Leslie F. Ruppert Tristan G. A. Youngs Tom Headen Justin E. Birdwell Michael Cheshire Martha Stokes Aaron M. Jubb 2023 <p id="sp0060">Petroleum within unconventional source-rock reservoirs is hosted in organic matter and mineral pore space as well as in voids and microfractures. Recent work has shown that for source-rock reservoirs in the dry gas window, significant portions of methane (CH₄), the main component of petroleum at elevated maturities, can be stored within fine (&lt;5 nm) organic matter porosity. However, within reservoirs at lower thermal maturities (e.g., peak oil or wet-gas conditions), the distribution and behavior of CH₄<span>&nbsp;</span>and the higher alkanes that comprise gas condensates across pore sizes is unclear, especially for pores with diameters &lt;50 nm. Understanding CH₄<span>&nbsp;</span>distribution within these settings provides insight for petroleum generation, movement, and recoverability, ultimately enabling increased accuracy of estimated ultimate recovery. Here wide Q-range total neutron scattering was used to evaluate perdeuterated methane (CD₄) behavior at reservoir pressures (200–750 bar) and temperature (60 °C) in a sample at the late oil/wet gas thermal maturity stage from the Late Cretaceous Niobrara Formation, an active petroleum producing formation within the Denver-Julesburg Basin, U.S.</p><p id="sp0065">Neutron scattering data show that mesopores within the Niobrara Formation sample exhibit mass fractal scattering, similar to previously measured U.S. marine shale samples. In the presence of CD₄, scattering intensities between Q = 0.02–0.1 Å⁻¹<span>&nbsp;</span>(corresponding to nominal pore diameters from 25 to 5 nm, respectively) decrease with increased pressure up to 750 bar where at least 80% of all pores with ~25 nm diameters are CD₄<span>&nbsp;</span>accessible. In contrast, between Q = 0.1–1 Å⁻¹<span>&nbsp;</span>(corresponding to nominal pore diameters from 5 to 0.5 nm, respectively), scattering intensity initially increased at the lowest CD₄<span>&nbsp;</span>pressure tested (200 bar) before decreasing with increasing pressure. These signal fluctuations with CD₄<span>&nbsp;</span>pressure are interpreted to arise from the creation of pores with diameters &lt;5 nm, likely through deformation of solid bitumen by supercritical CD₄, and/or the incorporation of CD₄<span>&nbsp;</span>within sample organic matter. This new porosity represents an increase of at least ~8% in available pore volume within the sample, although the majority of these pores do not persist following removal of CD₄. Additionally, there is strong evidence for densification of CD₄<span>&nbsp;</span>within the sample indicated by a shift in the CD₄<span>&nbsp;</span>intermolecular scattering peak to higher Q-values compared to bulk CD₄. These results provide insight into fluid properties within source-rock reservoirs at late oil/wet gas thermal maturities, especially as they relate to organic porosity interconnectivity, and are discussed with perspective toward pressure management of gas condensate wells.</p> application/pdf 10.1016/j.coal.2023.104349 en Elsevier Methane pore accessibility, densification, and accommodation by organic matter in the Niobrara Formation at wet-gas thermal maturity conditions article