Comparing “apples” and “oranges” in the Woodford Shale: Pitfalls of the thermal maturity gradient approach for constraining evolution of mudrock porosity

Abstract

The thermal maturity gradient approach is widely used in comparative studies of mudrocks to assess the evolution of the pore system. The fundamental assumption in this approach is that the compared samples share similar geochemical, mineralogical and textural properties, so that any differences between them can be reasonably attributed to the impact of thermal maturity and burial alteration. However, achieving the desired range of thermal maturity often requires sampling from widely spaced localities which may result in unrecognised microtextural differences owing to regional variations in depositional controls, making it challenging to isolate the effects of thermal alteration on pore system evolution. We use samples from the widely studied siliceous Woodford Shale to highlight the importance of sample screening in characterization studies of pore evolution with progressive thermal maturity. Samples were collected from different cores spanning a gradient of thermal maturity (0.5–1.65 %VRc) across the Cherokee Platform, Arkoma and Anadarko basins, and capturing the wider geographic extent of the formation across Oklahoma. Samples were selected from the same lithofacies type but varying stratigraphic position and screened using the standard bulk geochemical and mineralogical criteria that are widely employed to ensure properties are comparable across a sample set. Notwithstanding this screening step, microscale electron microscope characterization revealed substantial microtextural and compositional differences within the sample set. Our results show that total pore volume is significantly influenced by these seemingly subtle differences, likely because pore evolution pathways vary with the abundance and distribution of primary (i.e. biogenic vs detrital grains) and diagenetic sediment constituents. We conclude that standard sample screening approaches that are limited to bulk mineralogical and geochemical characterization, and cannot identify differences in detrital, biogenic and authigenic components or account for textural differences, are insufficient for thermal gradient studies. A combination of high-resolution imaging, mineral mapping, x-ray diffraction, and low-pressure gas adsorption experiments proved better suited for sample screening ensuring an “apples” to “apples” comparison, as well as for detailed characterization bringing to light the impact of microtextural heterogeneity between samples on measured rock properties.