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ABSTRACT: The GKN Discovery: A Wrench Fault Structure in Algeria

Djerroumi Zerrouk, Rekkeb Chabane, Nacer Bey Rabah

Structural style of Sonatrach's 1988 GKN discovery is different from earlier hydrocarbon discoveries in the southeast Constantine basin (e.g., Foua, Onk, and Ras Toumb). This new oil field is described and compared to previously published models, and its geologic evolution and migration history are discussed.

The densely faulted GKN structure is located on the northern upthrown side of the South Atlasic flexure. This flexure, which runs from Agadir to Gabes, separates the highly folded Aures-Nemenchas Massif from the less deformed Saharan Platform in this area. Wrench movement along the flexure produced northeast-southwest-trending en echelon drag folds. The geologic evolution of the area includes the following. (1) Pre-Eocene formation of a system of N140°E and N50°E faults, which influenced sedimentation and later folding. (2) Late Eocene northwest-southeast wrench movement that induced compression, producing the N50°E-trending Atlasic folds. (3) Post Miocene north-south compression, dislocating these structures and reviving wrench movement along the preexisting N140° faulting. (4) Miocene-Pliocene north-south compression that developed the predominant east-west trend, discernable all along the flexure.

The observed structure fits the wrench tectonics model as outlined by Wilcox. The productive reservoirs are organogenetic and micritic limestones of Cenomanian-Turonian age. Source rock sections are present in the Turonian and Vraconian.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990