Strata-bound Colonnade Fractures in Argillaceous Sediments of Qasr El Sagha Delta, Fayum-Egypt.

Abstract

Correponding author: Mohamed S. Hammed, Email: salehhammed@cu.edu.eg Received: 29/10/2021; Accepted: 07/11/2021 DOI: 10.21608/egjg.2021.103322.1010 ©2021 National Information and Documentation Center (NIDOC) THIS work reports spectacular and unique strata-bound columnar fractures in Late Eocene argillaceous sediments of Qasr El Sagha Delta, Fayum-Egypt. Polygonal fractures are widely distinguished as syn-sedimentary structures attributed to dewatering and simultaneous shrinkage of fine-grained sediments. Exposures permitted visualization of the progressive fracture growth along ca 2 meters high colonnade pillars. The increasing diameter and fracture apertures correlate well with the confined clay thickness. Tetragonal fractures are dominant in ca 3-10cm clay layers, while the hexagonal pattern is dominant in ca > 30cm thick clay beds. Fracture polygons of high frequency, order, and geometrical uniformity are better developed in montmorillonite-rich clays. Fractures have curved and en-passant patterns of ductile clay fracturing during the colloidal phase. Fracturing was developed in substrata conditions under few meters of burial sediments. Fracture apertures either increase or decrease toward the top and bottom layers of better permeability. A decrease of fracture apertures toward the top and bottom sand and silt indicates fracture initiation inside the colloid and limited fluid flows during fracturing. Fracture fills are of two natures: sand injection and deposition of unconsolidated laminated sands with delicate gastropod shells. Occasionally, clay layers form domes and sags with fracture rosettes and filamentous complex networks injected by sand. Mild and episodic seismicity was the main cause of dewatering (water escaping) and fracturing. This study might help during the evaluation of the subsurface non-conventional reservoirs in the Nile Delta, which underwent the same geological conditions.