Grain assemblages and diagenesis of the Mississippian organic-rich Caney Shale, Ardmore Basin, Oklahoma

Hydrocarbon production from the Mississippian Caney Shale, Ardmore Basin, is highly influenced by the heterogeneous rock physical properties. This research aims to establish a petrographic model to better understand the potential diagenetic pathways and evolution of bulk physical properties among different lithologies in the Caney Shale. The lithological variation, primary grain assemblages, diagenetic features, and evolution of bulk petrophysical properties were investigated based on detailed petrographic, geochemical, and petrophysical analysis of 93 core plug samples. The results suggest that the Caney Shale comprises five major lithologies: massive argillaceous mudstone, planar-laminated argillaceous mudstone, planar-laminated siliceous mudstone, massive calcareous mudstone, and massive to wavy-laminated wackestone and packstone. The primary grains in massive argillaceous mudstone are extremely terrigenous-rich (82 to 95 vol. %), whereas variable content of biogenic carbonate or silica allochems can be observed from the other four lithologies. The variations of grain components (extrabasinal versus intrabasinal) among different lithologies are likely caused by periodic sea level changes and the relative distance to the provenance. Due to this variation in grain assemblage composition, the sample suite displays a corresponding evolution in diagenetic variety and petrophysical properties. Massive argillaceous mudstone dominated by extrabasinal grains underwent intense mechanical compaction, resulting in low bulk porosity and permeability, whereas samples from other lithologies with greater amounts of intrabasinal grains underwent considerable cementation, resulting in variable but overall higher porosity and permeability. This study provides an example of new perspectives for linking micrometer-scale petrographic features to the evolution of bulk physical properties in shale.