounds among the steranes and significant amounts of 18 alpha(H)-17 alpha-methyl-28-norhopanes (C-2 7, C-29 and C-30 homologues) and 17 alpha(H)-15 alpha-methyl-27-norhop anes (C-30 homologue) are found in the hopanoid hydrocarbons. Converse ly, the proportion of these rearranged molecules in other sediment sam ples from the same formation decrease with increasing carbonate conten t. 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. (C) 1997 Elsevier Science Ltd.