Abstract: Triassic Depositional System and Reservoir Development, Ghadames Basin, Algeria

Robert P. Daniels, Richard C. Hook, Poul R. Sorensen, James J. Emme

A depositional model for Triassic sediments of the Ghadames Basin of Eastern Algeria was developed through core examination and regional well correlations. More than 700 meters of basal Triassic core from fourteen wells were interpreted and integrated with additional Triassic well log data.

Basal Triassic clastic sediments were deposited over a low relief erosional surface. The sediments were deposited in continental/fluvial environments which varied systematically through time, related to changes in depositional base level during a marine transgressive event. The basal clastics are conformably overlain by evaporites which represent inundation of the area by hypersaline marine conditions.

Initial Triassic deposits (TAGI) consist of stacked anastomosing fluvial sands and continental shales. These sands were localized within a broad (30-40 km wide) tectonically controlled erosional valley oriented SW-NE across the Ghadames Basin. Significant hydrocarbon accumulations have been found in this system at El Borma field (1 BBO recoverable), at Rhourde el Khrouf, and at Bir Rebaa West.

The continued rise of depositional base level shifted sabkha deposition over the back-filled valley system of the TAGI. Sabkha sedimentation was interrupted locally by tectonism and uplift of the Hassi Messaoud and possibly the Tihemboka ridges. These areas became clastic sources for alluvial fan sedimentation of the TAGI which record a tectonically controlled local marine regression. As the source area eroded, the large-scale marine transgression again predominated, as evidenced by the return to sabkha deposition. Complete submergence by hypersaline marine waters ensued, depositing Upper Triassic and Lower Jurassic anhydrite and salt.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994