Backscatter Electron Imaging of Viking Sandstones for Mapping Reactive Minerals

Reactive transport modeling for sedimentary materials relies on accurate estimates of the available surface areas of reactive minerals. Such estimates are frequently based on measurements of bulk mineral abundances which may or may not be representative of surface areas in pore fluids. We have employed backscatter electron (BSE) imaging and energy dispersive X-ray (EDX) analysis of Viking sandstone thin sections to examine the spatial patterns of reactive minerals. The Viking formation is in the Alberta sedimentary basin, has been a site for petroleum recovery, and is a potential aquifer for geological sequestration of carbon dioxide. An image processing algorithm was developed that categorizes minerals into three types, distinguishes pore space, and statistically characterizes the sizes, shapes and interfaces of mineral grains and pores. Kaolinite was found to cement quartz grains and line pore spaces, decreasing primary porosity. Minerals of mean atomic mass greater than quartz, which are likely to be reactive under conditions that perturb fluid chemistry, occur as entire grains or as inclusions, thus limiting their contact with pore fluids. Sandstones have high porosity and large pore sizes, while shaly sandstones have large quantities of non-quartz minerals and low porosity primarily in the form of fractures. These and other conclusions based on electron microscopy and image analysis are being used to support pore-scale network modeling of reactive transport in sedimentary rocks.