Petrography and authigenic chlorite in the Siegenian reservoir rocks, Berkine Basin, eastern Algerian Sahara

摘要

In Berkine Basin, the Siegenian stage is constituted by clayey sandstones and it contains several reservoir levels especially in the perimeter of Zemlet El Arbi, where the sandstone levels can reach 30 m of thickness. The Siegenian reservoir shows a tidal deposit environment, it is located at depths greater than 3200 m, its average thickness is 200 m in the south and it decreases to 30 m because it is strongly affected by Hercynian orogeny mainly in the northwest of the studied area. Petrographic and X-ray diffraction studies indicate the presence of authigenic chlorite, which is the most important cement in Siegenian sandstones. Chlorite appears in different forms: grain coating, pore lining, pore filling and rarely in the form of oolite. It often has a great impact on petrophysical reservoir qualities by inhibiting authigenic quartz overgrowth in porous space and preserving porosity. Other diagenetic phenomena observed are mechanical compaction, alteration and dissolution of feldspars and biotite, precipitation of clay minerals (smectite, berthierine, chlorite, kaolinite and illite) and precipitation of secondary silica, carbonates, pyrite and haematite.