Geochemical characterisation and of black shales from the Tarfaya-Layoune Basin (SW Morocco) : Implications for palaeoenvironmental controls on sulfurisation process of the organic matter
Organic geochemical and petrological investigations were carried out on
Cenomanian/Turonian black shales from three sample sites in the Tarfaya Basin (SW
Morocco) to characterize the sedimentary organic matter. These black shales have a
variable bulk and molecular geochemical composition reflecting changes in the quantity
and quality of the organic matter. High TOC contents (up to 18wt%) and hydrogen indices
between 400 and 800 (mgHC/gTOC) indicate hydrogen-rich organic matter (Type I-II
kerogen) which qualifies these laminated black shale sequences as excellent oil-prone
source rocks. Low Tmax values obtained from Rock-Eval pyrolysis (404-425°C) confirm an
immature to early mature level of thermal maturation.
Organic petrological studies indicate that the kerogen is almost entirely composed of
bituminite particles. These unstructured organic aggregates were most probably formed by
intensive restructuring of labile biopolymers (lipids and/or carbohydrates), with the incorporation of sulphur into the kerogen during early diagenesis. Total lipid analyses
performed after desulphurization of the total extract shows that the biomarkers mostly
comprise short-chain n-alkanes (C₁₆-C₂₂) and long-chain (C₂₅-C₃₅) n-alkanes with no
obvious odd-over-even predominance, together with steranes, hopanoids and acyclic
isoprenoids. The presence of isorenieratane derivatives originating from green sulphur
bacteria indicates that dissolved sulphide had reached the photic zone at shallow water
depths (~100m) during times of deposition. These conditions probably favoured intensive
sulphurization of the organic matter. Flash pyrolysis GC-MS analysis of the kerogen
indicates the aliphatic nature of the bulk organic carbon. The vast majority of pyrolysis
products are sulphur-containing components such as alkylthiophenes, alkenylthiophenes
and alkybenzothiophenes. Abundant sulphurization of the Tarfaya Basin kerogen resulted
from excess sulphide and metabolizable organic matter combined with a limited availability
of iron during early diagenesis. The observed variability in the intensity of OM
sulphurization may be attributed to sea level-driven fluctuations in the palaeoenvironment
during sedimentation
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