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Microporosity Quantification Using Confocal Microscopy

Abstract

Confocal microscopy provides high-resolution (0.2-microns/pixel) images of pores in sedimentary rocks. Such features are generally obscure or invisible using conventional petrography. Confocal microscopy, which is relatively new to the earth sciences, provides an innovative way to image pores and quantify pore-body size distributions. This information can be used to define pore types, examine pore connectivity, and better understand diagenesis. In practice, confocal microscopy is applied to thin sections of rocks that are vacuum-pressure impregnated with epoxy. The laser light source interacts with fluorescing dye within the epoxy. Reflected light intensity indicates the physical location of pores. Confocal microscopy uses point-by-point illumination. A pinhole is placed in front of a detector to eliminate out-of-focus light. Because each measurement is a single point, confocal microscopes scan along grids of parallel lines to provide images of sequential planes at specified depths within the sample. Confocal microscopy is used to generate 2D and 3D images of pore bodies and throats. These can be compared to laboratory-measured petrophysical properties and used to build pore-scale models. From these, we can compute petrophysical properties, such as capillary pressure, relative permeability, and recovery factors. Confocal work, performed within a stratigraphic framework in cyclic carbonates, can be used to improve mapping of pore types. These can be linked to logs to help enable mapping of diagenetic geobodies between wells.