Assessment of sedimentary environment from PAHs and aliphatic biomarkers: The case study of Fahdene black shales in northern Tunisia

Abstract

Polyaromatic and aliphatic biomarkers extracted from the Albian-Early Cenomanian source rock (Fahdene Formation) were evaluated by Gas Chromatography – Mass Spectrometry (GC-MS) and are compared to classic geochemical and mineralogical data. In addition to terpanes, hopanes, diasteranes and steranes, thirty-seven aromatic hydrocarbon elements were identified.The absence of the angiosperm-derived biomarker (18α(H)-Oleanane) and the low abundance of trimethylnaphthalenes isomers (1,2,5-TMN and 1,2,7-TMN) suggest that no land-plant traces were founded in the studied organic matter. In this fact, the Kerogene III identified by Rock Eval is not detected, and we can attribute these indices to the algae effect. The check of maturity by Tmax and the biomarker parameters (C27 18α-22,29,30-trinorneohopane (Ts), sterane isomerization ratio ββ/(αα+ββ), C32 22S/(22S + 22R) HH and the methylphenanthrene index (MPI-1)) indicates that these organic rich sediments are reached the early stage of oil window. In the other hand, aliphatic and polyaromatic biomarkers confirm classic data of X-ray diffraction and calcimetry and suggest a carbonate facies with some clayey intercalations in the middle part. The fracturing analysis of these carbonates prove the presence of high density of fractures with various sizes and directions. About filling, the fibrous calcite is the major element of mineralized veins, with the presence of bitumen traces. These fractures have two main orgin; the fluid pressure and the regional tectonic stress regime.Therefore, the Albian-Early Cenomanian fractured carbonates in the Mahjouba area were deposited, under suboxic conditions, in a marine environment affected by the eutrophication phenomenon, and have just reached the equilibrium stage of maturity.