New Seismic Neogene Clay Diapirs and Hydrocarbon Implications in the North-Eastern African Margin of Tunisia

Abstract

Subsurface and surface geological and geophysical studies performed on the Neogene series in the northern and oriental regions of Tunisia has allowed us to highlight the basin structuring and evolution. Neogene sequence deposits are distributed across graben, half graben, platforms, folds and syncline basins. Miocene, Pliocene and Quaternary series are made up of several thousand meters of mainly thick clay and sandstone packages. Clay and mud diapirs were recognized on a small scale in the Miocene outcrops of the Northern Atlassic and Sahel regions and in the Gulf of Hammamet, where they are well developed in the sub-surface and visible in the seismic scale. Clay-kinesis is induced by the lithostatic pressure of Plio-Quaternary thick deposits and by the transtensive and transpressive movements of North-South Grombalia-Enfidha- El Jem and East-West Hammamet-Maamoura, Kuriat and Boumerdès-Mahdia flower fault corridors. The occurrence of diapirs is fossilized on their flanks by synsedimentary Miocene, Pliocene and Quaternary high-angle subsiding graben and syncline basins accompanied by downlap prograding turbiditic sequences onlapped by aggrading and retrograding system tracts and pinchouts. Structural and stratigraphic unconformities are located on the flank diapirs. The clay diapirism accompanied the right and left lateral formation of basins and subsidence migration along the strike-slip fault corridors. The tops of diapir structures are marked by toplap erosional surfaces of the Upper Miocene, Pliocene or Quaternary sequences. Basin modeling is presented to visualize the structurating, the mechanisms and the distribution of clay diapirs around the basin and the tectonic structures. These Neogene diapirs were recognized and described for the first time in Tunisia in 1997. The existence of mud diapirism in the Miocene oil field area of the gulf of Hammamet allows us to devise a new geodynamic model comprising important petroleum implications in terms of structuration, system tracts, migration fluids, traps and seals.