CHARACTERIZATION OF ORGANIC MATTER AND METAL CONTENTS PRESERVED WITHIN THE ALBIAN-VRACONIAN AND CENOMANIAN-TURONIAN BLACK SHALES IN NORTH-CENTRAL TUNISIA

The Slata-Guern Halfaya area, located in the “diapiric” zone in Tunisia, is structured into NE-SW trending half-grabens and paleohighs structures generated as a result of an intra-Cretaceous extensive tectonic activity and contemporaneous halokinetic event.

The geochemical analysis indicates that the albian-vraconian facies (Fahdene formation) of Slata and cenomanian-turonian facies (Bahloul formation) of Guern-Halfaya are rich in organic matter with a total organic matter varying from 0.2%wt to 1.4%wt and from 0.62%wt to 5.46%wt, respectively. The diagram representing the Rocl-Evak parameter S₂ versus TOC and the evolution of the calculated Hydrogen Index parameter (HI) with depth, reveal that the organic content of the majority of the Fahdene and Bahloul samples is of Type II (marine, amorphous). The position of the samples in the diagram HI versus T_max registered values, suggest that the organic matter is mid-mature, at the oil-window zone for Fahdene Formation and at the sub-mature zone to the early mature zone for the Bahloul formation.

The correlations metals-organic matter contents are weak to absent and positive respectively for Fahdene and Bahloul formations. These contrasting correlations are explained by the degree of maturity of organic matter. In Fahdene formation, the thermal cracking of the kerogen has destabilized the organo-metal complexes resulting in the dissociation and expulsion of fluids including water, oil and metals. These metals, then, have migrated from the basin to the adjacent paleohigh aptian reefal carbonates where they encounter sulfur-rich fluid, most likely leached from nearby gypsum diapir, and precipitated to form Pb-Zn ore deposits of Slata. The metal content, in Bahloul formation, is rather preserved as organo-metal complexes within the remained immature kerogen macromolecules. This preservation of metal-organic matter is reflected by the presence of a positive correlation between metals and the organic content. This formation may paly a possible source of metals, elsewhere, if underwent thermal transformations.

In the diapir zone, the juxtaposition of a black organic-rich mature formation as a possible source of metals with diapiric evaporates, source of sulfur, and a permeable reservoir is an ideal trilogy for ore exploration in this area.