Abstract

Imaging micron- and nanometer-scale pores is critical for accurately evaluating reservoir quality of low-permeability reservoirs. Standard petrographic techniques have limited value due to the interference of multiple crystallographic layers in a thin section. In addition, the optical resolution of the technique hinders analysis of pores smaller than slide thickness, generally ~30 microns. Destructive focused ion-beam-milled scanning electron microscope (SEM) analysis overcomes this limitation and readily provides a 3D cube of a sample’s pore network. However, recent studies have shown surficial thermal alteration of organic matter due to the ion beam, highlighting a limitation of the technique. Micro-CT scanning is nondestructive, but imaging pores on the scale of a few microns to the sub-micron scale at a high resolution involves reducing sample size and increasing exposure time, resulting in small analytical volume and a slow, expensive analytical process.

Confocal laser scanning microscopy (CLSM) is a promising technique to visualize 3D pore networks in conventional and unconventional reservoirs. CLSM combines advantages of CT -scanning (it is nondestructive) with the better resolution of SEM analysis. Fluorescence in response to laser light excitation is the primary process for generating 3D data from within samples; thus, no additional physical cutting of the standard thin section sample is required. The resolution limitations are ~300 nanometers in the vertical and ~150 nanometers in the horizontal directions, providing an enhance ment over standard optical microscopy methods and CT-scanning.

Analysis of samples from low-permeability reservoir rocks from different American oil-producing basins provides promising results when characterizing pore networks in the size range of hundreds of nanometers to tens of microns using CLSM. Pore shapes and associated pore throat sizes were quantified in 3D across thin section samples, providing a novel in-depth understanding of the pore characteristics and their potential contribution to permeability. The results from this study suggest that CLSM can provide a fast and cost-effective alternative to well-accepted high-resolution petrographic imaging techniques such as SEM and micro-CT.