Relationship between ZnPb ore and oil accumulation processes: Example of the Bou Grine deposit (Tunisia)

Abstract

The “Bahloul formation” of Cenomanian-Turonian age combines qualities of a potential source rock for oil and of a rich lead-zinc mineralization in the Bou Grine area, on the border of the Lorbeus massif diapir (Tunisian Atlas). Analysis of 46 core samples from 7 boreholes drilled within or outside the mine area revealed the richness in organic matter of the Bahloul formation (∼ 4–5% TOC), its low maturity ( T max ∼ 423°C) and marine planktonic origin (HI ∼ 600 mg HC g −1 TOC). Abnormally high values of the Oil Production Index (up to about 30%) indicate the presence of migrated oil in the mine and in samples from the northeast boreholes. This information is confirmed by other observations: high relative amounts of hydrocarbons in the bitumen extracts, high Pr/Ph ratios and high amounts of light naphthenic hydrocarbons. The abundance of the latter components observed in chromatograms where n-alkanes are nearly absent may indicate the development of a biodegradation process during migration. However, this biological activity may have exerted the greatest intensity at the expense of indigenous organic constituents, in the mine area, where considerable amounts of hopanes and steranes were observed. All these data supported by geological observations allow us to propose the following genetic model for the Bou Grine deposit. The Bahloul formation which associates high amounts of organic matter, zinc and lead on a regional scale (Zn + Pb ∼ 0.5–2%) and a high porosity, could have served as a source for the migrated oil and for the metals necessary to the mineralization, as well as a conduit for the oil and metal-bearing brines. An intense biological activity developed at the expense of the organic content of the “Bahloul formation” and of the sulfate from the adjacent Triassic diapir may have produced the hydrogen sulfide responsible for ore deposition.