Geochemical Characterization of Cenomanian/Turonian Black Shales from the Tarfaya Basin (SW Morocco): Evidence for Palaeoenvironmental Controls on Early Sulfurization of Sedimentary Organic Matter

By

 Sadat Kolonic¹, J. S. Sinninghe Damsté², M. E. Böttcher³, M. M. M. Kuypers³, W. Kuhnt⁴, B. Beckmann¹, G. Scheeder⁵, T. Wagner¹

(1) University of Bremen, D-28334 Bremen, Germany (2) Netherlands Inst. for Sea Research (NIOZ), Texel, Netherlands (3) Max Planck Institute (MPI), Bremen, Germany (4) University of Kiel, Kiel, Germany (5) German Federal Inst. for Geosciences & Natural Resources, Hannover, Germany

 Organic geochemical and petrological investigations were carried out on Cenomanian/Turonian black shales from the Tarfaya Basin (SW Morocco). High TOC contents up to 18wt.% and HI between 400 and 800 indicate hydrogen-rich organic-matter of type I-II kerogen that qualify these laminated black shale sequences as excellent oil-prone source rocks. Low T_max values obtained from Rock-Eval Pyrolysis (404-425^oC) confirm an immature to early mature level of thermal maturation. Organic petrological investigations conducted on thin sections indicate that the kerogen is almost entirely composed of AOM. The formation of AOM is most likely related to intensive restructuring of labile biopolymers (lipids/carbohydrates) through incorporation of sulfur into the kerogen during early diagenesis. Total lipid analysis performed after desulfurisation of the total extract shows that the biomarkers predominantely comprise of short-chain n-alkanes (C₁₆-C₂₂) and long-chain (C₂₅-C₃₅) n-alkanes with no obvious odd-over-even predominance, steranes, hopanoids and acyclic isoprenoids. The presence of Isorenieratene derivatives indicates that dissolved sulfide had reached the photic zone at shallow water depths (~100m) during times of deposition. These conditions likely favoured intensive sulfurisation of the organic-matter. Flash pyrolysis GC-MS analysis of the kerogen indicates an aliphatic nature of the bulk organic-carbon. The vast majority of pyrolysis products are sulfur-containing components such as alkylthiophenes, alkenylthiophenes and alkybenzothiophenes. Abundant sulfurisation of the Tarfaya kerogen resulted from excess sulfate and metabolizable organic-matter over a limited availability of iron during early diagenesis. The observed variability in the intensity of sulfurisation of OM can be attributed to sea level-driven fluctuations in the paleoenvironment during sedimentation.