Advanced characterization of the Bakken Formation, an unconventional oil and gas play of the Williston Basin, was performed via newly developed analytical tools of microscopic investigation in concert with standard laboratory methods. Characterization of an unconventional formation to understand the composition and distribution of framework grains, organic matter (OM), clay minerals, and porosity is difficult because of the extremely lithified nature of the lithofacies within the formation and the small grain and particle sizes. In this study, corroborative methods aimed to define micro- and nanoscale fabrics that impact parameters such as maturity, recovery, clay content, micropore networks, and CO2 interactions for either storage or enhanced oil recovery (EOR). Lateral and vertical variations in the rock fabric across multiple wellsites were observed on a micro- to nanometer scale with innovative analytical technologies.

Detailed morphologies and chemical compositions of ion-milled samples were obtained with field emission scanning electron microscopy (FESEM) coupled with energy-dispersive spectroscopy (EDS). Furthermore, a new software suite, Advanced Mineral Identification and Characterization System (AMICS), was used to classify and quantify mineralogy, OM, and porosity from the FESEM images. For validation purposes, x-ray diffraction was used to obtain bulk mineral and clay mineral data and x-ray fluorescence to obtain bulk chemical compositions of the samples. Advanced image analysis was performed on high-resolution FESEM images as another corroborative approach to characterize key features of interest within the lithofacies. Each sample consisted of high-resolution FESEM backscattered electron (BSE) images taken at multiple magnifications to maximize particle morphology in the fine-grained rock of the unconventional reservoir.

The data highlighted trends related to factors that impact CO2 transport and sorption in unconventional reservoirs. Segmented BSE images from the FESEM using program parameters that included texture, gray scale, and other morphological properties made it possible to estimate OM, clays, and porosity for each sample. The compositional analysis, including matrix porosity, OM porosity, and mineralogical composition maps, provided context for the potential of organic-rich and tight rock formations as CO2- based EOR targets or CO2 storage targets.

Advanced image analysis techniques were applied to better understand and quantify factors that could affect CO2 storage in the Bakken Formation, with an ultimate goal of improved method development to estimate CO2 storage potential of unconventional reservoirs. Discernible differences in fabric, mineral, and elemental content in comparable lithofacies across wellsites provided insight into the nature of the Bakken Formation, which could serve as a proxy for other tight rock, organic-rich reservoirs that could be potential targets for both CO2-based EOR and CO2 storage.

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