Authigenic mineralizations at unconformities: Implication for reservoir characteristics

Abstract

Scanning electron microscopy coupled with petrographic and reservoir characteristic measurements were carried out on 447 core samples from seven different boreholes penetrating concealed unconformities. They include: Cretaceous/Devonian; Lower Lias/ Rhaeto-Liassic/Unknown Age/Carboniferous Limestone Series; Triassic/Lower Carboniferous; Triassic/Upper Coal Measures. Triassic/Permo-Trias/Permian; and multiple unconformities from the Middle East-Western Desert, Iraq. Reservoir characteristics at unconformities have been adversely affected by the development of authigenic minerals formed by the mixing of formation waters with infiltrated acidic waters into the paleo-surfaces, followed by extensive marine incursions and invasion of alkaline waters. The result is the formation of complex authigenic minerals in the form of overgrowths of quartz and rhombic feldspars, accompanied by clay-mineral neoformations as pore-fillings and coatings in the form of kaolinite, illite, mixed-layered illite-smectites and, to a lesser extent, chlorites. The final elimination of porosity was achieved by late-diagenetic calcite, colomite and sometimes gypsum cementation.