Gas genesis and storage in source rocks: Geochemistry, organic petrography, and porosimetry

Low-temperature (60 and 100 °C) and long-term (6 months to 5 years) heating of shales (Cretaceous Mowry Shale, Eocene Mahogany Shale, and Devonian to Mississippian New Albany Shale) and coals (Pennsylvanian Springfield Coal and Paleocene Wilcox Lignite) with low maturities demonstrated catalytic methane generation, and this finding may explain the occurrence of non-biogenic natural gas plays where insufficient thermal maturity is at odds with the conventional thermal cracking paradigm. Our heating experiments utilized source rock chips in the presence of hydrogen-isotope contrasting waters at hydrostatic pressures from 0.1 to 300 MPa. Higher temperature, longer heating time, and lower hydrostatic pressure enhanced catalytic methanogenesis. In contrast to earlier findings of hydrogen isotopic exchange between water and methane from hydrous pyrolysis experiments, the water hydrogen isotopic composition had limited to no influence on the delta2H value of methane generated catalytically in this study, which points to an anhydrous microenvironment where catalytic methanogenesis occurs in close contact with hydrophobic organic matter.;Maturity plays an important role in hydrocarbon generation. Vitrinite reflectance (VR0 in %) routinely evaluates thermal maturity in sedimentary basins. However, misidentifying solid bitumen as vitrinite leads to erroneous evaluations. A systematic and statistically sound basis for comparing VR0 values from different macerals has been developed for improved maceral differentiation and maturity assessment.;Porosity in shales is important for gas storage. As organic-rich shale thermally matures, generated bitumen and oil can fill pore spaces and reduce porosity. Soxhlet extraction of solvent-soluble bitumen and oil from shales opens additional pore space as documented by N2 and CO2 gas adsorption; however, specific effects on mesoporosity and microporosity depend on maturity, total organic carbon (TOC) content, solvent type, and grain size of samples. Comparative porosities of original and Soxhlet-extracted shales constrain porosity evolution along with maturation, as well as the effect of partial oil/bitumen filling and blocking of pores.