Source Rock Potential of an Eocene Carbonate Slope: The Armancies Formation of the South-Pyrenean Basin, Northeast Spain

Albert Permanyer, Dolors Valles, Carmen Dorronsorro

The Armancies Formation is an Eocene carbonate slope succession in the Catalonian South Pyrenean basin. It ranges from 500 to 700 m in thickness. The first 200 m are made of a thin-bedded facies of wackestones alternating with dark pelagic fauna of miliolid, ostracods, bryozoans, and planktonic foraminifers and show significant bioturbation. They also show a low organic content (< 0.5% TOC). The lime-mudstone beds show a massive structure or planar millimeter laminations. They may contain sparse pelagic fossils of planktonic foraminifers, ostracods, and dinoflagellates; they do not show any bioturbation, and have high TOC values, which can reach individual scores of about 14%. They qualify, therefore, as a typical oil shale.

Rock-Eval Pyrolysis analysis affords a mean S₂ value of 25 mg HC/g. Mean S₁ value is around 1.0 mg HC/g. As is typical of an initial oil window, T_max maturity parameter ranges from 432 to 440°C (mean = 434°C). This degree of evolution is in accordance with the very low value of carbonyl and carboxyl groups, as determined by IR spectrometry and NMR on Fischer assay extract. The proton NMR shows an aromatic/aliphatic hydrocarbon ratio of 1:4, as expected in earlier stages of catagenesis. N-alkane gas chromatography profiles show n-C₁₅ to n-C₁₉ prevalence and that neither even nor odd carbon numbers prevail. This distribution perfectly matches that of typical sediments of marine origin and also agrees with obtained hydrogen index values (mean HI = 500 mg HC/g TOC).

Sedimentological and geochemical results indicate an autochthonous marine organic matter and the potential of these slope shales is good oil-prone source beds.

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