Artificial maturation methods are used to induce changes in source rock thermal maturity without the uncertainties that arise when comparing natural samples from a particular basin that often represent different levels of maturation and different lithofacies. A novel uniaxial confinement clamp was used on Woodford Shale cores in hydrous pyrolysis experiments to limit sample expansion by simulating the effect of overburden present during thermal maturation in natural systems. These samples were then subjected to X-ray computed tomography (X-CT) imaging and low-field nuclear magnetic resonance (LF-NMR) relaxometry measurements. LF-NMR relaxometry is a non-invasive technique commonly used to measure porosity and pore-size distributions in fluid-filled porous media, but may also measure hydrogen present in hydrogen-bearing organic solids. Standard T1 and T2 relaxation distributions were determined and two dimensional T1-T2 correlation measurements were performed on the Woodford Shale cores. The T1-T2 correlations facilitate resolution of organic phases in the system. The changes observed in NMR-relaxation times correspond to bitumen and lighter hydrocarbon production that occur as source rock organic matter matures. The LF-NMR porosities of the core samples at maximum oil generation are significantly higher than porosities measured by other methods. This discrepancy likely arises from the measurement of highly viscous organic constituents in addition to fluid-filled porosity. An unconfined sample showed shorter relaxation times and lower porosity. This difference is attributed to the lack of fractures observed in the unconfined sample by X-CT.
Understanding the petroleum generation process is important for predicating the quantity and type of petroleum products present both in conventional and unconventional reservoirs. Comparisons of naturally matured samples at differing levels of thermal maturity are useful to identify changes that occur in reservoirs. Unfortunately obtaining samples from a particular stratigraphic interval at different thermal maturities is very difficult and comparing samples with different mineralogies and burial and thermal histories is not ideal. To complement research on naturally matured source rocks, artificial maturation is a valuable tool to study petroleum generation in a controlled manner. A commonly used artificial-maturation method is hydrous pyrolysis (HP; Lewan, 1985, 1997). This method involves placing samples in a reactor with water under moderate helium pressure. The reactor is heated to a pre-selected temperature, generally between 300°C and 365°C, and held for a specific period of time, usually 72 hours. The system is then allowed to cool and the products and residues are collected for mass-balance analysis and characterization.