Biodegradation of Hydrocarbons by Sulphate Reducing Bacteria in the Cretaceous Bahloul Formation (Tunisia)

Abstract

In the Atlas of Northern Tunisia stratiform lead and zinc sulphide mineralization occur in laminated black shales of the Cenomanian-Turonian Bahloul formation and as Bou Grine orebody along a fault system through Cretaceous series. The age and the mechanism of mineralization processes is still controversially discussed. The precipitation of base metals as sulphides in sediments requires organic matter as hydrogen source both in thermochemical and bacterial sulphate reduction.

In the present study the extractable organic matter of samples from the black shales (Bahloul) and the orebody (Bou Grine) has been investigated by organic geochemical methods in order to clarify the mechanism of involvement of the organic matter in the mineralization processes. Results have shown, that in high grade orebody (Pb+Zn=2–20%) the bitumen is severely biodegraded as indicated by the complete removal of nalkanes and of aromatic hydrocarbons from the naphthalene and phenanthrene series.

In low grade mineralized black shales of the Bahloul Formation (up to 1% Pb+Zn) the aromatic hydrocarbon fractions indicate severe biodegradation although the alkanes are only slightly biodegraded. Biomarker analyses (hopanes and steranes) and vitrinite reflectance data have shown that the maturity of the indigenous organic matter in the Bahloul formation reached the oil window. Consequently, in present case the degree of hydrocarbons biodegradation is linked with base metal mineralization.