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Petrographic and Spectroscopic Characterization of Boquillas Formation Samples Before and After Hydrous Pyrolysis: Implications for Storage Capacity and Expulsion Characteristics


Reservoir quality is a key factor in understanding petroleum systems. Shale oil plays described as self-sourced, indigenous, or in-situ, like those of the Late Cretaceous Eagle Ford Shale and Boquillas Formation, are unique in that the source rock also serves as the reservoir rock. Therefore, these mudrocks must contain localized storage capacity and exhibit limited apparent petroleum expulsion and migration upon thermal maturation. Micro-scale properties and processes that lead to formation of indigenous shale oil resources must be understood to assess macro-scale phenomena related to production. In this study, a combination of petrographic tools has been applied to a marginally mature (Ro ~0.5%) Boquillas sample before and after hydrous pyrolysis (HP) at 300 and 330 °C for 72 hours. These experiments allowed for examination of micro-scale changes in mineral matrix, organic matter composition, and porosity related to different degrees of thermal stress. Field-emission scanning electron and optical microscopy were applied along with fluorescence and micro-infrared and -Raman spectroscopic methods. Imaged areas were co-registered to allow for direct comparison of results generated by the different characterization tools. Changes in the rock matrix from the unheated sample to those altered by HP are consistent with previous work on similar samples showing alteration/dissolution of authigenic pyrite and kaolinite which infill void spaces in foraminifera and forming of other minerals such as talc along with interparticle void space. Multiple maceral types, including vitrinite, inertinite, solid bitumen, and micrinite were detected and showed shifts in spectral properties with increasing HP temperature consistent within previously observed effects of increased thermal stress. These changes included shifts in fluorescence wavelengths, changes in Raman D and G bands, and loss of aliphatic carbon moieties based on maceral-specific infrared spectra that all indicate increased aromaticity. The nature of production from the Eagle Ford play indicates that the usual expulsion and migration of petroleum from the source rock is mitigated, possibly by intrinsic micro-scale porosity. This porosity accommodates the volume change during conversion of solid organic matter to petroleum fluids. Changes in organic and mineral components during thermal maturation may also aid in accommodation of generated fluids while maintaining limited permeability.