Development of Sulfuric Acid Speleogenetic Deposits within Cavernous Middle Eocene Beds: Inference on Hydrocarbon Gas Seepages, Giza Pyramids Plateau, Egypt

Abstract

AbstractDevelopment of sulfuric acid speleogenetic mineral deposits within cavernous middle Eocene beds of the Pyramids plateau is linked to hydrocarbon gas seepages. The work carried out field observations, binocular, polarizing, scanning and transmitted electron microscopy investigation, X‐ray diffraction and X‐ray fluorescence analyses. The morphological and petrographic features and chemical composition of the studied mineral deposits reveal a hypogene sulfuric acid speleogenesis. A model comprised of the following stages can be used to explain the presence of these features. (1) Ascent of hydrogen sulfide (H2S) gas associated with hydrocarbon seepages from the Cretaceous reservoirs under reducing conditions followed by oxidation to sulfuric acid (H2SO4), (2) descent of carbonic acid (H2CO3)‐rich solution generated from surface sources, (3) reaction of H2SO4 and H2CO3 with the calcareous and argillaceous host rocks, (4) formation of H2SO4 speleogenetic by‐products represented by natroalunite, aluminium‐phosphate‐sulfate, hydrated halloysite and Fe/Mn oxides within the replacive gypsum, and (5) subsequent stresses due to the formation of nearby stratiform cavities gave rise to the development of fractures/veinlets filled with displacive fibrous satinspar gypsum. The study sets the paleokarst features of the Giza Pyramids plateau within a hypogene sulfuric acid karst system developed by the action of groundwater containing H2S, H2SO4 and H2CO3.