Nanoscale Imaging of Organic Matter and Wettability of Oil-Window Shales
Five shale samples preserved from the oil window of four unconventional reservoirs were solvent-extracted using toluene and methanol for high-resolution, secondary-electron imaging with field emission scanning electron microscopy (SEM) of the textures of organic matter (OM) present at or near induced fracture surfaces. The samples represent calcareous, siliceous, and mixed mineralogy mudrocks. Based on their nanoscopic textures, nine categories of OM could be distinguished: (1) nonporous particles, (2) low-porosity masses (sometimes  with mineral crystallites), (4) spherule aggregates, (5) intermediate-porosity granular masses, and (6) high-porosity foamy frameworks between mineral grains, together with (7) thick, (8) thin, or (9) very thin coats on mineral surfaces. The frequency of occurrence of these OM categories was qualitatively assessed from all images and compared to those from siliceous gas-window maturity Barnett Shale. The various morphologies are interpreted to comprise types of kerogen, bitumen, and asphaltenic residues, and transformations between them due to maturation and migration are proposed. After the first stage of cleaning and imaging, the samples were re-cleaned with stronger solvents for re-imaging at the same locations. Chloroform/methanol azeotrope only mobilized the very thin coats on clays, which presumably comprise asphaltenic nanoparticles deposited from crude oil as later-stage wettability alteration in the reservoir. Tetrahydrofuran effected only a slight swelling of mature bitumen, while pyridine (at higher temperature) apparently induced further bitumen production from kerogen and partially dissolved some preexisting bitumen but did not dissolve most of the OM present.