Investigation of water–rock interactions in Cambrian black shale via a flow-through experiment

Abstract

This paper summarizes the results of a flow-through experiment of our own design intended to investigate the water–rock interactions between pyrite embedded in black shale and dissolved oxygen in pore water. Deionized water in equilibrium with 3atm of PN2 or PO2 was flowed through a cylindrical rock specimen at 25°C. The electrical conductivity (EC) and pH values of the experimental outflow solutions were continuously monitored. First, original pore water was expelled by N2-saturated water followed by O2-saturated water. Concentrations of major ion species were determined for solutions periodically withdrawn from the experimental system. The changes in these components over time indicated that 1. The chemical composition of the original pore water in the black shale was rich in hydrogen ions, sulfate ions and iron species and that 2. Two major reactions, namely, ion exchange and pyrite oxidation, occurred during the water flow. The oxidation of pyrite in black shale was controlled by the concentration of dissolved oxygen asFeS2+7/2O2(aq)+H2O→Fe2++2SO42-+2H+,where the rate law for the reaction at pH=4.2–5.6 and T=25°C was determined to be Rsp=10−8.42±0.06 [O2]0.5 (molm−2s−1). This result is consistent with the reaction rate of pyrite grains with dissolved oxygen reported by previous researchers. Hydraulic conductivity decreased by approximately 12.5% during the nitrogen-inflow stage and by 69% during the oxygen-inflow stage. This change can be attributed to the oxidizing reaction within the rock and to the probable clogging of fine weathering products in the pore space. The geochemical modeling of the experimental outflowing water and seepage samples from the field using the PHREEQC program suggested that pore water develops into an acidic solution rich in iron and sulfate due to long-term pyrite oxidation and evaporation.