High-resolution large area scanning electron microscopy: an imaging tool for porosity and diagenesis of carbonate rock systems

Automated scanning electron microscopy image collection from geological polished thin sections, in conjunction with autonomous stitching, can be used to construct high-resolution (micron- to submicron-resolution) image montages over areas up to several square centimeters. The technique is here applied to an oolitic limestone and a carbonate laminite to illustrate its application as a tool to study carbonate porosity and diagenesis. Montages constructed from backscattered images are ideally suited to the extraction of data on microporosity, with possibilities including the construction of contoured maps to illustrate the spatial variation in porosity; the construction of porosity logs to illustrate trends in porosity across thin sections; and stochastic construction of digital rock models, for subsequent permeability calculation. Montages taken with a gaseous secondary electron detector in low-vacuum mode can utilize charge contrast imaging (CCI) at a variety of scales and were used here in examining the evolution of carbonate cementation. One example is oolitic limestone, illustrating the formation of grain-lining and pore-occluding cements, as well as recrystallization of the depositional fabric. CCI montages commonly suffer from a variety of contrast and brightness artifacts due to variation in charge distribution across the individual scanned image tiles. Several remedies are discussed that can reduce these artifacts, making it easier to apply image analysis techniques across such montages.