Alteration of chlorite coats and sandstone porosity reduction: Insights from reactive transport modeling

Abstract

Chlorite coats are widely recognized as a key element in preserving sandstone porosity because it can inhibit the growth of quartz cements. However, the alteration of chlorite coats and its potential influences on sandstone porosity are rarely discussed. Therefore, this work used reactive transport models under different petrographic and geochemical conditions to investigate the influence of chlorite coats on sandstone porosity in a major dissolution window (100 °C). The HCO3-rich (CO2-charged) and HCO3-depleted (organic acids-charged) waters were injected to induce mineral dissolution and precipitation. The results indicate that the alteration of chlorite coats may result in sandstone porosity reduction. The HCO3-rich water leads to a porosity decrease mainly through the precipitation of magnesite and siderite resulting from chlorite dissolution. In contrast, the HCO3-depleted water causes a porosity decrease mainly through the redistribution of kaolinite and quartz cements. Factors, including pCO2, organic acid concentration, coat coverage, coat thickness, and grain size, have secondary influences on net porosity change. In comparison, factors, including chlorite mineralogy, detrital lithology, and the reduction of K-feldspar dissolution rate caused by chlorite coat, have negligible influences. The alteration of chlorite coats may introduce significant mis-interpretation to the analysis of the relationship between chlorite coats and sedimentary facies. Moreover, the actual impact of pore-filling chlorite on porosity reduction may be either underestimated or overestimated. Therefore, the alteration of chlorite coats should be taken into consideration in future studies.