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Abstract: Reservoir Architecture and Depositional Environments of Triassic Fluvial Sandstones: Examples from Block 208, Ghadames Basin, Algeria

EVANS, MARTIN J., JULIAN M. GREEN and HAROLD H. JAMESON, Anadarko Algeria Corporation

Exploration activities in Block 208 of the Ghadames Basin, Algeria, have yielded significant oil discoveries in Triassic TAGI sandstone reservoirs. Field delineation is ongoing, providing a data base that serves to characterise the reservoir geology of the TAGI.

The Triassic succession of the Ghadames Basin was deposited on a regionally extensive sequence boundary, the Hercynian Unconformity. Increasing accommodation space related to relative sea level rise promoted accumulation of an onlapping wedge of fluvial sediments, deposited in a broad coastal plain setting.

The initial TAGI fluvial deposits back-filled a subtle erosional topography developed on the Hercynian Unconformity. Rates of sand supply to the fluvial system are inferred to be high, and coupled with a probable slowing in the rate of relative sea level rise, promoted the development of extensive braided river tracts. Regional correlation of well data indicates that the resultant tabular sand bodies are exceptionally large, separated vertically by correlatable mudstone intervals. Reservoir continuity is on the order of kilometres to 10s of kilometres in directions both transverse and parallel to paleoflow.

The TAGI is sand-rich fluvial system, with sand:shale commonly in the range 60-80%. Deposition occurred predominantly in shallow, low sinuosity braided streams, characterised by rapid lateral shifting of channel courses. Extensive reworking of the broad braided river tracts has resulted in well sorted, highly permeable and porous reservoirs, with limited preservation of fines and internal flow tarriers within individual sand bodies. Pressure transient analysis of well tests indicates that lateral hydraulic communication of reservoirs is excellent.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah