In carbonate rocks, pore diameters range in size over at least nine orders of magnitude, from submicrometer-scale voids to km-scale caves. This study is focused on micropores, which are defined as pore bodies with diameter ≤ 10 micrometers. Corresponding pore throats are generally ≤ 1 micrometer in diameter. To visualize and quantify microporosity, geologists commonly use pore casts, transmitted-light petrography, and scanning electron microscopy. Shortfalls exist in all of these techniques. Laser scanning confocal microscopy, a relatively new approach, provides a step change in our ability to image and quantify microporosity in carbonate rocks.
Laser scanning confocal microscopy provides high-resolution (0.2-micrometers/pixel) images of micropores. Such pores are generally obscure or invisible using conventional petrography. In practice, confocal microscopy is applied to polished rock chips or thin sections that have been vacuum-pressure impregnated with epoxy. The laser light source interacts with fluorescent dye within the epoxy. Emitted fluorescent light, recorded using point-by-point illumination, indicates the physical location of pores. A pinhole, placed in front of the detector, eliminates out-of-focus light. Because each measurement is a single point, confocal microscopes scan along grids of parallel lines to provide optical images of planes at specified depths within the sample.
Confocal microscopy is used to generate 2D and 3D images of pore bodies and throats. Results can be compared to laboratory-measured petrophysical properties, such as pore-throat diameters from mercury injection capillary pressure (MICP) data. Now, for the first time, we can compute pore-body to pore-throat size ratios without pore casts. These ratios are important, because they can be related to mercury recovery factors from imbibition MICP experiments.