Oil Seeps in Central-Northern Tunisia as Indicators of Actual Active Petroeum Systems

By

M. Saidi¹, R. Ghenima¹, L. Fourati¹, H. El Euchi¹

(1) ETAP, Tunis, Tunisia

 The occurrence of hydrocarbon seepages in Central-Northern Tunisia has been recognised since the start of the Century and, together with the presence of surface anticline features, provided a stimulus for early exploration in that area. Several wells drilled close to the surface oil seeps on the basis of only surface geologic survey, were encountered encouraging oil and condensate shows. Some of them were abandoned before reaching the objectives. Almost all the seeps are encountered in the Nappe zone and in the Triassic salt domes zone within Triassic to Miocene levels. They could be subdivided into 5 main categories based on their nature and origin: source rock or false seeps, updip seeps, seeps along fractures and oil along unconformities or cap-rock.

Potential source rocks identified in that zone belongs to the Albian (L. Fahdene Fm), Late Cenomanian-Early Turonian (Bahloul Fm), Ypresian (Bou Dabbous Fm) and Oligocene-Miocene (Numidian shales).

The origin of the oil seeps has been comprehensively investigated using different geochemical techniques. Oil/source rock correlation based on biomarker distribution and carbon isotope composition allowed grouping of these oil seeps into 4 main families related to the source rocks mentioned above.

The Aptian and Numidian sandstones as well as carbonate units of Aptian, Albian, Cenomanian, Turonian, Campanian-Maastrichtian and Ypresian are the most probable reservoirs that have hosted oil accumulations.

Structural studies based on outcrops and confirmed by more available seismic data indicate a variety of small and large potential traps both in the allochtonous and in the autochtonous which could be charged from mature source rocks.