Preservation of Reservoir Quality in Sandstones by Chlorite Coats - Insights from Viewing Ion-Milled Samples in SEM

Abstract

Summary The presence of continuous chlorite coats on detrital grains can preserve anomalously good reservoir quality in deeply buried sandstones by inhibiting quartz cementation. Two sandstones in the Gulf of Mexico basin—Upper Cretaceous Tuscaloosa sandstone in central and eastern Louisiana and Eocene Upper Wilcox sandstone in south Texas—have intervals that retain high porosity and permeability at temperatures >175°C because of extensive chlorite coats. However, significant variation exists in the morphology and orientation of the chlorite cement on detrital grains in these sandstones. Scanning electron microscope (SEM) observation of chlorite-cemented sandstone samples prepared by argon-ion-beam milling provides excellent views of grain/chlorite boundaries. This technique gives insight into the formation of the chlorite and development of quartz overgrowths within chlorite coats. Quartz overgrowths formed in gaps between chlorite crystals oriented perpendicular to detrital grains in Tuscaloosa sandstones of central Louisiana. In contrast, densely packed chlorite crystals oriented parallel to the detrital grains, such as those observed in Wilcox sandstones in south Texas, prevented precipitation of quartz overgrowths within the chlorite coats. Viewing ion-milled sandstone samples in SEM is a useful technique that can provide new insights into formation of diagenetic cements and preservation of reservoir quality in deeply buried sandstones.