Gothlandian Source Rock Discovered North of the Talemzane Arch, Central Tunisia

Susan M. Cunningham

The Gothlandian shales are well known throughout the Trias basin of Algeria and Tunisia as the primary hydrocarbon source rock of the Triassic and Cambrian-Ordovician sandstone reservoirs. Recent drilling in the relinquished Medenine and Douz permits of central Tunisia represents the first penetration of the Gothlandian shales north of the broad east-west Hercynian "Talemzane Arch," which spans the basin from Algeria to Tunisia.

Hydrocarbon source rock evaluation of the Gothlandian shales penetrated by nine wells shows that through a 55 to 180-m interval, (1) the shales are organically rich (the 19 samples have > 1.5% TOC), (2) the organic matter is type I and II kerogen, (3) the organic matter is thermally immature to mature, up to the peak oil stage of catagenesis, and (4) the rocks have produced hydrocarbons in the past and have the potential for producing additional hydrocarbons (genetic potential > 5 kg HC/tonne rock).

The observed thermal maturity of the organic matter and present-day geothermal gradient suggests that greater burial depths and/or a higher geothermal gradient previously existed in the northeast (Baguel area). Although the organic material presently at 2,600 m has reached thermal maturity and expelled hydrocarbons, hydrocarbon generation has ceased due to uplift and erosion.

The Sabria area to the west is currently reaching thermal maturity at depths of approximately 4,000 m.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.