DOKKA A. M., A. BOUDJEMA, * M. HAMEL, F. BENDERRA, A. AREZKI, and R. TITRAOUI, Sonatrach, Exploration Division, Algiers

ABSTRACT: Depositional Systems of the Lower Devonian Sandstone Reservoirs of the Western Central Sahara (ALGERIA)

Sedimentological study of the Siluro-Devonian outcrop sequences and a total of 2200 m cored sections and log data from 15 wells reveals two distinct facies successions separated by their bounding discontinuities.

The lower, transitional Silurian-Gedinian, succession overlies a regional lithostratigraphic marker bed of dark gray clayey bioclastic limestone with a concentration of orthoceras and brachiopod shells inferring a maximum flooding surface and parasequence boundary. This succession shows progradational facies characteristics of storm and wave influenced shallowmarine shelf and deltaic depositional systems. In both outcrop and subsurface sections, lower shoreface to delta front sandstones may be identified.

The upper facies succession (Siegenian) in the outcrops with its scoured basal contact displays peculiarities comparable to those of the Platte type braided fluvial complex in its lower half. Northward it is transitional to distal fluvial and grades upward into deltaic depositional system. In the subsurface, with no discernible upper boundary, this succession contains tidal and storm affected marginal marine sand bodies.

The Siegenian succession is overlain by transgressive shallow marine bioclastic sandstones and shales attributed to the Emsian with shelly polymictic conglomerates at its lower as well as upper contacts.

The lower Devonian sandstones show fairly good reservoir characteristics. Porosity is due to leaching of feldspars and dissolution of carbonate cements in addition to the interclastic, preserved by chlorite rims.

Good potential Silurian source rocks of hydrocarbons are known in the area. In view of the identified depositional environments, stratigraphic plays should be as important as the already known structural ones.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.