Chlorite cement and its effect on the reservoir quality of sandstones from the Panyu low-uplift, Pearl River Mouth Basin

Abstract

Based on porosity and permeability measurements, mercury porosimetry measurements, thin section analyses, SEM observations, X-ray diffraction (XRD) analysis and granulometric analyses, diagenetic features of reservoir sandstones taken from the Zhuhai formation in the Panyu low-uplift of the Pear River Mouth Basin were examined. This study shows that chlorite cements are one of the most important diagenetic features of reservoir sandstones. The precipitation of chlorite was controlled by multiple factors and its development occurred early in eo-diagenesis and continued till Stage A of middle diagenesis. The precipitation of chlorite at the early stage was mainly affected by the sedimentary environment and provenance. Abundant Fe- and Mg-rich materials were supplied during the deposition of distributary channel sediments in the deltaic front setting and mainly in alkaline conditions. With the burial depth increasing, smectite and kaolinite tended to be transformed into chlorite. Smectite cements were completely transformed into chlorite in sandstones of the studied area. Volcanic lithics rich in Fe and Mg materials were dissolved and released Fe2+ and Mg2+ into the pore water. These cations precipitated as chlorite cements in middle diagenesis in an alkaline diagenetic environment. Chlorite coatings acted as porosity and permeability, thus helping preserve cements in the chlorite cemented sandstones. The reservoir quality of chlorite cemented sandstones is much better than sandstones without chlorite cements. Chlorite cements play an important role in the reservoir evolution that was mainly characterized by preserving intergranular porosity and forming better pore-throat structures of sandstones.