COMBINED SEM AND REFLECTED LIGHT PETROGRAPHY OF ORGANIC MATTER IN THE NEW ALBANY SHALE (DEVONIAN-MISSISSIPPIAN) IN THE ILLINOIS BASIN: A PERSPECTIVE ON ORGANIC POROSITY DEVELOPMENT WITH THERMAL MATURATION

Organic matter (OM) pores are an important aspect of pore systems in unconventional gas shale reservoirs. Aside of thermal maturity, maceral type critically controls the development of secondary OM-hosted porosity. Three New Albany Shale samples (Devonian-Mississippian, kerogen type II sequence) with thermal maturities ranging from R_o (vitrinite reflectance) 0.55% to 1.42% were analyzed using a reflected light microscope and a field emission scanning electron microscope (SEM). Organic porosity was examined in specific organic macerals at different thermal maturities. Vitrinite and inertinite, derived from terrestrial woody materials, occur as discrete particles in the studied black shales. Within the resolution limits of the SEM, they do not show development of secondary porosity with thermal maturation. Cellular pores (0.2-10 μm) in inertinite were observed throughout the whole maturity range and were always filled with authigenic quartz or solid bitumen (SB). Because of its thermal degradation, amorphous organic matter (AOM) could not be observed at the maturity of R_o 0.80%. Alginite derived from Tasmanites cysts matures later than AOM and is still present at R_o 0.80%, but it is in the process of transformation to pre-oil bitumen and shows weak orange yellow fluorescence compared to strong greenish yellow fluorescence in the early mature samples. At the maturity of R_o 1.42%, alginite could no longer be detected because of its transformation to hydrocarbons and bitumen. Solid bitumen developed along original AOM networks and occurs as interparticle fills (space between quartz, dolomite, K-feldspar, clays and micas) and cavity fills (cellular pores in inertinite). Solid bitumen becomes smaller and more porous with thermal maturation, and thus becomes increasingly unidentifiable at optical microscope resolution. Solid bitumen-hosted porosity (20-1000 nm) is the main organic porosity type documented in the studied samples, and it is more common in shales within the gas widow than those that are still within the oil window. Secondary nanoporosity in the solid bitumen network plays a significant role in hydrocarbon storage and migration.