Abstract: Geology and Hydrocarbon Potential of the Lower Ordovician Play in the Hassi Messaoud Dome Flanks, Algeria

Messaoudi, M. and A. Khellif - Sonatrach

The Hassi Messaoud area is located on the central Sahara and is well known as a major oil production in Algeria from the Cambrian reservoir. Recent drilling in the Hassi Messaoud dome flanks has discovered significant oil production in the lower Ordovician formation (Hamra Quartzites reservoir). The reservoir unit that is eroded by the Hercynian orogeny on the central part of the Hassi Messaoud dome constitutes a potential petroleum play in a sinuous belt along the flanks where it is subcrop beneath the Hercynian unconformity. This belt constitutes an important regional pinch out against the Hassi Messaoud dome.

The lower Ordovician play is defined by the occurrence of Hamra Quartzites reservoir sandstones sealed by the overlying Liassic and Triassic shales, evaporites or eruptive. The reservoir is sourced from the mature basal silurian shales developed in the Oued Mya basin (west and north). The current model of hydrocarbon migration is combined with oil expelled from the silurian source rock and migrates vertically along faults to the Hercynian unconformity surface and then laterally to charge the Cambro-Ordovician reservoirs which are in up dip position. The migration pathways which are related on the presence of carrier beds, fractures, faults and connectivity with the Hamra quartzites reservoir sandstones is very complex and not well understood at the present time due to the poor seismic quality below the Hercynian unconformity.

The Hamra Quartzites reservoir sandstones are clean, well developed massive with an average thickness of 70 meters; porosities 2-10% and permeabilities 0-100 md. The reservoir quality is variable and due to the lateral facies changes and diagenetic effects. Most of the log data and core samples studied show that the reservoir has modest quality; but the distribution of the good reservoir with hydrocarbon production appears to be restricted to the area where the Hamra Quartzites reservoir subcrop beneath the Hercynian. This quality enhancement is interpreted to be the result of secondary solution porosity associated with the Hercynian erosion surface or fractures. The permeability can often be significantly enhanced by natural fractures related to the tectonic and featuring.

Combination structure-stratigraphic traps are the predominant hydrocarbon accumulation in the lower Ordovician reservoir. The fault blocks and faulted anticlines constitute the favorable traps for oil accumulation, but the faults should have sufficient throw to juxtapose Hamra Quartzites reservoir against no reservoir rocks as El Gassi shales units.

The subcrop lower Ordovician reservoir belt is a promising area for hydrocarbon exploration. Due to the reservoir type (Quartzitic sandstones and quartzites), the economic oil production may depend on the combination of matrix and fracture porosity. The horizontal drilling techniques can be utilized to full production.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil