Oil Source Rocks in Lacustrine Sequences from Tertiary Grabens, Western Mediterranean Rift System, Northeast Spain

Pedro Anadon, Stephen J. Cawley, Ramon Julia

Lacustrine sequences, 100-250 m thick, containing oil-prone, organic-rich mudstones (ORM) are exposed in five Tertiary basins in northeastern Spain. They were deposited in small lacustrine basins (up to 50 km²) that developed in grabens of the western Mediterranean rift system.

ORMs from the Rubielos basin comprise laminated gray mudstones with interbedded rhythmite intervals (up to 2.5 m thick) formed by couplets of organic- and carbonate-rich laminae (< 1 mm thick). In marginal zones, ORMs (up to 10 m thick) alternate with lean, bioturbated green marls (up to 5 m thick). ORMs (Rock-Eval yields ^approx 40 kg/MT, HI ^approx 850 mg HC/g TOC) had a dominant waxy terrestrial plant input, with significant and variable algal/bacterial input.

In the Cerdanya basin, the organic matter (OM) in ORMs and diatomites (yields ^approx 19 kg/MT, HI ^approx 442 mg HC/g TOC) was derived from waxy terrestrial plants, with relatively minor algal/bacterial contributions. A similar origin is proposed for sediments from Libros basin where organic-rich laminated diatomitic mudstones yields ^approx 10 kg/MT, HI ^approx 340 mg HC/g TOC) are interbedded in a limestone- and gypsum-dominated sequence. Dolostones form the organic-rich facies in the Ribesalbes basin (yields > 50 kg/MT, HI > 700 mg HC/g TOC); the OM has a mixed algal/plant origin. In the Campins basin, dolostones alternate with laminated ORM (yields ^approx 25 kg/MT; HI ^approx 500 mg HC/g TOC); the OM is derived from waxy terrestrial plants with subordinate algal/bacterial nput.

ORMs in these basins are immature for petroleum generation. Larger lacustrine basins similar to those described above, in more appropriate burial/thermal situations, can be envisioned as zones of potential interest for lacustrine oil exploration in the western Mediterranean.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.