Mapping of Microbial Habitats in Organic-Rich Shale

Abstract

Summary Microbial communities within unconventional hydrocarbon reservoirs in the oil to wet gas window are known to be present but sparse and believed also to be almost entirely dormant, due to heat, lack of space, and near-equilibrium redox conditions at the microscale – conditions that change when a rock is hydraulically fractured. In order to predict how microbial communities impact the geochemical response to such a perturbation, direct and indirect techniques are employed on select samples of the Point Pleasant Formation (OH and PA) to map organic filled pore space and migration pathways, and quantify the morphology, size, and surface mineralogy of fractures and pores. Samples containing these features have been selected for 3-D X-ray micro-computed tomographic (µXCT) imaging, performed at the Advanced Photon Source (APS). This effort has yielded microscale imaging, classification, and quantification of pores, organic matter, pyrite, and key pore-mineral associations. These data are complemented by Energy-dispersive X-ray spectroscopy (EDS) spot analysis and scanning electron microscope (SEM) backscattered electron (BSE) imaging to determine the spatial relationships of organic matter volumes with sufficient space for microbes. Mercury intrusion porosimetry (MIP) data permit quantification of pore throat size, vital to an understanding of nutrient, ribonucleic acid (RNA), and deoxyribonucleic acid (DNA) flux between otherwise isolated microbial communities. Results reported here include the quantification of the volumes able to accommodate microbes, the pore throat size distribution of the network connecting these volumes, a classification of organic matter and of its correlation with pyrite, and finally a discussion of certain implications for nutrient availability before and after the influx of drilling fluids. The findings of this study, although still preliminary, therefore contribute to our understanding and prediction of the biogeochemical evolution of a hydrocarbon-rich formation, and of its response to a perturbation such as hydraulic fracturing.